Vitamin D supplementation for women during pregnancy

Abstract

Background

Vitamin D supplementation during pregnancy may help improve maternal and neonatal health outcomes (such as fewer preterm birth and low birthweight babies) and reduce the risk of adverse pregnancy outcomes (such as severe postpartum haemorrhage).

Objectives

To examine whether vitamin D supplementation alone or in combination with calcium or other vitamins and minerals given to women during pregnancy can safely improve certain maternal and neonatal outcomes.

Search methods

We searched the Cochrane Pregnancy and Childbirth Trials Register (which includes results of comprehensive searches of CENTRAL, MEDLINE, Embase, CINAHL, ClinicalTrials.gov, the WHO International Clinical Trials Registry Platform, and relevant conference proceedings) (3 December 2022). We also searched the reference lists of retrieved studies.

Selection criteria

Randomised and quasi‐randomised trials evaluating the effect of supplementation with vitamin D alone or in combination with other micronutrients for women during pregnancy in comparison to placebo or no intervention.

Data collection and analysis

Two review authors independently i) assessed the eligibility of studies against the inclusion criteria, ii) assessed trustworthiness based on pre‐defined criteria of scientific integrity, iii) extracted data from included studies, and iv) assessed the risk of bias of the included studies. We assessed the certainty of the evidence using the GRADE approach.

Main results

The previous version of this review included 30 studies; in this update, we have removed 20 of these studies to 'awaiting classification' following assessments of trustworthiness, one study has been excluded, and one new study included. This current review has a total of 10 included studies, 117 excluded studies, 34 studies in awaiting assessment, and seven ongoing studies. We used the GRADE approach to assess the certainty of the evidence. This removal of the studies resulted in evidence that was downgraded to low‐certainty or very low‐certainty due to study design limitations, inconsistency between studies, and imprecision.

Supplementation with vitamin D compared to no intervention or a placebo

A total of eight studies involving 2313 pregnant women were included in this comparison. We assessed four studies as having a low risk of bias for most domains and four studies as having high risk or unclear risk of bias for most domains. The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to placebo or no intervention on pre‐eclampsia (risk ratio (RR) 0.53, 95% confidence interval (CI) 0.21 to 1.33; 1 study, 165 women), gestational diabetes (RR 0.53, 95% CI 0.03 to 8.28; 1 study, 165 women), preterm birth (< 37 weeks) (RR 0.76, 95% CI 0.25 to 2.33; 3 studies, 1368 women), nephritic syndrome (RR 0.17, 95% CI 0.01 to 4.06; 1 study, 135 women), or hypercalcaemia (1 study; no cases reported). Supplementation with vitamin D during pregnancy may reduce the risk of severe postpartum haemorrhage; however, only one study reported this outcome (RR 0.68, 95% CI 0.51 to 0.91; 1 study, 1134 women; low‐certainty evidence) and may reduce the risk of low birthweight; however, the upper CI suggests that an increase in risk cannot be ruled out (RR 0.69, 95% CI 0.44 to 1.08; 3 studies, 371 infants; low‐certainty evidence).

Supplementation with vitamin D + calcium compared to no intervention or a placebo

One study involving 84 pregnant women was included in this comparison. Overall, this study was at moderate to high risk of bias. Pre‐eclampsia, gestational diabetes, and maternal adverse events were not reported. The evidence is very uncertain about the effect of supplementation with vitamin D and calcium on preterm birth (RR not estimable; very low‐certainty evidence) or for low birthweight (RR 1.45, 95% CI 0.14 to 14.94; very low‐certainty evidence) compared to women who received placebo or no intervention.

Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D)

One study involving 1298 pregnant women was included in this comparison. We assessed this study as having a low risk of bias in all domains. Pre‐eclampsia was not reported. The evidence is very uncertain about the effect of supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D on gestational diabetes (RR 0.42, 95% CI 0.10 to 1.73; very low‐certainty evidence), maternal adverse events (hypercalcaemia no events and hypercalciuria RR 0.25, 95% CI 0.02 to 3.97; very low‐certainty evidence), preterm birth (RR 1.04, 95% CI 0.68 to 1.59; low‐certainty evidence), or low birthweight (RR 1.12, 95% CI 0.82 to 1.51; low‐certainty evidence).

Authors' conclusions

This updated review using the trustworthy assessment tool removed 21 studies from the previous update and added one new study for a total of 10 included studies. In this setting, supplementation with vitamin D alone compared to no intervention or a placebo resulted in very uncertain evidence on pre‐eclampsia, gestational diabetes, preterm birth, or nephritic syndrome. It may reduce the risk of severe postpartum haemorrhage; however, only one study reported this outcome. It may also reduce the risk of low birthweight; however, the upper CI suggests that an increase in risk cannot be ruled out. Supplementation with vitamin D and calcium versus placebo or no intervention resulted in very uncertain evidence on preterm birth and low birthweight. Pre‐eclampsia, gestational diabetes, and maternal adverse events were not reported in the only study included in this comparison. Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D) resulted in very uncertain evidence on gestational diabetes and maternal adverse events (hypercalciuria) and uncertain evidence on preterm birth and low birthweight. Pre‐eclampsia was not reported in the only study included in this comparison.

All findings warrant further research. Additional rigorous, high‐quality, and larger randomised trials are required to evaluate the effects of vitamin D supplementation in pregnancy, particularly in relation to the risk of maternal adverse events.

Plain language summary

Does vitamin D supplementation benefit, harm, or have no effects on pregnant women or their babies?

Key messages

Vitamin D supplementation during pregnancy may help improve certain health outcomes in mothers and their babies and reduce the risk of adverse (harmful) pregnancy outcomes.

Public health implications

Vitamin D deficiency during pregnancy has been associated with health complications in mothers and their babies. It is thought that additional vitamin D through supplementation during pregnancy might be needed to protect against these complications.

What did we want to find out?

To determine if vitamin D supplementation during pregnancy can safely improve certain health outcomes in the mothers and their babies (such as fewer preterm births and low birthweight babies) and reduce the risk of adverse pregnancy outcomes (such as excessive bleeding).

What did we do?

This is an update of a review that was first published in 2012 and subsequently updated in 2016 and 2019. We searched for clinical trials (December 2022) that looked at vitamin D supplementation, alone or in combination with calcium or other vitamins and minerals, during pregnancy, in comparison to placebo or no intervention. We reviewed each study using a tool to assess their trustworthiness. We compared and summarised the results of the studies and rated our confidence in the information, based on factors such as study methods and sizes.

What did we find?

The previous version of this review included 30 studies. In this update, we have removed 20 of these studies to 'awaiting classification' and excluded one study due to low trustworthiness. We have added one new study. This current review includes a total of 10 studies, 117 excluded studies, 34 studies in awaiting assessment, and seven ongoing studies.

Overall, we are not sure if supplementation with vitamin D alone compared to no intervention or a placebo (eight studies, 2313 women) prevents pregnancy‐associated hypertension, diabetes during pregnancy, babies born earlier than expected, and kidney disease in this setting. It may prevent severe excessive bleeding at birth (although this was based on a single study) and it may reduce the risk of having a baby with a low birthweight, but an increase in this risk cannot be ruled out at this stage. Also, we are not sure if supplementation with vitamin D and calcium versus placebo or no intervention (one study, 84 women) prevents babies being born earlier than expected and low birthweight. No other outcomes were reported in the only study included. Lastly, we are not sure if supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D) (one study, 1298 women) prevents diabetes during pregnancy, maternal adverse events, babies born earlier than expected, or low birthweight. Other outcomes were not reported.

What are the limitations of the evidence?

The quality and small to medium size of most studies were limitations in this review. Not all the studies provided data about the outcomes we were interested in. More well‐conducted research is needed so that we can be more certain of the effects of vitamin D supplementation in pregnancy on these outcomes. This includes research into possible adverse events, which is lacking in the current evidence.

How up‐to‐date is this evidence?

The evidence is up‐to‐date as of December 2022.

Summary of findings

Summary of findings 1. Vitamin D supplementation compared to placebo or no intervention for pregnancy and neonatal health outcomes.

Vitamin D supplementation compared to placebo or no intervention for pregnancy and neonatal health outcomes
Patient or population: pregnant women and their infants Setting: trials were carried from 2000 to 2015 in countries from Bangladesh, India, New Zealand, Pakistan, and the UK; most trials were conducted outside the tropics and in different seasons Intervention: vitamin D supplementation Comparison: placebo or no intervention
Outcomes	Anticipated absolute effects* (95% CI)		Relative effect (95% CI)	№ of participants (studies)	Certainty of the evidence (GRADE)	Comments
	Risk with placebo/control	Risk with vitamin D supplementation
Pre‐eclampsia	Study population		RR 0.53 (0.21 to 1.33)	165 (1 RCT)	⊕⊝⊝⊝ VERY LOWa,b	Included trial: Sablok 2015
	140 per 1000	74 per 1000 (29 to 187)
Gestational diabetes	Study population		RR 0.53 (0.03 to 8.28)	165 (1 RCT)	⊕⊝⊝⊝ VERY LOWa,b	Included trial: Sablok 2015
	18 per 1000	9 per 1000 (1 to 145)
Maternal adverse events: severe postpartum haemorrhage	Study population		RR 0.68 (0.51 to 0.91)	1134 (1 RCT)	⊕⊕⊝⊝ LOWc	Included trial: Harvey 2012
	169 per 1000	115 per 1000 (86 to 154)
Maternal adverse event: nephritic syndrome	Study population		RR 0.17 (0.01 to 4.06)	135 (1 RCT)	⊕⊝⊝⊝ VERY LOWb,d	Included trial: Yu 2008
	22 per 1000	4 per 1000 (0 to 90)
Maternal adverse event: hypercalcaemia	Study population		Not estimable	1134 (1 RCT)	⊕⊝⊝⊝ VERY LOWc,e	Included trial: Harvey 2012
	0 per 1000	0 per 1000
Preterm birth (less than 37 weeks' gestation)	Study population		RR 0.76 (0.25 to 2.33)	1368 (3 RCTs)	⊕⊝⊝⊝ VERY LOWd,f,g	Included trials: Grant 2013; Harvey 2012; Roth 2010
	42 per 1000	32 per 1000 (10 to 97)
Low birthweight (less than 2500 g)	Study population		RR 0.69 (0.44 to 1.08)	371 (3 RCTs)	⊕⊕⊝⊝ LOWd,g	Included trials: Bhutta 2011; Roth 2010; Sablok 2015
	220 per 1000	151 per 1000 (97 to 237)
*The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: confidence interval; RCT: randomised controlled trial; RR: risk ratio
GRADE Working Group grades of evidence High certainty: We are very confident that the true effect lies close to that of the estimate of the effect Moderate certainty: We are moderately confident in the effect estimate: The true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different Low certainty: Our confidence in the effect estimate is limited: The true effect may be substantially different from the estimate of the effect Very low certainty: We have very little confidence in the effect estimate: The true effect is likely to be substantially different from the estimate of effect

^aDowngraded two levels due to serious design limitations: single study with serious design limitations.

^bDowngraded two levels due to imprecision: wide confidence intervals crossing the line of no effect, single study, small sample size, and few events.

^cDowngraded two levels due to study design: single study with serious limitations due to reporting bias.

^dDowngraded one level due to study design limitations.

^eDowngraded one level due to imprecision: no events.

^fDowngraded one level due to inconsistency: I² > 60%.

^gDowngraded one level due to imprecision: wide confidence interval crossing the line of no effect.

Summary of findings 2. Vitamin D + calcium supplementation compared to placebo or no intervention for pregnancy and neonatal health outcomes.

Vitamin D + calcium supplementation compared to placebo/control for pregnancy and neonatal health outcomes
Patient or population: pregnant women and their infants Setting: trial was carried between 2009 and 2011 in Brazil; Brazil is within the tropics Intervention: vitamin D + calcium supplementation Comparison: placebo/control
Outcomes	Anticipated absolute effects* (95% CI)		Relative effect (95% CI)	№ of participants (studies)	Certainty of the evidence (GRADE)	Comments
	Risk with placebo/control	Risk with vitamin D + calcium supplementation
Pre‐eclampsia	‐	‐	‐	0 RCTs	‐	No trials reported on this outcome
Gestational diabetes	‐	‐	‐	0 RCTs	‐	No trials reported on this outcome
Maternal adverse events (not reported)	‐	‐	‐	0 RCTs	‐	No trials reported on this outcome
Preterm birth (less than 37 weeks' gestation)	Study population		Not estimable	84 (1 RCT)	⊕⊝⊝⊝ VERY LOWa,b	Included trial: Diogenes 2013
	0 per 1000	0 per 1000 (0 to 0)
Low birthweight (less than 2500 g)	Study population		RR 1.45 (0.14 to 14.94)	50 (1 RCT)	⊕⊝⊝⊝ VERY LOWa,c	Included trial: Diogenes 2013
	48 per 1000	69 per 1000 (7 to 711)
*The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: confidence interval; RCT: randomised controlled trial; RR: risk ratio
GRADE Working Group grades of evidence High certainty: We are very confident that the true effect lies close to that of the estimate of the effect Moderate certainty: We are moderately confident in the effect estimate: The true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different Low certainty: Our confidence in the effect estimate is limited: The true effect may be substantially different from the estimate of the effect Very low certainty: We have very little confidence in the effect estimate: The true effect is likely to be substantially different from the estimate of effect

^aDowngraded two levels due to study design: single study with serious limitations.

^bDowngraded two levels due to imprecision: small sample size, no events.

^cDowngraded two levels for imprecision: wide confidence intervals crossing the line of no effect, small sample size, few events.

Summary of findings 3. Vitamin D + calcium + other vitamins and minerals compared to calcium + other vitamins and minerals (but no vitamin D) for pregnancy and neonatal health outcomes.

Vitamin D + calcium + other vitamins and minerals compared to calcium + other vitamins and minerals (but no vitamin D) for pregnancy and neonatal health outcomes
Patient or population: pregnant women and their infants Setting: the only study included in this comparison was conducted in Bangladesh, which is located outside the tropics, and it was conducted in different seasons of the year Intervention: vitamin D + calcium + other vitamins and minerals Comparison: calcium + other vitamins and minerals (but no vitamin D)
Outcomes	Anticipated absolute effects* (95% CI)		Relative effect (95% CI)	№ of participants (studies)	Certainty of the evidence (GRADE)	Comments
	Risk with calcium + other vitamins and minerals (but no vitamin D)	Risk with vitamin D + calcium + other vitamins and minerals
Pre‐eclampsia	Study population		‐	(0 trials)	‐	No trials reported on this outcome
	See comment	See comment
Gestational diabetes	Study population		RR 0.42 (0.10 to 1.73)	1298 (1 RCT)	⊕⊝⊝⊝ VERY LOWa,b	Included trial: Roth 2013
	12 per 1000	5 per 1000 (1 to 20)
Maternal adverse event: hypercalcaemia	Study population		No events	1298 (1 RCT)	⊕⊝⊝⊝ VERY LOWb,c	Included trial: Roth 2013
	23 per 1000	64 per 1000 (28 to 147)
Maternal adverse event: hypercalciuria	Study population		0.25 (0.02 to 3.97)	1298 (1 RCT)	⊕⊝⊝⊝ VERY LOWa,b	Included trial: Roth 2013
	4 per 1000	1 per 1000 (0 to 15)
Preterm birth (less than 37 weeks' gestation)	Study population		RR 1.04 (0.68 to 1.59)	1298 (1 RCT)	⊕⊕⊝⊝ LOWb,d	Included trial: Roth 2013
	93 per 1000	96 per 1000 (63 to 147)
Low birthweight (less than 2500 g)	Study population		RR 1.12 (0.82 to 1.51)	1298 (1 RCT)	⊕⊕⊝⊝ LOWb,d	Included trial: Roth 2013
	162 per 1000	182 per 1000 (133 to 245)
*The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: confidence interval; RCT: randomised controlled trial; RR: risk ratio
GRADE Working Group grades of evidence High certainty: We are very confident that the true effect lies close to that of the estimate of the effect Moderate certainty: We are moderately confident in the effect estimate: The true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different Low certainty: Our confidence in the effect estimate is limited: The true effect may be substantially different from the estimate of the effect Very low certainty: We have very little confidence in the effect estimate: The true effect is likely to be substantially different from the estimate of effect

^aWe downgraded two levels for very serious limitations and imprecision with only one trial, with few events, and wide 95% confidence intervals (CI) crossing the line of no effect, contributing data.

^bWe downgraded one level for serious indirectness as there were multiple nutrient interventions in addition to vitamin D.

^cWe downgraded two levels for very serious limitations and imprecision with only one trial, with zero events, and wide 95% confidence intervals (CI) crossing the line of no effect, contributing data.

^dWe downgraded one level for serious limitations and imprecision due to only one trial with wide 95% confidence intervals (CI) crossing the line of no effect contributing data.

Background

Description of the condition

Vitamin D metabolism

Vitamin D is a fat‐soluble vitamin that comes primarily from exposure to sunlight, and is found naturally only in a few foods, such as fish‐liver oils, fatty fish, mushrooms, egg yolks, and liver (Holick 2007a; Holick 2008). There are two physiologically active forms of vitamin D collectively called calciferol: D₂ and D₃. Vitamin D₂ (also called ergocalciferol) is synthesised by plants while vitamin D₃ (also called cholecalciferol) is subcutaneously produced in humans from 7‐dehydrocholecalciferol upon exposure to ultraviolet light B (UVB) radiation (DeLuca 2004). Vitamin D in supplements is found as either vitamin D₂ or D₃. The latter may be three times more effective than vitamin D₂ in raising serum 25 hydroxyvitamin D (25(OH)D) concentrations, which is the way of measuring vitamin D status, and maintaining those levels for a longer time, particularly during the winter months; also, its metabolites have superior affinity for vitamin D‐binding proteins in plasma (Armas 2004; Logan 2013; McCullough 2007). As vitamin D has a short half‐life, adequate vitamin D intake is necessary in order to ensure sustained circulating levels.

Both D₂ and D₃ forms share a similar metabolism. They are first hydroxylated in the liver to form 25(OH)D (or calcidiol), and then in the kidney to 1,25 di hydroxyl vitamin D (1,25 (OH)₂ D or calcitriol) in response to parathyroid hormone (PTH) levels. Calcitriol is considered an important hormone with active metabolites that are involved in metabolic processes including bone integrity and calcium homeostasis (Wagner 2008).

The major sites of vitamin D action include the skin, intestine, bone, parathyroid gland, immune system, and pancreas as well as the small intestine and colon in the human fetus (Theodoropoulos 2003). Additionally, vitamin D helps maintain normal levels of glucose in the blood, by binding and activating the vitamin D receptor in the pancreatic beta cells, regulating the release of insulin in response to the level of circulating glucose (Clifton‐Bligh 2008; Maghbooli 2008; Palomer 2008; Xuan 2013). Vitamin D also indirectly affects glucose metabolism via the regulation of calcium homeostasis (Xuan 2013).

There is a unique relationship between vitamin D and calcium. PTH is responsible for raising the calcium concentration in the blood through bone resorption, while calcitriol inhibits PTH and allows an increase of serum calcium concentration from sources other than the bone. In the presence of calcitriol, renal and intestinal calcium and phosphorus absorption is augmented, leading to an improved calcium status.

Vitamin D status

Serum calcidiol or 25(OH)D can be used to assess vitamin D status, as it reflects the sum of the vitamin D produced cutaneously and that obtained from foods and supplements (Jones 2008). This metabolite is difficult to measure, with large variations between methods and among laboratories, even when the same methods are used, which may be explained by differences in sample pretreatment or the solvent extraction system used (Hollis 2004; Lankes 2015).

The Institute of Medicine (IOM) defined adequate vitamin D status as having serum 25(OH)D concentrations greater than 50 nmol/L (or 20 ng/mL) in both the general population and pregnant women (IOM 2011). Some investigators propose that concentrations around 80 nmol/L (32 ng/mL) are optimal, since they suppress PTH levels and lead to the greatest calcium absorption and the highest bone mass, reducing the rates of bone loss, falls, and fractures (Dawson‐Hughes 2005; Dawson‐Hughes 2008). It is uncertain whether these higher levels proposed for non‐pregnant adults are also adequate for pregnant women.

Vitamin D status is affected by factors that regulate its production in the skin (i.e. skin pigmentation, latitude, dressing codes, season, ageing, sunscreen use, and air pollution) and by factors affecting its absorption or metabolism (Holick 2007b; Maghbooli 2007). Melanin acts as a filter for ultraviolet (UV) rays, hence reducing the production of vitamin D by the skin. Hispanic and black populations in the USA may have a higher melanin content, and thus have reduced vitamin D photosynthesis (endogenous synthesis from exposure to sunlight) (Clemens 1982), explaining the variations in vitamin D concentration among ethnic groups living in the same geographical areas (Brooke 1980; Egan 2008; Ganji 2012; Matsuoka 1991; Nesby‐O'Dell 2002; Rockell 2005). An individual's skin phototype reflects the extent of sun‐burning versus subsequent tanning after an initial moderate sun exposure after a long period of little or no exposure (Gilchrest 2008). Phototypes I and II have rapid vitamin D photosynthesis after a minimal erythematic dose (MED). In contrast, phototype VI has little vitamin photosynthesis following the same MED dose (Clemens 1982). Differences in latitude have also been shown to influence the concentration of vitamin D, and individuals from countries at high and low latitudes have lower vitamin D levels. The importance of UV rays is further shown by the seasonal variation in the concentration of vitamin D between summer and winter, with higher levels during the summer compared with the winter months (Holick 2007b; Levis 2005). Vitamin D metabolism is also affected in obese individuals, as vitamin D is deposited in body fat stores, making it less bioavailable (Arunabh 2003). More recently, this low vitamin D status in obese individuals has been explained by a simple volumetric dilution of vitamin D in the fat mass (Drincic 2012), resulting in a higher prevalence of low levels of 25‐hydroxyvitamin D and these are more prevalent among overweight and obese individuals compared with normal weight individuals (Vilarrasa 2007; Vimaleswaran 2013; Wortsman 2000). In the same context, sedentary activity is also associated with low vitamin D levels as it may be linked with diminished sunlight exposure (Ohta 2009).

Magnitude of vitamin D deficiency

Vitamin D deficiency (VDD) is a common health problem worldwide (Cui 2023; Jiang 2023; Mogire 2020; Palacios 2014). There is a high prevalence of low vitamin D status in infants, children, adolescents, adults, and elderly people worldwide, even in countries with sun exposure all year round (Palacios 2014). The highest reported prevalence has been found in the Middle East, particularly in girls and women, but also Africa (Mogire 2020). In women, it has been estimated from a recent meta‐analysis including 308 studies with 7,947,359 participants from 81 countries, that the prevalence of VDD is on average 53.3%, while the prevalence of vitamin D insufficiency is on average 81.5% (Cui 2023). Such a high prevalence of VDD and vitamin D insufficiency is alarming and may have important health consequences, as described in the next sections.

In pregnancy, low serum 25(OH)D levels are also common. A review including 17 trials in pregnant and lactating women (two in America, six in Europe, one in Africa, seven in Asia, one in Oceania) found a prevalence of low vitamin D status (defined as concentrations lower than 50 nmol/L) of 33% in the USA and 24% Canadian pregnant women (Palacios 2014). In Europe, the prevalence of low vitamin D status was 45% in Belgium, 35% in the UK, 44% in the Netherlands, 20% in Spain, and 77% in Germany. In addition, the prevalence of VDD (defined as concentrations lower than 30 nmol/L) was 12% in Belgium, 4% in England, and 23% in the Netherlands. The only study reported in Africa reported a very low prevalence of low vitamin D status (1%) in a sample of 139 pregnant women from Tanzania. In Asia, the prevalence of low vitamin D status in pregnant women was very high: 90% in Turkey, 67% in Iran, 72% in Pakistan, 70% to 83% in Kuwait, 96% in India, and 69% in China. The prevalence of VDD was also very high: 50% in Turkey, 45% in Pakistan, 38% to 41% in Kuwait, and 60% in India. In Australia, low vitamin D status was found in 48% and VDD was found in 15% of pregnant women. Another review including 13 trials from seven countries found a prevalence of VDD and vitamin D insufficiency ranging from 39.4% to 76.5% (van der Pligt 2018). They also reported only VDD and found the highest prevalence among Chinese (100%), Turkish (95.6%), Iranian (89.4%), and Pakistani (89.0%) women.

Seasonal variation increases the risk of low serum 25(OH)D levels in pregnancy, with a greater prevalence of lower serum 25(OH)D levels during the winter months compared with the summer months (Nicolaidou 2006; O'Riordan 2008). Differences in latitude have also been shown to influence the concentration of vitamin D in a majority of pregnant women (Sloka 2009).

Maternal vitamin D status and health outcomes

Vitamin D status during pregnancy is the most important stage of the lifecycle, as the fetus completely relies on this source during this period for its development. During pregnancy, 1,25‐dihydroxyvitamin D increases early during pregnancy and continues to increase until delivery (Moller 2013). This large increase in 1,25‐dihydroxyvitamin D appears to be dependent on available 25‐dihydroxyvitamin D levels, but independent of calcium metabolism, which is a unique feature of pregnancy that allows such high levels of 1,25‐dihydroxyvitamin D (Pludowski 2013a). Therefore, maintaining high enough levels of 25‐dihydroxyvitamin D is important to sustain the increased levels of 1,25‐dihydroxyvitamin D during pregnancy. Such levels are still yet to be determined, but several trials have shown that maternal vitamin D status is significantly associated with fetal and neonatal vitamin D status (El Koumi 2013; Sachan 2005), and that maternal vitamin D status is associated with health outcomes during pregnancy and neonatal and infant development. These associations will be described below.

Vitamin D status and hypertensive disorders during pregnancy

Maternal VDD in pregnancy has been associated with an increased risk of pre‐eclampsia (new‐onset gestational hypertension and proteinuria after 20 weeks of gestation), a condition associated with an increase in maternal and perinatal morbidity and mortality (Aguilar‐Cordero 2020). This was evidenced in a meta‐analysis that included 55 studies, indicating that VDD was associated with a higher risk of developing pre‐eclampsia, but this statistical significance depended on the study design used (Aguilar‐Cordero 2020). Another meta‐analysis including 27 trials found that vitamin D supplementation during pregnancy significantly reduced the risk of pre‐eclampsia, which was greater if supplementation started before the 20th week of gestation (Fogacci 2020).

Women with pre‐eclampsia have lower concentrations of 25‐hydroxyvitamin D compared with women with normal blood pressure (Yuan 2021). The low levels of urinary calcium (hypocalciuria) in women with pre‐eclampsia may be due to a reduction in the intestinal absorption of calcium impaired by low levels of vitamin D (August 1992; Halhali 1995). Additionally, pre‐eclampsia and low serum 25(OH)D levels are directly and indirectly associated through biologic mechanisms including immune dysfunction, placental implantation, abnormal angiogenesis, excessive inflammation, and hypertension (Bodnar 2007; Cardus 2006; Evans 2004; Hewison 1992; Li 2002). Vitamin D may influence early placental development and, thus, the development of pre‐eclampsia through its role in gene regulation and expression; however, more studies are needed to confirm this.

Vitamin D status and other maternal conditions

Low serum 25(OH)D levels in early pregnancy have been associated with an elevated risk of gestational diabetes mellitus, as evidenced in a meta‐analysis that included 29 prospective and nested case‐control studies (Milajerdi 2021). This meta‐analysis also showed that vitamin D insufficiency and deficiency significantly increased the risk of developing gestational diabetes mellitus; the risk was higher among those with vitamin D levels of 40 to 90 nmol/L (Milajerdi 2021).

VDD may also lead to a high bone turnover, bone loss, osteomalacia (softening of the bones), and myopathy (muscle weakness) in the mother, in addition to neonatal and infant VDD (El Koumi 2013; Glerup 2000; Lips 2001). Lastly, poor control of maternal diabetes early in pregnancy is inversely correlated with low bone mineral content in infants, as is low maternal vitamin D status (Namgunga 2003).

An adequate vitamin D status may also protect against other adverse pregnancy outcomes. For example, maternal VDD has been linked to caesarean section (Merewood 2009; Scholl 2012), but the mechanisms involved are unclear. It has been suggested that VDD during pregnancy may reduce pelvic muscle strength and control (Scholl 2012), but this needs to be confirmed.

Low prenatal and perinatal maternal vitamin D concentrations can affect the function of other tissues, leading to a greater risk of multiple sclerosis, cancer, insulin‐dependent diabetes mellitus, and schizophrenia later in life (McGrath 2001).

Vitamin D status and preterm birth and low birthweight

Maternal vitamin D status has been found to be associated with various infant birth outcomes in a few meta‐analyses (Aguilar‐Cordero 2020; Fang 2021; Luo 2022; van der Pligt 2018; Zhao 2022). A meta‐analysis that included 55 studies found that women with maternal vitamin D concentrations < 75 nmol/L had higher risk of prematurity (Aguilar‐Cordero 2020). Another meta‐analysis including 71 studies found that the highest vitamin D concentrations were inversely associated with the risk of low birthweight, preterm birth, and small for gestational age compared to the lowest levels (Zhao 2022). This meta‐analysis also showed that each 25 nmol/L increase in vitamin D levels was associated with a 6% reduction in the risk of preterm birth and a 10% reduction in the risk of small for gestational age. Another meta‐analysis including 16 studies found that maternal VDD resulted in a significantly increased risk of low birthweight (Fang 2021). Another meta‐analysis including 23 studies with 5390 participants found that vitamin D supplementation during pregnancy was associated with a significantly increased length at birth and that vitamin D supplementation during the second trimester of pregnancy was associated with a significant increase in birthweight (Luo 2022). Lastly, a systematic review including 13 studies from seven countries found that VDD during pregnancy was associated with low birthweight in four out of seven studies (van der Pligt 2018). However, another meta‐analysis among 24 observational studies did not show that VDD at any time during pregnancy increased the risk of preterm birth (Lian 2021). Since moderate heterogeneity was found in that meta‐analysis in the second trimester, a subgroup analysis was done and the results showed that VDD in the second trimester may significantly increase this risk.

There is not much information on maternal vitamin D status and low birthweight or preterm birth in children born from HIV‐infected pregnant women (Mehta 2009). Studies have reported a high prevalence of VDD among HIV‐infected pregnant women (Eckard 2013; Mave 2012).

Vitamin D status and postnatal growth

Some observational studies suggest that vitamin D levels during pregnancy influence fetal bone development and children's growth (Bodnar 2010; Brooke 1980; Ioannou 2012; Mahon 2010; Morley 2006). A recent meta‐analysis including five trials found that bone mineral density and content in children at age four to six years was higher among mothers supplemented with vitamin D (Moon 2023).

With respect to head circumference, there are inconsistent associations with maternal vitamin D status, as found in a systematic review of nine observational studies (Harvey 2014). However, a study found that head circumference in children nine years of age was significantly associated with maternal calcidiol levels (Gale 2008).

It is not clear if maternal VDD leads to neonatal rickets, since rickets is usually identified later in childhood. Early studies indicate a possible risk for neonatal rickets in the offspring of women with osteomalacia (abnormal softening of the bone because of deficiency of phosphorus, calcium, or vitamin D) (Ford 1973). More recent studies have found that VDD was identified in 92% of rachitic (having rickets) Arab children and 97% of their mothers compared with 22% of nonrachitic children and 52% of their mothers (Dawodu 2005). A positive correlation was found between maternal and child vitamin D levels.

In addition, analyses using data from pregnant women participating in the Southampton Women's Survey, a prospective longitudinal study, found in fetuses of mothers with low vitamin D status a greater femoral metaphyseal cross‐sectional area and a higher femoral splaying index at 19 and 34 weeks' gestation (Mahon 2010), and a significant association between fetal femur volume and vitamin D status (Ioannou 2012), which has been suggested to be possibly related to early rickets development (Harvey 2014).

Vitamin D status and immune response

Vitamin D has direct effects on both the adaptive and the innate immune systems (Miller 2010; Walker 2009). In children, vitamin D insufficiency is linked to autoimmune diseases such as type 1 diabetes mellitus, multiple sclerosis, allergies and atopic diseases (Bener 2009; Miller 2010; Pierrot‐Deseilligny 2010). Various studies have also shown that VDD is strongly associated with tuberculosis, pneumonia, and cystic fibrosis (Chocano‐Bedoya 2009; Hall 2010; Nnoaham 2008; Williams 2008), and both prenatal and perinatal vitamin D deprivation might influence early‐life respiratory morbidity as this vitamin is important for lung growth and development (Devereux 2007; Litonjua 2009). However, a meta‐analysis published in 2020 including six trials with 2898 subjects did not find a significant association between vitamin D intake during pregnancy and the occurrence of asthma in offspring or in the risk of assessed childhood respiratory problems due to maternal supplementation of vitamin D during pregnancy. Similarly, a meta‐analysis including six trials found no effect of vitamin D supplementation during pregnancy on the incidence of eczema in childhood or food allergy (Zeng 2023). Vitamin D may have positive effects on the immune system by up‐regulating the production of the antimicrobial peptides by macrophages and endothelial cells (Wang 2004), which may inactivate viruses and suppress inflammation (Cantorna 2008), and subsequently reduce the severity of infections, but more studies are needed to confirm if vitamin D supplementation during pregnancy has these effects in infants.

Vitamin D toxicity

Vitamin D excess leads to hypercalcaemia (calcium levels are 10.5 mg/dL or higher) and hypercalciuria (urinary excretion of calcium exceeds 250 mg/day in women), which is associated with renal and kidney stones (Heaney 2008). Toxicity in adults usually appear at doses of vitamin D higher than 10,000 international units (IU)/day (250 μg/day), although most of the evidence is based on short‐term exposures (less than six months) (Hathcock 2007; Heaney 2008; IOM 2011; Vieth 1999). Single‐dose supplements providing 7.5 mg (300,000 IU) or more may also be harmful (Roth 2011a).

The potential for vitamin D‐induced teratogeneses (birth defects) and adverse events in the offspring (e.g. growth restriction, delayed ossification, craniofacial hypoplasia) has been suggested by a few studies in rats and rabbits (Ariyuki 1987; Chan 1979; Friedman 1969; Ornoy 1968; Ornoy 1969). However, there are considerable limitations in extrapolating such findings to humans, in whom adverse fetal effects have not reportedly occurred following maternal ingestion of maintenance doses as high as 5 mg (200,000 IU) of vitamin D per day. Overall, animal and human studies show that fetal excess of vitamin D metabolites is unlikely to occur when maternal concentrations are within a normal range (Roth 2011a).

Description of the intervention

The World Health Organization (WHO) currently does not recommend provision of vitamin D supplements during pregnancy as part of routine antenatal care (WHO 2016), mainly due to lack of evidence and only in cases of VDD, which is in alignment with the American Congress of Obstetricians and Gynecologists guidelines (ACOG 2015).

There is ongoing controversy regarding the serum 25(OH)D levels that are considered adequate or optimal for overall health. The US Institute of Medicine has determined that concentrations greater than 50 nmol/L or 20 ng/mL are adequate based on the current studies available (IOM 2011), although many investigators consider that optimal levels should be higher (greater than 75 nmol/L or 30 ng/mL) (Dawson‐Hughes 2005; Hollick 2009). Vitamin D recommendations to maintain adequate levels of serum 25(OH)D also differ among different organisations. The Recommended Nutrient Intake (RNI) established by the WHO/Food and Agriculture Organization of the United Nations is 200 IU/day (5 μg/day) of vitamin D for pregnant women (WHO 2004). The European Food Safety Authority (EFSA) and the Institute of Medicine in the USA recommend 600 IU/day (15 μg/day) of vitamin D for pregnant women (EFSA 2016; IOM 2011). The Royal College of Obstetricians and Gynaecologists recommend 400 IU/day (10 μg/day) for all pregnant women (RCOG 2014). For high‐risk women (dark skin, reduced exposure to sunlight, or those who are socially excluded or obese), they recommend at least 1000 IU/day (25 μg/day). In addition, for women at high risk of pre‐eclampsia, they recommend at least 800 IU/day (20 μg/day), combined with calcium. An expert panel in Central Europe recommended 1500 to 2000 IU/day (37.5 to 50.0 μg/day) (Pludowski 2013b). This is consistent with a carefully designed trial among mostly white‐skinned women during pregnancy at a northern latitude, in which the estimated vitamin D intake required to maintain serum 25(OH)D at ≥ 50 nmol/L in most women was about 29 μg/day (O'Callaghan 2018). More studies are needed among dark‐skinned pregnant women and women from other latitudes to determine the vitamin D intake level needed to maintain the serum 25(OH)D at ≥ 50 nmol/L.

Recommendations on the use of vitamin D supplements during pregnancy also vary, ranging from 200 to 400 IU/day (5 to 10 μg/day) (Canadian Paediatric Society 2007; UK Department of Health 2009). The American Academy of Pediatrics suggests that healthcare professionals who provide obstetric care should consider monitoring maternal vitamin D status by measuring its concentrations in pregnant women (Wagner 2008). Different investigators have suggested that a supplemental dose of vitamin D of 1000 to 1600 IU (25 to 40 μg/day) might be necessary to achieve the optimal level of this vitamin in the body (Dawson‐Hughes 2005). This dose is expected to raise serum 25(OH)D by 1.2 nmol/L for every μg (40 IU) of vitamin D₃ given orally to individuals with low 25‐hydroxyvitamin D levels; those with higher baseline concentrations would have smaller increments with the same dose (Dawson‐Hughes 2005). Others have suggested that doses around 1000 IU/day may be needed in order for pregnant women to maintain a serum 25(OH)D level of more than 50 nmol/L (20 ng/mL) (Heaney 2003; Hollis 2004; Hollis 2007; Vieth 2001). Higher doses have also been suggested, such as weekly doses of 5000 IU (125 μg/week) (Utiger 1998) or a single dose of 200,000 IU (5 mg) or greater (Mallet 1986; Sahu 2009; Yu 2009).

Since vitamin D can also be synthesised by the skin upon exposure to sunlight, increasing casual sun exposure for reaching the optimal serum levels has been recommended (Holick 2002). However, as excessive UV radiation is a carcinogen, it might be worth obtaining additional vitamin D from foods or supplements.

How the intervention might work

There is strong evidence that maternal vitamin D supplementation significantly increases maternal vitamin D status and there is good evidence that it also improves infant vitamin D status (Gallo 2020). This increase in maternal status may in turn have an effect on various health outcomes. Several recent meta‐analyses have found a potential effect of vitamin D supplementation for preventing gestational diabetes mellitus, pre‐eclampsia, preterm birth, low birthweight, and other infant outcomes (Aguilar‐Cordero 2020; Fang 2021; Luo 2022; van der Pligt 2018; Zhao 2022). However, these meta‐analyses have generally not included a measure to exclude low‐quality trials. This is important to include in meta‐analyses due to the increase in the number of publications from untrustworthy and potentially fraudulent trials.

Why it is important to do this review

Currently, most countries do not include vitamin D supplementation as part of their routine antenatal care. As stated by the Working Group convened by the Sackler Institute for Nutrition Science at the New York Academy of Sciences and the Bill & Melinda Gates Foundation (in co‐ordination with a scientific organising committee to assess the global prevalence and disease burden of vitamin D deficiency), vitamin D affects pregnancy and birth outcomes, but the evidence is conflicting (Roth 2018).

This review updates the previous Cochrane review on vitamin D supplementation in pregnancy (Palacios 2019). The 2019 review included 30 trials (7033 women) and concluded that supplementing pregnant women with vitamin D alone probably reduces the risk of pre‐eclampsia, gestational diabetes, and low birthweight, and may reduce the risk of severe postpartum haemorrhage. Also, the 2019 review found that supplementing with vitamin D and calcium probably reduces the risk of pre‐eclampsia but may increase the risk of preterm births (these findings warrant further research), while supplementing with vitamin D and other nutrients probably has no effect on some of these outcomes. However, as pointed out by Kiely 2020, there were only a few trials with limited sample sizes that lacked sufficient information to confidently establish the reliability of their findings.

Therefore, the present review is not only an update, since the previous review was published in 2019, but it is also part of a pilot project assessing Cochrane Pregnancy and Childbirth’s screening tool for scientific integrity/trustworthiness (Weeks 2023). This trustworthiness screening was implemented to ensure that high‐quality, relevant, and accessible systematic reviews are produced in response to the increase in the number of publications from untrustworthy and potentially fraudulent trials. This is important as these reviews are used for evidence‐informed health decision‐making for pregnant women and their infants.

The use of this trustworthiness screening resulted in the removal of 21 studies from the previous version of the review. As a result, in this update of the review from 2019, only one study was included that also passed the trustworthiness screening. Therefore, more high‐quality studies are needed to help establish practice guidelines at the population level. Also, information on the most effective and safe dosage, the optimal dosing regimen (daily, intermittent, or single doses), the timing of initiation of vitamin D supplementation, and the effect of vitamin D when combined with other vitamins and minerals is also needed to inform policymaking. This is being reviewed in another Cochrane review (Palacios 2019b).

Objectives

To examine whether vitamin D supplementation alone or in combination with calcium or other vitamins and minerals given to women during pregnancy can safely improve maternal and neonatal outcomes.

Methods

Criteria for considering studies for this review

Types of studies

We intended to include randomised and quasi‐randomised trials with randomisation at either individual or cluster level, but we only found randomised controlled trials with individual randomisation. We did not include cross‐over trials or any other observational designs (e.g. cohort or case‐control studies) in this meta‐analysis, but we considered such evidence in the discussion, where relevant. Abstracts were included if they had enough information to extract the data.

Types of participants

Pregnant women of any gestational or chronological age, parity (number of births) and number of fetuses, living in any country. Pregnant women with pre‐existing conditions were excluded.

Types of interventions

Vitamin D supplementation during pregnancy irrespective of dose, duration or time of commencement of supplementation or type of supplementation (oral or by injection). We included trials testing vitamin D alone or in combination with other micronutrients as long as the intervention and the control group were treated similarly. Specifically, we assessed the following comparisons.

1. Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals)

2. Supplementation with vitamin D + calcium versus placebo or no intervention (no vitamins or minerals)

3. Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamin and mineral supplementation (but no vitamin D)

4. Supplementation with vitamin D + calcium versus calcium supplementation (but no vitamin D)

5. Supplementation with vitamin D + calcium + other vitamins and minerals versus other vitamin and mineral supplementation (but no vitamin D + calcium)

Types of outcome measures

Maternal antenatal clinical and laboratory outcomes and infant clinical and laboratory outcomes as described below.

Primary outcomes

Maternal

1. Pre‐eclampsia (as defined by trialists).

2. Gestational diabetes (as defined by trialists).

3. Adverse events as reported by trial authors (e.g. severe postpartum haemorrhage, nephritic syndrome, hypercalcaemia, hypercalciuria, kidney stones, etc.).

Infant

1. Preterm birth (less than 37 weeks' gestation).

2. Low birthweight (less than 2500 g).

Secondary outcomes

Maternal

1. Impaired glucose tolerance (as defined by trialists).

2. Caesarean section.

3. Gestational hypertension (as defined by trialists).

4. Maternal death (death while pregnant or within 42 days of termination of pregnancy).

5. Vitamin D concentration at term (25‐hydroxyvitamin D, 25(OH)D, in nmol/L).

Infant

1. Birth length (cm).

2. Head circumference at birth (cm).

3. Birthweight (g).

4. Admission to special care (including intensive care) during the neonatal period (within 28 days after delivery).

5. Stillbirth (as defined by trialists).

6. Neonatal death (within 28 days after delivery).

7. Apgar score less than seven at five minutes.

8. Neonatal infection (e.g. respiratory infections within 28 days after delivery).

9. Very preterm birth (less than 32 weeks' gestation).

Search methods for identification of studies

The following search methods section of this review is based on a standard template used by Cochrane Pregnancy and Childbirth.

Electronic searches

We searched the Cochrane Pregnancy and Childbirth Trials Register in collaboration with their Information Specialist (3 December 2022).

The Register is a database containing over 34,000 reports of controlled trials in the field of pregnancy and childbirth. It represents the contemporary results of over 30 years of searching, including handsearched journals and conference proceedings, and journals reviewed via a current awareness service. For the detailed search strategies used to populate Cochrane Pregnancy and Childbirth's Trials Register for the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, Embase, and CINAHL, see Appendix 1; Appendix 2; Appendix 3; Appendix 4.

Briefly, Cochrane Pregnancy and Childbirth’s Trials Register was maintained by their Information Specialist and contains trials identified from the following.

• Monthly searches of the Cochrane Central Register of Controlled Trials (CENTRAL). CENTRAL contains Cochrane's centralised searches of PubMed, Embase, CINAHL, ClinicalTrials.gov, and WHO's International Clinical Trials Registry Platform (ICTRP).

• Weekly searches of MEDLINE (Ovid).

• Weekly searches of Embase (Ovid).

• Monthly searches of CINAHL (EBSCO).

• Regular scanning of 30 journals and the proceedings of major conferences.

• Weekly current awareness alerts for a further 44 journals plus monthly BioMed Central email alerts.

Search results are screened and the full text of all relevant trial reports identified through the searching activities described above is reviewed. Based on the intervention described, each trial report is assigned a number that corresponds to a specific Pregnancy and Childbirth review topic (or topics), and is then added to the Register. The Information Specialist searches the Register for each review using this topic number rather than keywords. This results in a more specific search set that has been fully accounted for in the relevant review sections (Included, Excluded, Awaiting Classification, or Ongoing).

In addition, we searched ClinicalTrials.gov and the World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP) (3 December 2022) for unpublished, planned, and ongoing trial reports using the search methods detailed in Appendix 5.

We did not apply any language or date restrictions.

Searching other resources

We also searched the reference lists of retrieved studies.

Data collection and analysis

For methods used in the previous version of this review, seePalacios 2019. For this update, the following methods were used for assessing the reports that were identified as a result of the updated search. The following methods section of this review is based on a standard template used by Cochrane Pregnancy and Childbirth.

Selection of studies

Two review authors (AC, JW) independently assessed for inclusion all the references identified through the search. All the papers were assessed in duplicate, and we resolved any disagreements through discussion; if required, we consulted a third review author (CP or LK). For studies that were published only as abstracts or study reports that contained little information on methods, we attempted to contact the authors to obtain further details of study design and results. We were able to screen all the potentially eligible studies. We created a study flow diagram to map out the number of records identified, included, and excluded (Figure 1).

1.

Study flow diagram

Screening eligible studies for scientific integrity/trustworthiness

We applied trustworthiness checks following Cochrane Pregnancy and Childbirth editorial policy. The Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool had four criteria, or domains, at the time of our screening process (Weeks 2023). These were on governance, baseline data, feasibility, and results. We refined the tool by developing a list of questions for each criterion, to facilitate the screening process, and to allow for historical changes in the expectations for research methods. All studies meeting our inclusion criteria were evaluated by at least two authors using the tool (AC, JW). A third author was consulted in cases where the decision was not immediately clear to select studies that, based on available information, were deemed to be sufficiently trustworthy to be included in the analysis. We only included data from abstracts if, in addition to the trustworthiness assessment, the study authors confirmed in writing that the data to be included in the review came from the final analysis and would not change. If such information was not available/provided, the study remained in ‘awaiting classification’.

The detailed criteria are described below and in Figure 2:

2.

Process for using the Cochrane Pregnancy and Childbirth criteria for assessing the trustworthiness of a study (Weeks 2023).

1. Research governance: Are there any retraction notices or expressions of concern listed on the Retraction Watch Database relating to this study? Was the study prospectively registered (for those studies submitted during or after 2010)? And have the authors provided a plausible reason? Did the trial authors provide/share the protocol and/or ethics approval letter? For the authors that were reached out to, did they engage in communication with Cochrane Pregnancy and Childbirth within the agreed timelines? Did the trial authors provide individual patient data (IPD)? If not, was there a plausible reason?

2. Baseline characteristics: Is the study free from characteristics of the study participants that appear too similar, e.g. the distribution of the mean (SD) excessively narrow or excessively wide, as noted by Carlisle 2017.

3. Feasibility: Is the study free from characteristics that could be implausible (e.g. large numbers of women with a rare condition recruited from a single centre within 12 months)? In cases with (close to) zero losses to follow‐up, is there a plausible explanation?

4. Results: Is the study free from results that could be implausible? (e.g. massive risk reduction for main outcomes with small sample size, evidence of copying). Compared with other studies in the review, is the study free from very different results? (e.g. huge benefits with no complications or side effects, but others show that the benefits are not so pronounced and there are complications.)

5. Cases of insufficient data: Data from abstracts or posters were only included if, in addition to the trustworthiness assessment, the study authors confirmed in writing that the data to be included in the review came from the final analysis and would not change.

6. Allowing for historical changes in methodological/reporting expectations: Studies submitted before 1990 were judged to have passed the trustworthiness criteria even if study dates, ethics approval, a clear description of randomisation and blinding process, and dropout rates were not explicitly stated. For studies submitted before 2010, mention of the trial registration was not required and either the study dates or the ethics/consent information could be missing from the write‐up. For studies submitted before 1980, we did not try to contact the authors because of the age of the publications and the age of the authors; rather, we made trustworthiness/GRADE decisions based on what information was available in the public domain.

Data extraction and management

We designed a form to extract data. For included studies, all review authors extracted the data using the agreed form. AC entered data into Review Manager software (RevMan Web), and CP and LK checked for accuracy.

We analysed dichotomous data in terms of average risk ratio, and we analysed continuous data in terms of mean difference. There was no need to use the standardised mean difference as trials did not report outcomes using different scales.

Assessment of risk of bias in included studies

Two review authors (AC, JW) independently assessed the risk of bias for each study using the criteria outlined in the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011). We resolved any disagreement by discussion and consulted the third author (CP, LK).

(1) Random sequence generation (checking for possible selection bias)

We described for each included study the method used to generate the allocation sequence in sufficient detail to allow an assessment of whether it should produce comparable groups.

We assessed the method as:

• low risk of bias (any truly random process, e.g. random number table; computer random number generator);

• high risk of bias (any non‐random process, e.g. odd or even date of birth; hospital or clinic record number);

• unclear risk of bias.

(2) Allocation concealment (checking for possible selection bias)

We described for each included study the method used to conceal the allocation sequence and assessed whether intervention allocation could have been foreseen in advance of, or during recruitment, or changed after assignment.

We assessed the methods as:

• low risk of bias (e.g. telephone or central randomisation; consecutively numbered, sealed, opaque envelopes);

• high risk of bias (open random allocation; unsealed or non‐opaque envelopes);

• unclear.

(3.1) Blinding of participants and personnel (checking for possible performance bias)

We described for each included study the methods used (if any) to blind study participants and personnel from knowledge of which intervention a participant received. We considered that studies were at low risk of bias if they were blinded, or if we judged that the lack of blinding was unlikely to affect results. We assessed blinding separately for different outcomes or classes of outcomes.

We assessed the methods as:

• low, high, or unclear risk of bias for participants;

• low, high, or unclear risk of bias for personnel.

We classified blinding as 'high risk of bias' if the blinding status of a trial was unclear or the trial was open.

(3.2) Blinding of outcome assessment (checking for possible detection bias)

We described for each included study the methods used, if any, to blind outcome assessors from knowledge of which intervention a participant received. We assessed blinding separately for different outcomes or classes of outcomes.

We assessed the methods used for blinding the outcome assessment as:

• low risk of bias;

• high risk of bias;

• unclear.

(4) Incomplete outcome data (checking for possible attrition bias through withdrawals, dropouts, protocol deviations)

We assessed losses to follow‐up and post‐randomisation exclusions systematically for each trial.

We described for each included study, and for each outcome or class of outcomes, the completeness of data including attrition and exclusions from the analysis. We noted whether attrition and exclusions were reported, the numbers included in the analysis at each stage (compared with the total randomised participants), reasons for attrition or exclusion where reported, and whether missing data were balanced across groups or were related to outcomes. We assessed the methods as:

• low risk of bias;

• high risk of bias;

• unclear.

We considered follow‐up to be 'low risk of bias' if more than 80% of participants initially randomised in a trial were included in the analysis and any loss was balanced across groups, unclear if the percentage of initially randomised participants included in the analysis was unclear, and 'high risk of bias' if less than 80% of those initially randomised were included in the analysis or if loss was imbalanced in different treatment groups.

(5) Selective reporting (checking for reporting bias)

We described for each included study how we investigated the possibility of selective outcome reporting bias and what we found.

We assessed the methods as:

• low risk of bias (where it is clear that all of the study’s pre‐specified outcomes and all expected outcomes of interest in the review have been reported);

• high risk of bias (where not all the study’s pre‐specified outcomes have been reported; one or more reported primary outcomes were not pre‐specified; outcomes of interest are reported incompletely and so cannot be used; study fails to include results of a key outcome that would have been expected to have been reported);

• unclear risk of bias.

(6) Other sources of bias

We assessed whether each study was free of other problems that could put it at risk of bias, and we noted for each included study any important concerns we had about other possible sources of bias:

• low risk of further bias;

• high risk of further bias;

• unclear whether there is a risk of further bias.

(7) Overall risk of bias

We summarised the risk of bias at two levels: within studies (across domains) and across studies.

For the first, we made explicit judgements about whether studies were at high risk of bias, according to the criteria given in the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011), and for primary outcomes, we explored the impact of the level of bias through undertaking a Sensitivity analysis.

Measures of treatment effect

Dichotomous data

For dichotomous data, we presented results as an average risk ratio with 95% confidence intervals.

Continuous data

For continuous data, we used the mean difference as the outcomes were measured in the same way between trials; there was no need to use the standardised mean difference to combine trials.

Unit of analysis issues

Cluster‐randomised trials

We planned to include cluster‐randomised trials in the analyses along with individually randomised trials, but we did not find eligible studies with this design. We planned to adjust the standard errors of the results from cluster‐randomised studies using the methods described in the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011), if sufficient information was available to allow for this. We planned to use an estimate of the intracluster correlation coefficient (ICC) derived from the trial (if possible), or from another source. If ICCs from other sources were used, we planned to report this and to conduct sensitivity analyses to investigate the effect of variation on the ICC.

If we had identified both cluster‐randomised trials and individually randomised trials, we would have combined the results from both if there was little heterogeneity between the study designs and the interaction between the effect of intervention and the choice of randomisation unit would be considered as unlikely.

Studies with more than two treatment groups

For studies with more than two intervention groups (multi‐arm studies), we combined groups to create a single pair‐wise comparison (Higgins 2011) and included the disaggregated data in the corresponding subgroup category. When the control group was shared by two or more study arms, we divided the control group (events and total population) over the number of relevant subgroup categories to avoid double counting the participants. The details are described in the Characteristics of included studies tables.

Cross‐over trials

We did not consider cross‐over trials eligible for inclusion.

Dealing with missing data

For included studies, we noted levels of attrition. We explored the impact of including studies with high levels of missing data in the overall assessment of treatment effect by using sensitivity analysis.

For all outcomes, we carried out analyses, as far as possible, on an intention‐to‐treat basis, i.e. we attempted to include all participants randomised to each group in the analyses, and analysed all participants in the group to which they were allocated, regardless of whether or not they received the allocated intervention. The denominator for each outcome in each trial was the number randomised minus any participants whose outcomes are known to be missing.

Assessment of heterogeneity

We assessed statistical heterogeneity in each meta‐analysis using the Tau², I², and Chi² statistics. We regarded heterogeneity as substantial if the I² value was greater than 30% and either the Tau² was greater than zero, or there was a low P value (less than 0.10) in the Chi² test for heterogeneity.

Assessment of reporting biases

We planned to investigate reporting biases (such as publication bias) by using funnel plots for the primary outcomes with 10 or more studies. We planned to assess funnel plot asymmetry visually.

Data synthesis

We carried out statistical analysis using the Review Manager software (RevMan Web). We intended to use fixed‐effect meta‐analysis for combining data where it would be reasonable to assume that studies were estimating the same underlying treatment effect: i.e. where trials were examining the same intervention, and we judged the trials’ populations and methods sufficiently similar.

Since we detected substantial heterogeneity, we used random‐effects meta‐analysis to produce an overall summary of an average treatment effect across trials. We treated the random‐effects summary as the average range of possible treatment effects, and we discussed the clinical implications of treatment effects differing between trials. If the average treatment effect was not clinically meaningful, we did not combine trials.

As we used random‐effects analyses, we present the results as the average treatment effect with its 95% confidence interval, and the estimates of Tau² and I².

Subgroup analysis and investigation of heterogeneity

We planned to investigate any substantial heterogeneity in the primary outcomes by using subgroup analyses among outcomes with more than three studies as follows:

1. by start of supplementation: less than 20 weeks versus 20 weeks of pregnancy or more versus unknown/mixed;

2. by pre‐gestational body mass index (BMI) (kg/m²): underweight (lower than 18.5) versus normal weight (18.5 to 24.9) versus overweight (25 or higher) versus unknown/mixed;

3. by supplementation scheme/regimen: single versus daily versus weekly versus unknown/mixed;

4. by skin pigmentation based on the Fitzpatrick skin tone chart (Fitzpatrick 1988): three or less versus four or more versus mixed/unknown;

5. by latitude: between the Tropics of Cancer and Capricorn versus north of the Tropic of Cancer or south of the Tropic of Capricorn versus unknown/mixed;

6. by season at the start of pregnancy: summer versus winter versus mixed/unknown/unreported.

Sensitivity analysis

We planned to carry out sensitivity analyses to explore the effect of study risk of bias assessed by concealment of allocation, high attrition rates, or both, with high risk of bias studies being excluded from the analyses in order to assess whether this makes any difference to the overall result. In this version of the review there were insufficient data to allow for these planned analyses.

Summary of findings and assessment of the certainty of the evidence

We used the GRADE approach as outlined in the GRADE Handbook in order to assess the certainty of the body of evidence relating to the following outcomes for the main comparisons (1. Vitamin D supplementation compared to placebo or no intervention for pregnancy and neonatal health outcomes; 2. Vitamin D + calcium supplementation compared to placebo or no intervention for pregnancy and neonatal health outcomes; 3. Vitamin D + calcium + other vitamins and minerals compared to calcium + other vitamins and minerals (but no vitamin D) for pregnancy and neonatal health outcomes):

1. Pre‐eclampsia

2. Gestational diabetes

3. Maternal adverse events as reported by trial authors (e.g. severe postpartum haemorrhage, nephritic syndrome, hypercalcaemia, hypercalciuria, kidney stones, etc.)

4. Preterm birth (less than 37 weeks' gestation)

5. Low birthweight (less than 2500 g)

We used the GRADEpro Guideline Development Tool to import data from Review Manager 5.3 (RevMan Web) in order to create summary of findings tables. We produced a summary of the intervention effect and a measure of certainty for each of the above outcomes using the GRADE approach. The GRADE approach uses five considerations (study limitations, consistency of effect, imprecision, indirectness, and publication bias) to assess the certainty of the body of evidence for each outcome. We downgraded the evidence from 'high certainty' by one level for serious (or by two levels for very serious) limitations, depending on assessments for risk of bias, indirectness of evidence, serious inconsistency, imprecision of effect estimates, or potential publication bias.

Results

Description of studies

We identified a total of 163 new reports from the electronic databases; the number of potentially relevant studies found from other sources was zero, with no duplicates. We retrieved a total of 163 papers in full text. We reassessed two studies that were in 'awaiting classification' and six ongoing studies from the previous version of this review. We also reassessed 30 trials that had been included in the previous version of this review. In this current update, we have included 10 studies and excluded a total of 117 studies; 34 studies are awaiting classification and seven are ongoing (see the PRISMA flow diagram: Figure 1).

Results of the search

We assessed the 163 new reports from the updated search and also reassessed six ongoing or unpublished studies (Baird 2016; Jelsma 2013; Judkins 2010; Lindqvist 2010; Mosalanejad 2016; Rasmussen 2009), and the two studies awaiting classification in the previous version of the review (Bimson 2017; Das 2009). Following the assessment, Bimson 2017, Das 2009, Judkins 2010, Lindqvist 2010, Mosalanejad 2016, and Rasmussen 2009 were excluded (reasons detailed in Characteristics of excluded studies). Baird 2016 is still ongoing. Jelsma 2013 is awaiting classification.

Two of the 163 new reports had been retracted, so they were excluded before screening for trustworthiness (Asemi 2013b; Gurkan 2022). Following the assessment of the new reports, one new trial was included (Vafaei 2019), 51 were excluded, 6 were classified as ongoing, and 13 were classified as awaiting classification (reasons detailed in Characteristics of studies awaiting classification). The remaining 91 reports were additional references belonging to new and existing studies.

Overall, 10 studies are now included, 117 are excluded, 34 are awaiting classification, and seven are ongoing.

Screening eligible studies for trustworthiness

We reassessed 30 studies that had been included in the previous version of this review using the trustworthiness criteria. Nine reassessed studies continued to be included in this review: eight studies continued to be included because they met the trustworthiness criteria on the first assessment (Benson 2009; Bhutta 2011; Diogenes 2013; Grant 2013; Harvey 2012; Roth 2010; Roth 2013; Yu 2008); one trial continued to be included because it met the trustworthiness criteria after we communicated with the authors (Sablok 2015).

Out of the 21 studies removed from ‘included’ in the previous update, one was moved to ‘excluded’ because it had been retracted (Asemi 2013a). The other 20 studies were moved to 'awaiting assessment' because they did not pass the trustworthiness assessment. Of these 20, 16 had issues in multiple domains and four in a single domain. For the four studies for which there were issues in only one domain, the reason for all four was a lack of governance. The most common multiple domains were domain 1 (governance; n = 20) and domain 3 (feasibility; n = 16). No study was affected by domain 4 (results). The main issues were over governance (for instance, no prospective clinical trial registration of trials submitted for publication in or after 2010) and feasibility (for instance, the trial dates overlapped with when the paper was sent for publication, or there was less than six months between the end of the trial and its publication, without reasonable explanation from the authors). See Table 4.

1. Trustworthiness issues: why studies were moved from ‘included’ in the previous update to ‘awaiting assessment’ in this update.

	Did not meet trustworthiness criteria (20 studiesa)
Domain	Trustworthiness issues affecting a single domain (4 studies)	Trustworthiness issues affecting multiple domains (16 studies)	Total number of trustworthiness issues
Governance	4	20	24
Baseline		10	10
Feasibility		16	16
Comparison of results		0	0
Lack of information		2	2

^aWe moved 21 studies from ‘included’ in the previous update, but one study (Asemi 2013) was moved from ‘included’ to ‘excluded’ because of retraction, so there are only 20 studies in this table.

For studies published in 1980 or later, we contacted the authors to ask them about their trials' trustworthiness issues, but with no success. For one trial, the author replied by saying they were trying to find the information needed, but at the time of writing this review, this had not yet arrived (Kaur 1991). For 10 studies, the authors did not respond to our queries about trustworthiness issues (Li 2000a; Mallet 1986; Mirghafourvand 2013; Sabet 2012; Samimi 2016; Samimi 2017; Shahgheibi 2016; Singh 2015; Taherian 2002; Vaziri 2016). For eight studies, we were unable to contact the authors about the trustworthiness issues (Asemi 2012; Asemi 2013b; Brooke 1980; Delvin 1986; Mazurkevich 2013; Naghshineh 2016; Sasan 2017; Tehrani 2014). For two studies, we were able to correspond with the authors but the information they sent did not satisfy the trustworthiness criteria (Marya 1987; Marya 1988). One study, which was in the ongoing studies section in the previous review update (Jelsma 2013), was moved to awaiting classification because we are unclear about the intervention. We contacted the authors in March 2023 but have yet to receive a reply.

Of the 163 reports identified in the new search (December 2022), after the trustworthiness tool was applied, one was included because it met the trustworthiness criteria after we communicated with the authors (Vafaei 2019). Thirteen reports were kept in awaiting classification because they did not meet the trustworthiness criteria: for two published reports the authors did not respond to our trustworthiness queries (Chen 2022; Dabbaghmanesh 2019), and we were unable to contact the authors for another two published reports about which we had trustworthiness queries (Persad 2019; Sircar 2021). We are still in correspondence with the authors of one report (Basutkar 2020). Five reports had no published papers and there were no responses to our queries (CTRI/2015/07/006039 (first received 2015); CTRI/2017/12/010850 (first received 2017); IRCT20140317017034N6 (first received 2018); NCT03743922 (first received 2018 Nov 16); NCT04591847 (first received 2020 Oct 19)); another three had no published papers, and we were unable to find any contact details (ChiCTR1900024080 (first received 2019); ChiCTR‐IOQ‐16009309 (first received 2016); Klar 2020).

In total, 10 studies passed the trustworthiness test for inclusion (see Figure 2).

See: Characteristics of included studies; Characteristics of excluded studies; Characteristics of studies awaiting classification; Characteristics of ongoing studies for further details.

Included studies

We included 10 studies (involving 3867 women and their infants) in this updated review (Benson 2009; Bhutta 2011; Diogenes 2013; Grant 2013; Harvey 2012; Roth 2010; Roth 2013; Sablok 2015; Vafaei 2019; Yu 2008). Details of these studies are provided in the Characteristics of included studies table.

Settings

The studies included in this review were carried out from the 2000s to 2019. Studies were conducted in Australia (Benson 2009), Bangladesh (Roth 2010; Roth 2013), Brazil (Diogenes 2013), India (Sablok 2015), Iran (Vafaei 2019), New Zealand (Grant 2013), Pakistan (Bhutta 2011), and the UK (Harvey 2012; Yu 2008).

Latitude

Most studies were conducted either above or below the Tropics of Cancer and Capricorn (Benson 2009; Bhutta 2011; Harvey 2012; Roth 2010; Roth 2013; Sablok 2015; Vafaei 2019; Yu 2008). Only one study was conducted between the Tropics of Cancer and Capricorn (Grant 2013), and one study was conducted just where the Tropic of Capricorn lies (Diogenes 2013).

Seasonality

The reporting of seasons varied among studies, with some occurring during the summer (Roth 2010; Vafaei 2019; Yu 2008), two not reporting the season (Benson 2009; Bhutta 2011), and five reporting mixed seasons (Diogenes 2013; Grant 2013; Harvey 2012; Roth 2013; Sablok 2015).

Participants

The sample size from all the studies ranged between 78 women (Benson 2009) and 1560 women (Roth 2013). Pre‐gestational body mass index (BMI) of the participants, an important determinant of vitamin D status, was reported only in two studies (Diogenes 2013; Sablok 2015). The rest of the studies did not report this. None of the studies used the Fitzpatrick skin tone chart (Fitzpatrick 1988), another important determinant of vitamin D status; however, several studies reported the ethnicity/race of participants. Most studies were among women from the Middle East (Bhutta 2011; Vafaei 2019) or Asia (Roth 2010; Roth 2013; Sablok 2015). Two studies reported that participants were from mixed ethnicity (Benson 2009; Yu 2008), 94% of women in one trial were white (Harvey 2012), one was among white women or black women (Diogenes 2013), and another was among Pacific, European, and Maori women (Grant 2013).

Interventions and comparisons

A total of eight studies compared provision of vitamin D supplement in comparison with placebo or no intervention (Comparison 1: Benson 2009; Bhutta 2011; Grant 2013; Harvey 2012; Roth 2010; Sablok 2015; Vafaei 2019; Yu 2008). One study compared provision of oral vitamin D plus calcium supplements versus no intervention or placebo (Comparison 2: Diogenes 2013). One study compared oral vitamin D plus calcium, iron and folic acid versus calcium, iron and folic acid but no vitamin D (Comparison 3: Roth 2013). No studies evaluated the effects of either oral vitamin D plus calcium supplements versus calcium (Comparison 4), nor oral vitamin D + calcium + other vitamins and minerals supplements versus other oral vitamins and mineral supplements (but no vitamin D + calcium) (Comparison 5).

Start of supplementation

A total of four studies started supplementation before week 20 (Benson 2009; Bhutta 2011; Harvey 2012; Vafaei 2019). The rest of the studies started supplementation at 20 or more weeks' gestation (Diogenes 2013; Grant 2013; Roth 2010; Roth 2013; Sablok 2015; Yu 2008).

Dose of vitamin D used

The dose of vitamin D provided varied in the included studies as well as the regimen. Studies differed in the frequency of supplementation, with some studies using daily doses, weekly doses, monthly doses, or single doses. Some studies had more than one group of vitamin D intervention. For daily, weekly, and monthly dosage, we calculated the total amount in international units (IU) per day. The daily doses used were 200 IU vitamin D in one study (Diogenes 2013); 600 IU vitamin D in one study (Roth 2013); 800 IU vitamin D in another study (Yu 2008); 1000 IU vitamin D in three studies (Grant 2013; Harvey 2012; Vafaei 2019); 2000 IU vitamin D in one study (Grant 2013); 2400 IU vitamin D in one study (Roth 2013); and 4000 IU vitamin D in two studies (Bhutta 2011; Roth 2013). One study started supplementation at 2000 IU per day and if 25(OH)D levels were below 75 nmol/L by week 28 of pregnancy, the dose was doubled to 4000 IU per day (Benson 2009). The study by Roth 2013 gave three different doses during pregnancy as mentioned above: 4200 IU per week or 600 IU/day; 16,800 IU per week or 2400 IU/day; 28,000 IU per week or 4000 IU/day. We combined the data from these groups and, on average, this group received 16,333 IU per week or 2333 IU/day. Grant 2013 also gave one arm 1000 IU vitamin D and another 2000 IU vitamin D. These groups were combined in our analyses.

One study gave single‐dose supplementation of vitamin D: 200,000 IU vitamin D in a group in Yu 2008. One trial used a dose of 35,000 IU vitamin D every week (Roth 2010). For the study Sablok 2015, the dose depended upon the level of serum 25(OH)D levels at baseline; it varied from one dose of 60,000 IU (if serum 25(OH)D levels were > 50 nmol/L), two doses of 120,000 IU (if serum 25(OH)D levels were 25 to 50 nmol/L), or four doses of 120,000 IU (if serum 25(OH)D levels < 25 nmol/L).

Overall, the total provision of supplemental vitamin D provided throughout pregnancy varied. Five studies provided 56,000 IU vitamin D or less (Benson 2009; Diogenes 2013; Grant 2013; Harvey 2012; Roth 2013); four studies provided more than 56,000 to 200,000 IU vitamin D (Bhutta 2011; Roth 2013; Sablok 2015; Yu 2008); and four studies provided more than 200,000 IU of vitamin D (Roth 2010; Roth 2013; Sablok 2015; Vafaei 2019) throughout pregnancy.

Vitamin D form used

The vitamin D was provided in the form of cholecalciferol‐D3 in 20 studies (Benson 2009; Diogenes 2013; Grant 2013; Harvey 2012; Roth 2010; Roth 2013; Sablok 2015) and as ergocalciferol‐D2 in one trial (Yu 2008). Two studies did not report the vitamin D form used (Bhutta 2011; Vafaei 2019).

Doses of calcium in the studies providing vitamin D and calcium supplementation

The doses of calcium provided along with the vitamin D were 500 mg (Roth 2013) and 600 mg (Diogenes 2013).

Health worker cadre

The studies were mostly carried out in the context of antenatal care and the administration of the supplements and the antenatal care was provided by the researchers themselves or through health allied personnel. The outcome measurements were carried out by different groups according to the nature of the outcome, whether it was clinical, biochemical, anthropometric, or dietary assessments. A more detailed description of the health worker cadre is presented in Characteristics of included studies.

Laboratory methodology for the assessment of vitamin D status

Different laboratory methods were used to measure vitamin D status as serum 25(OH)D concentrations. One trial used an immunoassay ELISA kit for their determinations (Sablok 2015); five studies used a chemiluminescent enzyme‐labelled immunometric assay (Benson 2009; Bhutta 2011; Diogenes 2013; Harvey 2012; Vafaei 2019); another trial used isotope‐dilution liquid chromatography‐tandem mass spectrometry (Grant 2013). Only two studies used high‐performance liquid chromatography tandem mass spectroscopy (LCMS/MS) (Roth 2010; Roth 2013). In one study, the laboratory method was not reported (Yu 2008).

Outcomes

Most studies reported laboratory‐assessed outcomes such as vitamin D concentrations (Benson 2009; Bhutta 2011; Grant 2013; Harvey 2012; Roth 2010; Roth 2013; Sablok 2015). Birthweight (g) was reported by all studies except Vafaei 2019. Of our primary outcomes, pre‐eclampsia was reported by Sablok 2015 only, gestational diabetes by Roth 2013 and Sablok 2015, maternal adverse events by Harvey 2012, Roth 2013, and Yu 2008, preterm birth by Diogenes 2013, Grant 2013, Harvey 2012, Roth 2010, and Roth 2013, and low birthweight by Bhutta 2011, Diogenes 2013, Roth 2010, Roth 2013, and Sablok 2015. Generally, other outcomes were reported by just one or two studies.

Funding sources

Studies were funded mainly by research grants from universities, health institutions, and non‐government organisations, sometimes in combination. The Luke Proposch Perinatal Research Scholarship from the Australian and New Zealand College of Obstetrics and Gynaecology Research Foundation supported Benson 2009. The Pakistan Initiative for Mothers and Newborns (PAIMAN) supported Bhutta 2011. The Conselho Nacional de Desenvolvimento Cientıfico e Tecnologico and the Fundacao Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro supported Diogenes 2013. The Health Research Council of New Zealand and Cure Kids supported Grant 2013. The Arthritis Research UK, Medical Research Council, Bupa Foundation, and National Institute for Health Research supported Harvey 2012. The Thrasher Research Fund supported Roth 2010 and the Bill & Melinda Gates Foundation supported Roth 2013. The Institute of Obstetrics and Gynaecology Trust and the Wolfson and Weston Research Centre for Family Health supported Yu 2008. Sablok 2015 was self‐funded. Vafaei 2019 was financed by the Vice Chancellor for Research of the Shiraz University of Medical Science, Shiraz, Iran (Grant No. 94-01-01-9357). No studies had funding sources of concern, e.g. vitamin D manufacturers or similar.

Declarations of interest

The following studies reported that none of the authors had conflict of interests: Benson 2009; Diogenes 2013; Grant 2013; Roth 2010; Roth 2013; Sablok 2015; Vafaei 2019. The following studies did not include the conflict of interest statement in their publication: Bhutta 2011; Yu 2008. Only one trial reported conflict of interests: Harvey 2012.

SeeCharacteristics of included studies for a detailed description of the studies, including vitamin D doses used and regimens compared.

Excluded studies

We excluded 117 studies. The main reason for exclusion was that all pregnant women received vitamin D either as different regimens, or as standard care, without placebo or no treatment control (Baqui 2009; Bhatia 2012; Bhatia 2010; Bisgaard 2009; ChiCTR1900027679 (first received 2019); CTRI/2013/10/004056 (first received 2013); CTRI/2022/01/039091 (first received 2022); Das 2010; Dawodu 2013; de Menibus 1984; Enkhmaa 2018 Dec; Gerais 2015; Gupta 2018; Hashemipour 2014; Hollis 2011; Jefferson 2019; Judkins 2010; Kachhawa 2014; Karamali 2015; Khatiwada 2020; Kiely 2015; Lalooha 2012; March 2010; Marya 1981; McLean 2012; Mirzaei‐Azandaryani 2022; Mojibian 2015; Mosalanejad 2016; Mutlu 2013; Nausheen 2014; NCT02272387 (first received 2014); NCT02713009 (first received 2016 Jan 14); NCT04825093 (first received 2021 April 01); Rostami 2018; Rostami 2020; Shakiba 2013; Singh 2021; Soheilykhah 2013; Stephensen 2011; Thiele 2014; Wagner 2006; Wagner 2013; Wagner 2020; Weiss 2009; Xiaomang 2021; Yap 2014; Zhao 2019). Also, we excluded three studies because the treatment groups differed in other nutrients given in the supplements, other than vitamin D (Asemi 2015; Azami 2017; Pandey 2015), two studies had no group with vitamin D (Atkinson 2010; NCT02706158 (first received 2016 March 11)), and one study was investigating calcium supplementation (Cullers 2019). We excluded one study because treatment groups differed more than by vitamin D supplementation (Hossain 2012). Other studies' interventions and comparisons did not fit the scope of the review: measuring vitamin D levels early in pregnancy (Fang 2019), comparing sunlight exposure (Hajhashemi 2017), and comparing diet (Kermack 2017; Li 2016).

In addition, seven studies were not randomised trials (Ala‐Houhala 1986; Bhatia 2010; Cockburn 1980; Das 2010; DRKS00005421 (first received 2013); Hanieh 2014; Ito 1994). Three were retracted (Asemi 2013b; Gurkan 2022; Qian 2015). One study unfortunately had all its data destroyed (Lindqvist 2010).

Five studies were conducted on non‐pregnant women (Czech‐Kowalska 2013; Niramitmahapanya 2017; Rasmussen 2009; Taheri 2014; von Hurst 2009), three were conducted among postpartum women (Chandy 2016; Trivedi 2020; Wheeler 2017), and one study was conducted among couples for fertility purposes (Kermack 2017). Studies were also carried out in pregnant women with existing conditions that did not fit within the scope of our review such as: glucose intolerance or gestational diabetes (Asemi 2013a; Asemi 2014; Baidya 2022; Bhavya 2020; Camarena 2022; ChiCTR‐TRC‐14005235 (first received 2014); CTRI/2014/12/005343 (first received 2014); CTRI/2019/01/017185 (first received 2019); Gunasegaran 2020; Hosseinzadeh 2020; Huang 2021; IRCT20100102002954N11 (first received 2018); IRCT20120718010324N59 (first received 2020); IRCT2012101611144N1 (first received 2012); IRCT20130616013678N29 (first received 2019); IRCT201306253140N11 (first received 2013); IRCT2015022714275N2 (first received 2015); IRCT20150607022585N3 (first received 2018); IRCT2015122725725N1 (first received 2016); Jamilian 2016; Jamilian 2017a; Jamilian 2017b; Jamilian 2019; Karamali 2014; Li 2016; Moghaddam 2012; Mozzafari 2010; NCT03645109 (first received 2018 Aug 24); Razavi 2017; Simsek 2011; SLCTR/2018/020 (first received 2018); Valizadeh 2016; Yazdchi 2016; Zhang 2016), other chronic conditions (Etemadifar 2015; Shi 2017; Sudfeld 2017), vitamin D deficiency (Bimson 2017), periodontitis (Adegboye 2020), and those with sleep disorders (IRCT20120718010324N61 (first received 2021)). One reference referred to a trial registered in 1986 on the Oxford Database of Perinatal Trials and reported the recruitment and follow‐up completed in 1979, but there were no reports available, and we were unable to locate the author who registered the trial (MacDonald 1986).

For more detailed descriptions of excluded studies along with the reasons for exclusion, seeCharacteristics of excluded studies.

Risk of bias in included studies

Overall, risk of bias varied across and within the studies. We assessed no studies to be at low risk of bias in every domain.

Allocation

Sequence generation

We assessed 10 studies as having adequate methods for generating the randomisation sequence. Seven studies used computer‐generated random number sequences (Diogenes 2013; Grant 2013; Harvey 2012; Roth 2010; Roth 2013; Sablok 2015; Yu 2008), two used permuted block randomisation (Bhutta 2011; Vafaei 2019), and one study used a random numbers table to randomise the intervention groups (Benson 2009).

Allocation concealment

We judged that seven studies had adequate methods of allocation concealment (Benson 2009; Bhutta 2011; Grant 2013; Harvey 2012; Roth 2010; Roth 2013; Yu 2008). In the case of Harvey 2012, participants at 28 weeks had their serum 25(OH)D measured and if below 75 nmol/L, the dose was doubled to 4000 IU. The others did not report the methods of concealment (Diogenes 2013; Sablok 2015; Vafaei 2019).

Blinding

Blinding of participants, staff, and outcome assessors

Investigators in six studies reported that they used a double‐blinded design and specified that both participants and those conducting the assessments were blinded (Bhutta 2011; Grant 2013; Harvey 2012; Roth 2010; Roth 2013; Vafaei 2019). One trial was reported as single‐blinded, being blinded for participants only; personnel and assessors were unblinded (Diogenes 2013). One trial reported being single‐blind, but since one of the groups received no supplementation, it was assumed that it was not blinded to participants but to the assessment team (Sablok 2015). Benson 2009 and Yu 2008 were not blinded.

Incomplete outcome data

Five studies had complete data: Grant 2013; Harvey 2012; Roth 2010; Roth 2013; Yu 2008. Vafaei 2019 reported loss to follow‐up due to obstetric complications. The others did not report on attrition, missing data, and lack of intention‐to‐treat analyses.

Selective reporting

We did not have access to the study protocol for Sablok 2015 or Vafaei 2019, and protocols for Harvey 2012 and Yu 2008 did not pre‐specify outcomes, so formally assessing reporting bias was not possible. The remaining six studies reported all the outcomes pre‐specified in their protocols (Benson 2009; Bhutta 2011; Diogenes 2013; Grant 2013; Roth 2010; Roth 2013). However, it should be noted that all except Roth 2013 specified only their laboratory outcomes such as bone mass and vitamin D levels. Insufficient studies contributed data to allow us to carry out exploration of possible publication bias by using funnel plots.

Other potential sources of bias

This varied in the different studies. For example, Harvey 2012 reported that participants were allowed to continue taking their own multivitamin, but they did not specify who took those supplements and who did not take them during the study. There were some issues of unclear reporting in Vafaei 2019 and Yu 2008.

We have also included figures that summarise our risk of bias assessments (Figure 3; Figure 4).

3.

Risk of bias graph: review authors' judgements about each risk of bias item presented as percentages across all included studies.

4.

Risk of bias summary: review authors' judgements about each risk of bias item for each included study.

Effects of interventions

See: Table 1; Table 2; Table 3

In this updated review we included 10 studies assessing a total of 3697 women. One study did not report on any outcomes relevant to this review (Vafaei 2019). We organised the summary results by comparison and by primary and secondary outcomes.

In the Data and analyses tables, we set up three of the five prespecified comparisons as there were no outcome data for Comparison 4 (Supplementation with vitamin D + calcium versus calcium supplementation (but no vitamin D)) or 5 (Supplementation with vitamin D + calcium + other vitamins and minerals versus other vitamins and minerals supplementation (but no vitamin D + calcium)). For comparisons 1, 2, and 3, we set up tables for all primary outcomes (even where no data were available), not only to highlight gaps in the current research evidence, but also to be able to add any data that may become available in future updates.

SeeData and analyses for detailed results for primary and secondary outcomes.

For each of the comparisons, we have indicated the number of studies contributing data and the total number of women recruited in these studies. However, for some outcomes, only one or two studies provided data and due to loss to follow‐up denominators for particular outcomes may have been considerably lower than the randomised sample. Therefore, we have indicated the number of studies contributing data and the number of women included in that analysis.

Because no outcome was reported by more than three studies, we were not able to conduct subgroup analyses. Also, as we only considered one study of high quality, we did not perform sensitivity analysis. We considered a study to be of high quality if it was assessed as having a low risk of bias in both the randomisation and allocation concealment domains and additionally a low risk of bias for either blinding or losses to follow‐up.

Comparison 1. Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals)

Eight studies involving 2313 pregnant women were included in this comparison (Benson 2009; Bhutta 2011; Grant 2013; Harvey 2012; Roth 2010; Sablok 2015; Vafaei 2019; Yu 2008). Vafaei 2019 did not contribute any data, so it is not included in the following analysis.

We assessed the following studies as having low risk of bias for allocation and blinding: Bhutta 2011; Grant 2013; Harvey 2012; Roth 2010. We assessed Sablok 2015 as having unclear risk of bias for allocation and high risk for blinding. We assessed Benson 2009 and Yu 2008 to be at low risk for allocation but high risk for blinding. See Table 1.

Primary outcomes

Maternal

Pre‐eclampsia (as defined by trialists)

The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to placebo or no intervention on pre‐eclampsia (risk ratio (RR) 0.53, 95% confidence interval (CI) 0.21 to 1.33; 1 study, 165 women; very low‐certainty evidence; Analysis 1.1).

1.1. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 1: Pre‐eclampsia (ALL)

Gestational diabetes (as defined by trialists)

The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to placebo or no intervention on gestational diabetes (RR 0.53, 95% CI 0.03 to 8.28; 1 study, 165 women; very low‐certainty evidence; Analysis 1.2).

1.2. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 2: Gestational diabetes (ALL)

Maternal adverse events

The evidence suggests that supplementation with vitamin D during pregnancy compared to placebo or no intervention may reduce the risk of severe postpartum haemorrhage; however, only one study reported this outcome (RR 0.68, 95% CI 0.51 to 0.91; 1 study, 1134 women; low‐certainty evidence). The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to the control group on nephritic syndrome (RR 0.17, 95% CI 0.01 to 4.06; 1 study, 135 women; very low‐certainty evidence). The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to the control group on hypercalcaemia, as there was only one study with no cases in any of the groups (Analysis 1.3).

1.3. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 3: Maternal adverse events

Infant

Preterm birth (less than 37 weeks' gestation)

The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to placebo or no intervention on preterm birth (RR 0.76, 95% CI 0.25 to 2.33; 3 studies, 1368 infants; very low‐certainty evidence; Analysis 1.4).

1.4. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 4: Preterm birth (less than 37 weeks' gestation) (ALL)

Low birthweight (less than 2500 g)

The evidence suggests that supplementation with vitamin D during pregnancy compared to placebo or no intervention may reduce the risk of low birthweight; however, the upper CI suggests that an increase in risk cannot be ruled out (RR 0.69, 95% CI 0.44 to 1.08; 3 studies, 371 infants; low‐certainty evidence;Analysis 1.5).

1.5. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 5: Low birthweight (less than 2500 g) (ALL)

Secondary outcomes

Maternal

Caesarean section

The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to placebo or no intervention on the risk of caesarean section (RR 0.95, 95% CI 0.77 to 1.19; 3 studies, 470 women; very low‐certainty evidence; Analysis 1.6).

1.6. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 6: Caesarean section

Gestational hypertension

The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to placebo or no intervention on the risk of gestational hypertension (RR 0.79, 95% CI 0.42 to 1.49; 2 studies, 1130 women; very low‐certainty evidence; Analysis 1.7).

1.7. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 7: Gestational hypertension

Maternal death

The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to placebo or no intervention on the risk of maternal death, as there was only one study with no cases in any of the groups (Analysis 1.8).

1.8. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 8: Maternal death (death while pregnant or within 42 days of termination of pregnancy) (ALL)

Impaired glucose tolerance

No trial reported this outcome.

Maternal vitamin D concentration at term (25(OH)D in nmol/L)

Evidence from six studies (1807 women) indicated that supplementation with vitamin D during pregnancy compared to placebo or no intervention showed a mean difference in 25(OH)D concentration of 37.23 nmol/L (95% CI 8.60 to 65.86) compared to the control group (Analysis 1.9). This result should be interpreted cautiously as the response to supplementation was highly heterogeneous (Tau² = 1249.10, I² = 99%, Chi² test for heterogeneity P < 0.00001) and ranged from 0.8 nmol/L (95% CI ‐0.97 to 2.57) to 96.00 nmol/L (95% CI 87.40 to 104.60). If the studies reporting the lowest or highest levels are removed, heterogeneity remains similar.

1.9. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 9: Maternal vitamin D concentration at term (25‐hydroxyvitamin D) (nmol/L) (ALL)

Infant

Length at birth (cm)

Evidence from two studies (302 infants) indicated that supplementation with vitamin D during pregnancy compared to placebo or no intervention showed a mean difference in birth length of 0.44 cm (95% CI ‐0.05 to 0.93; Analysis 1.10).

1.10. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 10: Birth length (cm)

Head circumference at birth (cm)

Evidence from three studies (1262 infants) indicated that supplementation with vitamin D during pregnancy compared to placebo or no intervention showed a mean difference in head circumference at birth of 0.24 cm (95% CI ‐0.43 to 0.92; Analysis 1.11). There was heterogeneity in the response to the supplementation (Tau² = 0.31; I² = 89%, Chi² test for heterogeneity P = 0.0001); therefore, the results should be interpreted with caution.

1.11. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 11: Head circumference at birth (cm)

Birthweight (g)

Evidence from six studies (1744 infants) indicated that supplementation with vitamin D during pregnancy compared to placebo or no intervention showed a mean difference in birthweight of 30.44 g (95% CI ‐50.06 to 110.95; Analysis 1.12). There was some substantial heterogeneity among studies in terms of the size of the treatment (Tau² = 6155.02; I² = 71%, Chi² test for heterogeneity P = 0.004).

1.12. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 12: Birthweight (g)

Stillbirth (as defined by trialists)

The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to the control group on the risk of stillbirth (RR 0.35, 95% CI 0.06 to 1.98; 3 studies, 584 infants; Analysis 1.13). Only one case of stillbirth out of 364 was reported in the vitamin D group and three cases out of 220 in the control group.

1.13. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 13: Stillbirth

Neonatal death (within 28 days after delivery)

The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to placebo or no intervention on the risk of neonatal death (RR 0.27, 95% CI 0.04 to 1.67; 2 studies, 326 infants; Analysis 1.14). Only one neonatal death out of 193 was reported in the vitamin D group and four neonatal deaths were reported out of 133 in the no intervention or placebo group.

1.14. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 14: Neonatal death

Apgar score less than seven at five minutes

The evidence is very uncertain about the effect of supplementation with vitamin D during pregnancy compared to placebo or no intervention on the risk of Apgar scores less than seven at five minutes (RR 0.53, 95% CI 0.11 to 2.53; 1 study, 165 infants; Analysis 1.15).

1.15. Analysis.

Comparison 1: Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals), Outcome 15: Apgar score less than seven at five minutes

Other infant secondary outcomes

No studies reported on the other pre‐specified infant secondary outcomes: admission to special care (including intensive care) during the neonatal period (within 28 days after delivery), neonatal infection (e.g. respiratory infections), or very preterm birth (less than 34 weeks' gestation).

Comparison 2. Supplementation with vitamin D + calcium versus placebo/no intervention (no vitamins or minerals)

One study involving 84 women made this comparison (Diogenes 2013). We assessed this study to have a high risk of bias. See Table 2.

Primary outcomes

Maternal

Pre‐eclampsia (as defined by trialists)

This outcome was not reported.

Gestational diabetes (as defined by trialists)

This outcome was not reported.

Maternal adverse events

This outcome was not reported.

Infant

Preterm birth (less than 37 weeks' gestation)

This was not estimable because no events were reported for this outcome.

Low birthweight (less than 2500 g)

The evidence is very uncertain about the effect of supplementation with vitamin D and calcium during pregnancy compared to placebo or no intervention on the risk of low birthweight (RR 1.45, 95% CI 0.14 to 14.94; 1 study, 50 infants; very low‐certainty evidence; Analysis 2.1).

2.1. Analysis.

Comparison 2: Supplementation with vitamin D + calcium versus placebo or no intervention (no vitamin or minerals), Outcome 1: Low birthweight (less than 2500 g) (ALL)

Secondary outcomes

Maternal

No secondary maternal outcomes were reported.

Infant

Length at birth (cm)

Evidence from one study (50 infants) indicated that supplementation with vitamin D and calcium during pregnancy compared to placebo or no intervention showed a mean difference in birth length of ‐0.30 cm (95% CI ‐1.68 to 1.08; Analysis 2.2).

2.2. Analysis.

Comparison 2: Supplementation with vitamin D + calcium versus placebo or no intervention (no vitamin or minerals), Outcome 2: Birth length (cm)

Head circumference at birth (cm)

Evidence from one study (54 infants) indicated that supplementation with vitamin D and calcium during pregnancy compared to placebo or no intervention showed a mean difference in head circumference of ‐0.30 (95% CI ‐1.05 to 0.45; Analysis 2.3).

2.3. Analysis.

Comparison 2: Supplementation with vitamin D + calcium versus placebo or no intervention (no vitamin or minerals), Outcome 3: Head circumference at birth (cm)

Birthweight (g)

Evidence from one study (54 infants) indicated that supplementation with vitamin D and calcium during pregnancy compared to placebo or no intervention showed a mean difference in birthweight of ‐100.00 g (95% CI ‐380.80 to 180.80; Analysis 2.4).

2.4. Analysis.

Comparison 2: Supplementation with vitamin D + calcium versus placebo or no intervention (no vitamin or minerals), Outcome 4: Birthweight (g)

Other infant secondary outcomes

No other pre‐specified infant secondary outcomes were reported: neonatal death; admission to special care (including intensive care) during the neonatal period (within 28 days after delivery); stillbirths (as defined by trialists); Apgar score less than seven at five minutes; neonatal infection (e.g. respiratory infections); or very preterm birth (less than 34 weeks' gestation).

Comparison 3. Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D)

One study (1300 women) was included in this comparison (Roth 2013). We assessed it as having a low risk of bias.

Primary outcomes

Maternal

Pre‐eclampsia (as defined by trialists)

The included study under this comparison did not report on this outcome.

Gestational diabetes (as defined by trialists)

The evidence is very uncertain about the effect of supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D on gestational diabetes (RR 0.42, 95% CI 0.10 to 1.73; 1 study, 1298 women; very low‐certainty evidence;Analysis 3.1).

3.1. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 1: Gestational diabetes (ALL)

Maternal adverse events

The evidence is very uncertain about the effect of supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D on maternal hypercalciuria (RR 0.25, 95% CI 0.02 to 3.97; 1 study, 1298 women; very low‐certainty evidence;Analysis 3.2). No confirmed cases were reported for maternal hypercalcaemia.

3.2. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 2: Maternal adverse events

Infant

Preterm birth (less than 37 weeks' gestation)

The evidence is uncertain about the effect of supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D on the risk of preterm birth (RR 1.04, 95% CI 0.68 to 1.59; 1 study, 1298 infants; low‐certainty evidence;Analysis 3.3).

3.3. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 3: Preterm birth (less than 37 weeks' gestation) (ALL)

Low birthweight (less than 2500 g)

The evidence is uncertain about the effect of supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D on the risk of low birthweight (RR 1.12, 95% CI 0.82 to 1.51; 1 study, 1298 infants; low‐certainty evidence;Analysis 3.4).

3.4. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 4: Low birthweight (less than 2500 g) (ALL)

Secondary outcomes

Maternal

Caesarean section

The evidence is uncertain about the effect of supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D on the risk of caesarean section (RR 1.10, 95% CI 0.95 to 1.27; 1 study, 1298 women; Analysis 3.5).

3.5. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 5: Caesarean section

Gestational hypertension

The evidence is uncertain about the effect of supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D on the risk of gestational hypertension (RR 0.93, 95% CI 0.31 to 2.79; 1 study, 1298 women; Analysis 3.6).

3.6. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 6: Gestational hypertension

Maternal death

The evidence is uncertain about the effect of supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D on the risk of maternal death (RR 0.25, 95% CI 0.02 to 3.98; 1 study, 1300 women; Analysis 3.7). Only one maternal death out of 1040 was reported in the vitamin D, calcium, and other vitamins and minerals group and one maternal death was reported out of 260 in the no intervention or placebo group.

3.7. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 7: Maternal death (death while pregnant or within 42 days of termination of pregnancy)

Maternal vitamin D concentration at term (25‐hydroxyvitamin D in nmol/L)

Evidence from one study (635 women) indicated that supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D showed a mean difference in 25(OH)D concentration of 75.17 nmol/L (95% CI 71.97 to 78.37; Analysis 3.8).

3.8. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 8: Maternal vitamin D concentration at term (25‐hydroxyvitamin D) (nmol/L) (ALL)

Infant

Birth length (cm)

Evidence from one study (1297 infants) indicated that supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D showed a mean difference in birth length of 0.00 cm (95% CI ‐0.28 to 0.28; Analysis 3.9).

3.9. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 9: Birth length (cm)

Head circumference at birth (cm)

Evidence from one study (1297 infants) indicated that supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D showed a mean difference in head circumference of 0.00 cm (95% CI ‐0.17 to 0.17; Analysis 3.10).

3.10. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 10: Head circumference at birth (cm)

Birthweight (g)

Evidence from one study (1297 infants) indicated that supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D showed a mean difference in birthweight of ‐7.00 g (95% CI ‐55.95 to 41.95; Analysis 3.11).

3.11. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 11: Birthweight (g)

Stillbirth (as defined by trialists)

The evidence is uncertain about the effect of supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D on the risk of stillbirth (RR 0.66, 95% CI 0.29 to 1.46; 1 study, 1300 women; Analysis 3.12). A total of 21 stillbirths out of 1040 were reported in the vitamin D, calcium, and other vitamins and minerals group and eight stillbirths were reported out of 260 in the no intervention or placebo group.

3.12. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 12: Stillbirth

Neonatal death (within 28 days after delivery)

The evidence is uncertain about the effect of supplementation with vitamin D, calcium, and other vitamins and minerals during pregnancy compared to no vitamin D on the risk of neonatal death (RR 0.69, 95% CI 0.22 to 2.14; 1 study, 1298 women; Analysis 3.13). A total of 11 neonatal deaths out of 1039 were reported in the vitamin D, calcium, and other vitamins and minerals group and four neonatal deaths were reported out of 259 in the no intervention or placebo group.

3.13. Analysis.

Comparison 3: Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D), Outcome 13: Neonatal death

Comparison 4. Supplementation with vitamin D + calcium versus calcium (but no vitamin D)

No studies were included in this comparison.

Comparison 5. Supplementation with vitamin D + calcium + other vitamins and minerals versus other vitamins and minerals (but no vitamin D + calcium)

No studies were included in this comparison.

Discussion

Summary of main results

This updated review evaluated the effects of supplementation with vitamin D alone or in combination with calcium and other vitamins and minerals during pregnancy, using the Cochrane Pregnancy and Childbirth trustworthiness assessment tool. The use of this tool resulted in the removal of 21 studies from the previous update (Palacios 2019), and only one new study with no relevant data was added. This significantly impacted the conclusions of our review. We have now graded the evidence for the efficacy of vitamin D supplementation in this setting as either low‐ or very low‐certainty and most reported outcomes have only one, two, or three studies contributing data.

Supplementation with vitamin D compared to no intervention or a placebo (8 studies, 2313 women)

• Very uncertain evidence of the effect of supplementation with vitamin D on pre‐eclampsia, gestational diabetes, preterm birth, or nephritic syndrome.

• Supplementation with vitamin D may reduce the risk of severe postpartum haemorrhage; however, only one study reported this outcome. It may also reduce the risk of low birthweight; however, the upper CI suggests that an increase in risk cannot be ruled out.

Supplementation with vitamin D and calcium versus placebo/no intervention (one study, 84 women)

• Very uncertain evidence of the effect of supplementation with vitamin D and calcium on preterm birth or low birthweight.

• Pre‐eclampsia, gestational diabetes, and maternal adverse events were not reported.

Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D) (one study, 1298 women)

• Very uncertain evidence of the effect of supplementation with vitamin D, calcium, and other vitamins and minerals on gestational diabetes or maternal adverse events (hypercalciuria).

• Uncertain evidence of the effect of supplementation with vitamin D, calcium, and other vitamins and minerals on preterm birth or low birthweight.

• Pre‐eclampsia was not reported.

No studies evaluated the effects of vitamin D + calcium + other vitamins and minerals versus other vitamins and minerals (but no vitamin D + calcium).

Overall completeness and applicability of evidence

The aim of the present review was to compare studies providing any dose of vitamin D supplementation during pregnancy with placebo or no intervention for improving maternal and neonatal outcomes. In this update, the number of studies included decreased by two‐thirds in comparison to the previous version (Palacios 2019), due to the trustworthiness assessment process removing studies that could be untrustworthy. We implemented this trustworthiness screening to ensure that high‐quality, relevant, and accessible systematic reviews are produced to inform health decision‐making for pregnant women and their infants. The updated review showed that results are less consistent, less precise, and have a higher degree of uncertainty overall. In general, the studies included did not report important maternal outcomes, such as adverse events, impaired glucose tolerance, gestational hypertension, or death, as well as infant outcomes such as neonatal death, admission to special care in the neonatal period, Apgar score less than seven at five minutes, neonatal infection, or very preterm birth. Therefore, more studies are needed to determine the effectiveness and safety of the intervention.

What is missing from the review?

More high‐quality trials are needed for each of the main outcomes, as most outcomes were only reported by a few studies (three or fewer studies), and with larger sample sizes. Most studies were small to medium size, with two studies including fewer than 100 participants (Benson 2009; Diogenes 2013), six studies including 100 to 500 participants (Bhutta 2011; Grant 2013; Roth 2010; Sablok 2015; Vafaei 2019; Yu 2008), and only two studies including more than 500 participants (Harvey 2012; Roth 2013). Many studies did not adjust for baseline serum 25(OH)D levels, which is an important determinant for potential effects on health outcomes (Heaney 2014). The effects of vitamin D supplementation may be more profound among women with vitamin D deficiency. Another missing factor was the lack of specification of pre‐gestational body mass index (BMI) and skin pigmentation, two important determinants of vitamin D status. Most studies provided vitamin D alone or with calcium. Only one study compared vitamin D with other nutrients (Roth 2013), which is what in practice most women would be taking. This is important to evaluate as there could be interactions between nutrients in dietary supplements that should be tested. Furthermore, more trials are needed starting earlier in pregnancy, as only four studies started supplementation before week 20 (Benson 2009; Bhutta 2011; Harvey 2012; Vafaei 2019). The effects of vitamin D may be more important if it starts early in pregnancy as the enzyme 1‐alpha‐hydroxylase, which catalyses the synthesis of 1,25 dihydroxy vitamin D3, has the highest level of expression in the first trimester and it is reduced towards the third trimester, highlighting its possible role early in pregnancy (Zehnder 2002). It also takes about six to eight weeks from the start of the vitamin D supplementation to achieve a steady state in serum 25(OH)D concentrations. Although the study Roth 2013 is the largest so far (> 1500 participants) and tested different vitamin D doses in combination with other nutrients among women with 64% vitamin D deficiency, it started in mid‐pregnancy. This could explain the lack of significant effects on these health outcomes. It is suggested that future trials take into consideration the criteria as reported by Heaney 2014, such as using serum vitamin D baseline levels as an inclusion criterion for entry into the study, using a large enough dose to change vitamin D levels, measuring the change in vitamin D status during the trial, testing the hypothesis of a change in vitamin D status, and assessing the status of other key nutrients to ensure that vitamin D is the only tested nutrient. A limitation of this present review was that bone‐related outcomes were not included, but this was out of the scope of the initial protocol, which was to review the specified outcomes to inform the WHO guidelines on vitamin D supplementation during pregnancy. Also, because only a few studies were included in the updated review, we could not perform subgroup analyses.

With respect to safety, the studies reporting on maternal and infant safety‐related outcomes may suggest that vitamin D supplementation may be safe during pregnancy. However, this was assessed differently in the studies, such as by evaluating haemorrhages, nephritic syndrome, hypercalcaemia, and hypercalciuria. Additional testing may be needed to confirm cases of hypercalcaemia, such as measuring cord blood or neonatal blood, particularly for studies reporting high average maternal serum 25(OH)D levels at term. Therefore, the safety of this intervention still needs further studies. Also, most secondary outcomes defined in this review (maternal death, neonatal admission to intensive care unit, Apgar score less than seven at five minutes, neonatal infection, or very preterm birth) were not reported by any of the studies or only one study, so no conclusions can be drawn. More trials are needed to report on these safety‐related outcomes to have a definite conclusion.

Many studies were removed or not included in this updated review because they did not pass the trustworthiness assessment tool (Weeks 2023). Also, 117 studies were excluded mainly because the comparisons were among different doses of vitamin D without a placebo or no supplementation group; this is being reviewed in another systematic review (Palacios 2019b). We did not include studies with different doses and no placebo, as most countries do not have the policy to include vitamin D in their prenatal supplementation guidelines. Therefore, it is important to first determine if vitamin D supplementation is beneficial against a placebo or no intervention group. Some of the studies were also excluded because they were undertaken in pregnant women with glucose intolerance, gestational diabetes, or other chronic conditions.

Quality of the evidence

The risk of bias was high for blinding in four studies and for attrition in four studies. In addition, the results varied considerably between studies and this could be related to the variability in vitamin D regimens used. For example, two studies used doses of about 200 to 600 IU of vitamin D per day. Although these could be considered low doses, these are the recommended doses for pregnancy by several organisations (EFSA 2016; IOM 2011; RCOG 2014; WHO 2004). Five studies used medium doses of 800 to 2000 IU of vitamin D per day, and four studies used doses > 2000 IU of vitamin D per day. There was also a large variability in the frequency of the supplementation, with six studies providing vitamin D daily and four studies providing the supplementation weekly. Finally, as mentioned before, only four studies started supplementation before week 20. The effects of vitamin D may be more important if it starts early in pregnancy as the enzyme 1‐alpha‐hydroxylase, which catalyses the synthesis of 1,25 dihydroxy vitamin D3, has the highest level of expression in the first trimester and it is reduced towards the third trimester, highlighting its possible role early in pregnancy (Zehnder 2002). These differences may have influenced the results observed.

Based on the risk of bias and the results from the studies included, we evaluated the certainty of the body of evidence for the primary outcomes with the GRADE methodology for Comparison 1 (vitamin D alone versus placebo/no intervention; Table 1), Comparison 2 (vitamin D + calcium versus placebo/no intervention; Table 2), and Comparison 3 (vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals but no vitamin D; Table 3). We considered that publication bias was unlikely, but the risk of bias of the studies, inconsistency, and imprecision resulted in many outcomes with very low‐certainty evidence because of indirectness, imprecision of results, and limitations in study design.

Potential biases in the review process

We identified several potential biases in the review process. They were minimised in two ways: (1) assessment of eligibility for inclusion and data extraction were carried out independently by two review authors and (2) assessments of risk of bias and data entry were also assessed independently by two review authors. However, reviewing the assessments of eligibility and risk of bias required that we make a number of subjective judgements. Others may have reached different decisions regarding these issues. We would encourage readers to examine the Characteristics of included studies table to assist in the interpretation of results.

Agreements and disagreements with other studies or reviews

This review updates the previous Cochrane reviews on vitamin D supplementation in pregnancy (De‐Regil 2012; De‐Regil 2016; Palacios 2019). The 2012 review included six studies with a total of 1023 women, excluded eight studies, and six studies were still ongoing; the 2016 review included 15 studies assessing a total of 2833 women, excluded 27 studies, and 23 studies were still ongoing or unpublished; the 2019 review included 30 studies with a total of 7033 women, excluded 60 studies, identified six as ongoing/unpublished studies, and two studies were awaiting assessment. In the 2019 review, the authors concluded that supplementing pregnant women with vitamin D alone probably reduces the risk of pre‐eclampsia, gestational diabetes, and low birthweight, may reduce the risk of severe postpartum haemorrhage, and may not have an effect on the risk of preterm birth. Supplementing pregnant women with vitamin D and calcium probably reduces the risk of pre‐eclampsia but may increase the risk of preterm births. Supplementing pregnant women with vitamin D and other nutrients may not have an effect on the risk of preterm birth or low birthweight. In this update, the results are very different because the trustworthiness assessment process removed many studies that were originally included in the 2019 review but were assessed as untrustworthy. When these studies were removed, this review showed that the effects of vitamin D supplementation for most outcomes assessed are very uncertain.

Our results are in part in agreement with other similar meta‐analyses. For example, Gallo 2020 compared the effects of vitamin D supplementation with placebo or a lower level of vitamin D supplementation on maternal and neonatal health outcomes. Maternal vitamin D supplementation significantly increased maternal 25(OH)D concentrations (13 studies), although heterogeneity was significant, similar to the present study. They found fair evidence for the effects of vitamin D supplementation on increasing infant birth weight (nine studies), which is different to the present review. No significant effects were observed for pre‐eclampsia, caesarean section, infant gestational age, and birth length. Another meta‐analysis conducted by Oh 2020 found among 11 studies that provided vitamin D supplementation compared to placebo (lower level of vitamin D supplementation) or no vitamin D during pregnancy that vitamin D supplementation may reduce the risk of preterm births (RR 0.64, 95% CI 0.40 to 1.04), contrary to the results of the present review. Another meta‐analysis, including 43 trials, compared vitamin D supplementation versus a placebo group (lower level of vitamin D supplementation) or no vitamin D (Roth 2017). Vitamin D supplementation significantly increased maternal 25(OH)D at term, but the dose‐response effect was weak. Also, women assigned to vitamin D supplementation had infants with higher mean birthweight compared to controls, but no effect was found on the risk of pre‐eclampsia, gestational diabetes, preterm birth, low birthweight, gestational hypertension, caesarean section, admission to hospital, neonatal death, stillbirth, or other adverse events, similar to the present review. Furthermore, a meta‐analysis conducted by Fogacci 2020 among 27 trials comparing vitamin D supplementation during pregnancy with no supplementation or a lower level of vitamin D supplementation found that women supplemented with vitamin D had a reduced risk of pre‐eclampsia (odds ratio (OR) 0.37, 95% CI 0.26 to 0.52). However, many of the trials included in the meta‐analysis by Fogacci 2020 were removed from the present review once the trustworthiness screening was used. Another meta‐analysis by Liu 2022 among 42 trials found that vitamin D supplementation during pregnancy was associated with a lower risk of neonatal death (RR 0.69, 95% CI 0.48 to 0.99; 13 trials) and a higher length at birth (MD 0.27 cm, 95% CI 0.02 to 0.51), but it was not associated with higher birthweight (MD 37.07 g, 95% CI ‐9.67 to 83.80) or head circumference (MD 0.15 cm, 95% CI ‐0.02 to 0.32), or lower risk of preterm birth (RR 0.93, 95% CI 0.79 to 1.09) or low birthweight (RR 0.90, 95% CI 0.66 to 1.24). Lastly, another meta‐analysis of 27 trials found that vitamin D supplementation in pregnancy with more than 2000 IU/day had a positive effect only on gestational diabetes mellitus (RR 0.70, 95% CI 0.51 to 0.95; 7 trials), while a dose of 2000 IU/day or less was associated with a lower risk of pre‐eclampsia (RR 0.29, 95% CI 0.09 to 0.95; 3 trials) (Irwinda 2022). No differences in preterm birth risk or birthweight were found with the supplementation.

Overall, the aforementioned meta‐analyses have shown improvements in certain maternal and infant outcomes with vitamin D supplementation during pregnancy, contrary to the present review. This could be explained by differences in the inclusion/exclusion criteria of trials used in the reviews. For example, all of these reviews included trials in which the control group had either placebo (no intervention) or a lower vitamin D dose (such as 200 to 600 IU/day). In the present review, we only included those with no vitamin D supplementation or no intervention in the control group. This excluded many trials that were conducted in the US, Canada, and in certain countries in Europe, as there are published guidelines on the amounts of vitamin D recommended during pregnancy; therefore, trials conducted in those countries could not include a control group without any vitamin D supplementation. Also, these reviews did not exclude studies based on their methodological quality or whether they had high risk of bias, while the present review excluded many of these trials when using the trustworthiness assessment tool. These differences in the inclusion/exclusion criteria resulted in a low number of high‐quality trials compared to the other reviews. For all outcomes included in this review, there were only one to three trials that provided data. Therefore, future reviews with higher‐quality trials may provide different results.

Authors' conclusions

Implications for practice.

In this updated review, we added one new study with no relevant data and, following the trustworthiness assessment, removed data from 21 studies from the existing analyses. Therefore, only 10 studies were included. This significantly impacted the conclusions of our review. We have now graded the evidence as either low‐ or very low‐certainty and most reported outcomes have only one, two, or three studies contributing data.

Overall, the updated review showed that in this setting, vitamin D alone compared to no intervention or a placebo (eight studies, 2313 women) resulted in very uncertain evidence on the risk of pre‐eclampsia, gestational diabetes, preterm birth, and nephritic syndrome. It may reduce the risk of severe postpartum haemorrhage; however, only one study reported this outcome. It may reduce the risk of low birthweight; however, the upper CI suggests that an increase in risk cannot be ruled out. Supplementation with vitamin D and calcium versus placebo/no intervention (one study, 84 women) resulted in very uncertain evidence on the risk of preterm birth and low birthweight; pre‐eclampsia, gestational diabetes, and maternal adverse events were not reported. Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D) (one study, 1298 women) resulted in very uncertain evidence on the risk of gestational diabetes and maternal adverse events (hypercalciuria), and uncertain evidence on the risk of preterm birth and low birthweight. Pre‐eclampsia was not reported. More information on bone‐related outcomes and on adverse events is needed to inform practice.

Implications for research.

Additional rigorous, high‐quality and larger randomised trials are required to evaluate the effects of supplementation with vitamin D alone or in combination with other nutrients during pregnancy on maternal and neonatal health outcomes. It would be helpful if future trials were to evaluate the increase of serum 25(OH)D concentration with supplementation based on the baseline levels to adjust for initial maternal status and to start early in pregnancy among a diverse group of pregnant women with different degrees of body mass index (BMI), skin pigmentation, and settings. Also, trials are needed to evaluate potential interactions between supplements, as many countries currently include several micronutrients as part of their antenatal care and to evaluate systematically maternal adverse events to confirm the safety of the supplementation. There is also the need to be consistent when reporting maternal adverse events, as this was done differently in the trials, and to include other measures to further evaluate safety, such as cord blood. Other outcomes should also be reported, such as bone health‐related outcomes.

Information on the most effective and safe dosage, the optimal dosing regimen (daily, intermittent, or single doses), the timing of initiation of vitamin D supplementation, and the effect of vitamin D when combined with other vitamins and minerals is also needed to inform policy‐making.

What's new

Date	Event	Description
30 July 2024	New citation required and conclusions have changed	The previous version of this review concluded that supplementing pregnant women with vitamin D alone probably reduces the risk of pre‐eclampsia, gestational diabetes, and low birthweight, and may reduce the risk of severe postpartum haemorrhage. Supplementing pregnant women with vitamin D and calcium probably reduces the risk of pre‐eclampsia but may increase the risk of preterm births at < 37 weeks. However, the conclusions in this updated review of 10 studies have changed so that supplementation with vitamin D alone compared to no intervention or a placebo (eight trials, 2313 women) resulted in very uncertain evidence on pre‐eclampsia, gestational diabetes, preterm birth, and nephritic syndrome. It may still reduce the risk of severe postpartum haemorrhage; however, only one trial reported this outcome. It may also reduce the risk of low birthweight. Supplementation with vitamin D and calcium versus placebo or no intervention (one trial, 84 women) resulted in very uncertain evidence on preterm birth and low birthweight. Pre‐eclampsia, gestational diabetes, and maternal adverse events were not reported in the only trial included in this comparison.
30 July 2024	New search has been performed	The previous version of this review included 30 studies; in this update, 20 of these studies have been removed to 'awaiting classification' and one has been excluded following assessments of trustworthiness. One new study has been included. This current review has a total of 10 included studies, 117 excluded studies, 34 studies in awaiting assessment, and 7 ongoing studies.

History

Protocol first published: Issue 12, 2010
Review first published: Issue 2, 2012

Date	Event	Description
12 July 2018	New citation required and conclusions have changed	Supplementation with vitamin D alone (22 trials in total, 13 new trials added in this update) during pregnancy probably reduces the risk of pre‐eclampsia, gestational diabetes, and low birthweight. Supplementation with vitamin D + calcium (nine trials in total, three new trials added in this update) during pregnancy probably reduces the risk of pre‐eclampsia but may increase the risk of preterm births. Supplementation with vitamin D + other nutrients (1 trial added in this update) in pregnancy may make little or no difference to the risk of preterm birth or low birthweight. In general, more data are needed to conclude about the risk of maternal adverse events.
12 July 2018	New search has been performed	Search updated.
30 June 2015	New search has been performed	Search and methods updated. We included a new comparison to assess the effects of vitamin D + calcium + other vitamins and minerals versus other vitamins and minerals (but no vitamin D + calcium). We also moved adverse effects to primary outcomes.
30 June 2015	New citation required and conclusions have changed	Nine trials included for this update. The few trials that reported on the effects of vitamin D supplementation during pregnancy on low birthweight and preterm delivery suggest a lower risk on these outcomes with vitamin D in a single or continued dose. However, this result should be interpreted with caution due to the small number of trials and pregnant women included. Also, the quality of the evidence was low in most studies, with high heterogeneity.
10 May 2012	Amended	Error in 'Plain language summary' corrected: "Data from three trials involving 463 women show a trend for women who receive vitamin D supplementation during pregnancy to less frequently have a baby with a birthweight below 2500 grams than those women receiving no treatment or placebo".

Appendices

Appendix 1. CENTRAL search strategy

#1 MeSH descriptor: [Pregnancy] explode all trees
#2 MeSH descriptor: [Pregnancy Complications] explode all trees
#3 MeSH descriptor: [Infant, Newborn] explode all trees
#4 MeSH descriptor: [Fetus] explode all trees
#5 MeSH descriptor: [Fetal Development] explode all trees
#6 MeSH descriptor: [Prenatal Diagnosis] explode all trees
#7 MeSH descriptor: [Fetal Monitoring] explode all trees
#8 MeSH descriptor: [Fetal Therapies] explode all trees
#9 MeSH descriptor: [Heart Rate, Fetal] explode all trees
#10 MeSH descriptor: [Extraembryonic Membranes] explode all trees
#11 MeSH descriptor: [Placenta] explode all trees
#12 MeSH descriptor: [Placental Function Tests] explode all trees
#13 MeSH descriptor: [Uterine Monitoring] explode all trees
#14 MeSH descriptor: [Pelvimetry] explode all trees
#15 MeSH descriptor: [Oxytocics] explode all trees
#16 MeSH descriptor: [Tocolytic Agents] explode all trees
#17 MeSH descriptor: [Tocolysis] explode all trees
#18 MeSH descriptor: [Maternal Health Services] explode all trees
#19 MeSH descriptor: [Peripartum Period] explode all trees
#20 MeSH descriptor: [Parity] explode all trees
#21 MeSH descriptor: [Perinatal Care] explode all trees
#22 MeSH descriptor: [Postpartum Period] explode all trees
#23 MeSH descriptor: [Labor Pain] explode all trees
#24 MeSH descriptor: [Anesthesia, Obstetrical] explode all trees

#25 MeSH descriptor: [Obstetric Surgical Procedures] explode all trees
#26 MeSH descriptor: [Analgesia, Obstetrical] explode all trees
#27 MeSH descriptor: [Obstetric Nursing] explode all trees
#28 MeSH descriptor: [Maternal‐Child Nursing] explode all trees
#29 MeSH descriptor: [Midwifery] explode all trees
#30 MeSH descriptor: [Apgar Score] explode all trees
#31 MeSH descriptor: [Breast Feeding] explode all trees
#32 MeSH descriptor: [Bottle Feeding] explode all trees 
#33 MeSH descriptor: [Milk, Human] explode all trees
#34 {OR #1‐#33}
#35 pregnan*
#36 fetus
#37 foetus
#38 fetal
#39 foetal
#40 newborn
#41 "new born"
#42 birth
#43 childbirth
#44 laboring
#45 labour*
#46 antepart*
#47 prenatal*
#48 antenatal*
#49 perinatal*

#50 postnatal*
#51 postpart*
#52 caesar*
#53 cesar*
#54 obstetric*
#55 tocoly*
#56 oxytoci*
#57 placent*
#58 parturi*
#59 preeclamp*
#60 eclamp*
#61 intrapart*
#62 puerper*
#63 episiotom*
#64 amnio*
#65 matern*
#66 gestation*
#67 lactati*
#68 breastfe*
#69 breast NEXT fe*
#70 preconcept*
#71 periconcept*
#72 interconcept*
#73 {OR #35‐#72}
#74 #34 OR #73

Appendix 2. MEDLINE search strategy

1. randomized controlled trial.pt.
2. controlled clinical trial.pt.
3. randomized.ab.
4. placebo.ab.
5. drug therapy.fs.
6. randomly.ab.
7. trial.ab.
8. groups.ab.
9. or/1‐8
10. exp Pregnancy/
11. exp Pregnancy Complications/
12. exp Maternal Health Services/
13. exp Fetus/
14. exp Fetal Therapies/
15. exp Fetal Monitoring/
16. exp Prenatal Diagnosis/
17. Perinatal Care/
18. Labor pain/
19. Analgesia, Obstetric/
20. exp Obstetric Surgical Procedures/
21. Infant, Newborn/
22. exp Postpartum Period/
23. Breastfeeding/
24. or/10‐23
25. 9 and 24
26. exp animals/ not humans.sh.
27. 25 not 26

Appendix 3. Embase search strategy

1. CROSSOVER PROCEDURE/
2. DOUBLE BLIND PROCEDURE/
3. SINGLE BLIND PROCEDURE/
4. RANDOMIZED CONTROLLED TRIAL/
5. crossover$.ti,ab
6. (cross ADJ over$).ti,ab
7. placebo$.ti,ab
8. (doubl$ ADJ blind$).ti,ab
9. allocat$.ti,ab
10. random$.ti,ab
11. trial$.ti
12. 1 OR 2 OR 3 OR 4 OR 5 OR 6 OR 7 OR 8 OR 9 OR 10 OR 11
13. exp PREGNANCY/
14. exp PREGNANCY DISORDER/
15. exp OBSTETRIC PROCEDURE/
16. exp BREAST FEEDING/ OR exp BREAST FEEDING EDUCATION/
17. exp CHILDBIRTH/ OR exp CHILDBIRTH EDUCATION/
18. exp LABOR PAIN/
19. (antenatal* OR prenatal* OR puerper* OR postnatal* OR post‐natal* OR post ADJ
natal* OR postpartum OR post‐partum OR post ADJ partum).ti,ab
20. (prepregnancy OR pre‐pregnancy OR pre ADJ pregnancy OR preconcept* OR pre‐
concept* OR pre ADJ concept* OR periconcept* OR peri‐concept* OR peri ADJ
concept*).ti,ab
21. ((preterm OR premature) AND (labour OR labor)).ti,ab
22. (eclamp* OR preeclamp* OR pre ADJ eclamp*).ti,ab
23. amniocentes*.ti,ab
24. (chorion* ADJ vill*).ti,ab
25. (breastfe* OR breast‐fe* OR breast ADJ fe* OR lactation).ti,ab
26. (caesarean OR cesarean OR caesarian OR cesarian OR cesarien OR caesarien).ti,ab
27. (newborn OR new ADJ born).ti,ab
28. (pregnancy OR pregnant OR pregnancies).ti
29. episiotom*.ti,ab
30. 13 OR 14 OR 15 OR 16 OR 17 OR 18 OR 19 OR 20 OR 21 OR 22 OR 23 OR 24 OR
25 OR 26 OR 27 OR 28 OR 29
31. 12 AND 30

Appendix 4. CINAHL search strategy

1. exp CLINICAL TRIALS/
2. (clinic* ADJ trial*).ti,ab
3. (trebl* ADJ mask*).ti,ab
4. (tripl* ADJ blind*).ti,ab
5. (tripl* ADJ mask*).ti,ab
6. (doubl* ADJ blind*).ti,ab
7. (doubl* ADJ mask*).ti,ab
8. (singl* ADJ blind*).ti,ab
9. (singl* ADJ mask*).ti,ab
10. (randomi* ADJ control* ADJ trial*).ti,ab
11. RANDOM ASSIGNMENT/
12. (random* ADJ allocat*).ti,ab
13. placebo*.ti,ab
14. PLACEBOS/
15. QUANTITATIVE STUDIES/
16. (allocat* ADJ random*).ti,ab
17. breastfeeding.ti,ab
18. breastfed.ti,ab
19. exp BREAST FEEDING/
20. breast‐fe*.ti,ab
21. exp PREGNANCY/
22. exp PREGNANCY COMPLICATIONS/
23. (prenatal OR antenatal OR antepartum OR postpartum OR postnatal).ti,ab
24. (pregnant OR pregnancy).ti
25. ((preterm OR premature) AND (labor OR labour)).ti,ab
26. (midwife OR midwifery).ti,ab
27. CHILDBIRTH EDUCATION/
28. exp PREGNANCY, MULTIPLE/ OR exp PREGNANCY TRIMESTERS/
29. exp MATERNAL‐CHILD CARE/
30. (prenatal* OR pre‐natal* OR perinatal* OR peripartum OR antenatal* OR postnatal*
OR post‐natal* OR postpart* OR post‐part* OR puerper* OR prepregnancy OR pre‐
pregnancy OR preconcept* OR pre‐concept* OR periconcept* OR peri‐
concept*).ti,ab
31. OBSTETRIC EMERGENCIES/
32. OBSTETRIC NURSING/
33. exp SURGERY, OBSTETRICAL/
34. exp DIAGNOSIS, OBSTETRIC/
35. 1 OR 2 OR 3 OR 4 OR 5 OR 6 OR 7 OR 8 OR 9 OR 10 OR 11 OR 12 OR 13 OR 14
OR 15 OR 16
36. 17 OR 18 OR 19 OR 20 OR 21 OR 22 OR 23 OR 24 OR 25 OR 26 OR 27 OR 28 OR
29 OR 30 OR 31 OR 32 OR 33 OR 34

Appendix 5. Search terms used for additional author searching

Authors searched the ClinicalTrials.gov and the WHO‐hosted International Clinical Trials Registry Platform (ICTRP) for any ongoing or planned trials (3 December 2022) using the terms "vitamin D supplementation and pregnancy".

Data and analyses

Comparison 1. Supplementation with vitamin D alone versus placebo or no intervention (no vitamins or minerals).

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
1.1 Pre‐eclampsia (ALL)	1	165	Risk Ratio (M‐H, Random, 95% CI)	0.53 [0.21, 1.33]
1.2 Gestational diabetes (ALL)	1	165	Risk Ratio (M‐H, Random, 95% CI)	0.53 [0.03, 8.28]
1.3 Maternal adverse events	2		Risk Ratio (M‐H, Random, 95% CI)	Subtotals only
1.3.1 Severe postpartum haemorrhage	1	1134	Risk Ratio (M‐H, Random, 95% CI)	0.68 [0.51, 0.91]
1.3.2 Nephritic syndrome	1	135	Risk Ratio (M‐H, Random, 95% CI)	0.17 [0.01, 4.06]
1.3.3 Hypercalcaemia	1	1134	Risk Ratio (M‐H, Random, 95% CI)	Not estimable
1.4 Preterm birth (less than 37 weeks' gestation) (ALL)	3	1368	Risk Ratio (M‐H, Random, 95% CI)	0.76 [0.25, 2.33]
1.5 Low birthweight (less than 2500 g) (ALL)	3	371	Risk Ratio (M‐H, Random, 95% CI)	0.69 [0.44, 1.08]
1.6 Caesarean section	3	470	Risk Ratio (M‐H, Fixed, 95% CI)	0.95 [0.77, 1.19]
1.7 Gestational hypertension	2	1130	Risk Ratio (M‐H, Fixed, 95% CI)	0.79 [0.42, 1.49]
1.8 Maternal death (death while pregnant or within 42 days of termination of pregnancy) (ALL)	1	180	Risk Ratio (M‐H, Random, 95% CI)	Not estimable
1.9 Maternal vitamin D concentration at term (25‐hydroxyvitamin D) (nmol/L) (ALL)	6	1807	Mean Difference (IV, Random, 95% CI)	37.23 [8.60, 65.86]
1.10 Birth length (cm)	2	302	Mean Difference (IV, Random, 95% CI)	0.44 [‐0.05, 0.93]
1.11 Head circumference at birth (cm)	3	1262	Mean Difference (IV, Random, 95% CI)	0.24 [‐0.43, 0.92]
1.12 Birthweight (g)	6	1744	Mean Difference (IV, Random, 95% CI)	30.44 [‐50.06, 110.95]
1.13 Stillbirth	3	584	Risk Ratio (M‐H, Random, 95% CI)	0.35 [0.06, 1.98]
1.14 Neonatal death	2	326	Risk Ratio (M‐H, Random, 95% CI)	0.27 [0.04, 1.67]
1.15 Apgar score less than seven at five minutes	1	165	Risk Ratio (M‐H, Random, 95% CI)	0.53 [0.11, 2.53]

Comparison 2. Supplementation with vitamin D + calcium versus placebo or no intervention (no vitamin or minerals).

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
2.1 Low birthweight (less than 2500 g) (ALL)	1	50	Risk Ratio (M‐H, Random, 95% CI)	1.45 [0.14, 14.94]
2.2 Birth length (cm)	1	50	Mean Difference (IV, Random, 95% CI)	‐0.30 [‐1.68, 1.08]
2.3 Head circumference at birth (cm)	1	54	Mean Difference (IV, Random, 95% CI)	‐0.30 [‐1.05, 0.45]
2.4 Birthweight (g)	1	50	Mean Difference (IV, Random, 95% CI)	‐100.00 [‐380.80, 180.80]

Comparison 3. Supplementation with vitamin D + calcium + other vitamins and minerals versus calcium + other vitamins and minerals (but no vitamin D).

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
3.1 Gestational diabetes (ALL)	1	1298	Risk Ratio (M‐H, Random, 95% CI)	0.42 [0.10, 1.73]
3.2 Maternal adverse events	1		Risk Ratio (M‐H, Random, 95% CI)	Subtotals only
3.2.1 Hypercalcaemia	1	1298	Risk Ratio (M‐H, Random, 95% CI)	Not estimable
3.2.2 Hypercalciuria	1	1298	Risk Ratio (M‐H, Random, 95% CI)	0.25 [0.02, 3.97]
3.3 Preterm birth (less than 37 weeks' gestation) (ALL)	1	1298	Risk Ratio (M‐H, Random, 95% CI)	1.04 [0.68, 1.59]
3.4 Low birthweight (less than 2500 g) (ALL)	1	1298	Risk Ratio (M‐H, Random, 95% CI)	1.12 [0.82, 1.51]
3.5 Caesarean section	1	1298	Risk Ratio (M‐H, Random, 95% CI)	1.10 [0.95, 1.27]
3.6 Gestational hypertension	1	1298	Risk Ratio (M‐H, Random, 95% CI)	0.93 [0.31, 2.79]
3.7 Maternal death (death while pregnant or within 42 days of termination of pregnancy)	1	1300	Risk Ratio (M‐H, Random, 95% CI)	0.25 [0.02, 3.98]
3.8 Maternal vitamin D concentration at term (25‐hydroxyvitamin D) (nmol/L) (ALL)	1	635	Mean Difference (IV, Random, 95% CI)	75.17 [71.97, 78.37]
3.9 Birth length (cm)	1	1297	Mean Difference (IV, Fixed, 95% CI)	0.00 [‐0.28, 0.28]
3.10 Head circumference at birth (cm)	1	1297	Mean Difference (IV, Random, 95% CI)	0.00 [‐0.17, 0.17]
3.11 Birthweight (g)	1	1297	Mean Difference (IV, Random, 95% CI)	‐7.00 [‐55.95, 41.95]
3.12 Stillbirth	1	1300	Risk Ratio (M‐H, Random, 95% CI)	0.66 [0.29, 1.46]
3.13 Neonatal death	1	1298	Risk Ratio (M‐H, Random, 95% CI)	0.69 [0.22, 2.14]

Characteristics of studies

Characteristics of included studies [ordered by study ID]

Benson 2009.

Study characteristics
Methods	Randomised controlled trial
Participants	78 pregnant women between 14 and 18 weeks' gestation at risk, defined as: dark skinned, veiled; with vitamin D deficiency that has not commenced treatment prior to recruitment Exclusion criteria: women taking barbiturates or anticonvulsants (decreased vitamin D absorption) and severe renal failure
Interventions	Participants were individually randomised to 1 of 2 groups: group 1 (n = 38): 2000 IU of cholecalciferol orally daily commencing between 14 and 18 weeks' gestation (if still deficient at 28 weeks the dose was doubled to 4000 IU orally daily until birth); group 2 (n = 40): no treatment during pregnancy. The mother received 300,000 IU cholecalciferol orally immediately and the baby 150,000 IU cholecalciferol orally immediately after birth. Health worker cadre: in order to facilitate compliance, encouragement was given from midwifery/medical staff at each 2‐ to 4‐weekly antenatal visit with additional intervening telephone calls to women with poor compliance. Pill counts were not performed.
Outcomes	Maternal: vitamin D level Infant: vitamin D level Laboratory method used for assessment of vitamin D concentrations: serum 25‐OH vitamin D concentrations were determined by direct competitive chemiluminescence immunoassay for quantitative determination of total serum 25‐OH vitamin D (LIAISON®) Diasorin 25‐OH vitamin D assay (Stillwater, MN, USA)
Notes	Total dose of supplementary vitamin D during pregnancy: more than 20,000 IU vitamin D Start of supplementation: 14 to 18 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): mixed/unknown Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: south of Tropic of Capricorn Season at the start of pregnancy: all year round Source of funding: J.E. Benson was a recipient of the Luke Proposch Perinatal Research Scholarship from the Australian and New Zealand College of Obstetrics and Gynaecology Research Foundation, enabling her to undertake this research. Study was funded by research grant. Dates of the study and location: between 2008 and 2009, Melbourne, Australia Declarations of interest among primary researchers: the authors have no conflict of interest to disclose
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomly allocated (envelopes in a tamper‐proof box, ratio 1:1)
Allocation concealment (selection bias)	Low risk	Envelopes in a tamper‐proof box, ratio 1:1
Blinding of participants and personnel (performance bias) All outcomes	High risk	The trial reported that it was single‐blinded. It is assumed that it was not blinded to participants as one of the groups did not receive any supplementation.
Blinding of outcome assessment (detection bias) All outcomes	High risk	Open‐label trial (masking not used)
Incomplete outcome data (attrition bias) All outcomes	High risk	57.9% intervention and 57.5% control data reported
Selective reporting (reporting bias)	Low risk	All pre‐specified outcomes reported
Other bias	Low risk	The study appears to be free of other sources of bias

Bhutta 2011.

Study characteristics
Methods	Randomised, parallel assignment, double‐blind trial
Participants	115 pregnant females from 12 to 20 weeks of gestation who agreed to participate in the study with presence of at least 20 natural teeth in mouth excluding third molars. For controls: non‐pregnant, healthy females matched with pregnant women with respect to age and education. Exclusion criteria: pregnant females with high vitamin D levels, women with metabolic diseases such as diabetes (type 1 or 2), presence of acute dental or periodontal disease, presence of systemic disease and/or medication affecting the periodontium; receipt of systemic antibiotic treatment or dental prophylaxis in the previous 3 months and those who do not provide informed consent.
Interventions	Participants were individually randomised to 1 of 2 groups: group 1 (n = 36): vitamin D3 4000 mg per day (given as 1 tablespoon syrup per day); group 2 (n = 49): placebo (given as 1 tablespoon syrup per day) for approximately 6 months. Health worker cadre: CHWs were responsible for the delivery of supplementation to the study participants. The CHWs were assigned to visit study participants on a fortnightly basis. The first supplementation was provided by the physician at the time of recruitment; later on, the CHWs continued to replenish the supply fortnightly.
Outcomes	Maternal: periodontal probing depth, interleukin 6 (IL‐6), IL‐2, IL‐4, IL‐10, TNF, IFN‐ɣ and IL‐17 levels Laboratory method used for assessment of vitamin D concentrations: vitamin D levels were analysed on DiaSorin‐LIASON Inc. kit
Notes	Total dose of supplementary vitamin D during pregnancy: more than 200,000 IU Start of supplementation: 12 to 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: unknown Source of funding: the study was supported with a research grant from Pakistan Initiative for Mothers and Newborns (PAIMAN). Dates of the study and location: launched in 2004, Jhelum, Pakistan Declarations of interest among primary researchers: none declared
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	The study participants were randomised in blocks.
Allocation concealment (selection bias)	Low risk	Allocation codes for vitamin D and placebo were kept in a sealed envelope in a locked cabinet at the Aga Khan University until the completion of the study.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	The investigators, study staff, and the participants were blinded to the group allocation.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Allocation codes for vitamin D and placebo were kept in a sealed envelope in a locked cabinet at the Aga Khan University until the completion of the study.
Incomplete outcome data (attrition bias) All outcomes	High risk	Only recorded birthweight from 63/85 (74.1%) participants
Selective reporting (reporting bias)	Low risk	All pre‐specified outcomes reported
Other bias	Low risk	The study appears to be free of other sources of bias.

Diogenes 2013.

Study characteristics
Methods	Randomised, placebo‐controlled trial; 2‐arm design with individual randomisation
Participants	84 pregnant adolescents (13 to 19 years of age) primigravidae (pregnant for the first time) with singleton pregnancies and 23 to 29 weeks of gestation attending prenatal care at the Maternidade Escola, Universidade Federal do Rio de Janeiro, Brazil (latitude: 22.9083° S, 43.1964° W) from September 2009 to June 2011 and intending to exclusively or predominantly breastfeed. Women with chronic health problems, pregnancy complications, smokers, users of nutritional supplements besides iron plus folate supplements provided during standard prenatal care, and mothers who decided not to breastfeed were excluded from the study.
Interventions	Participants were randomly assigned to: 1 of 2 groups: group 1 (n = 43) received a commercially available supplement (Rexall Sundown®) containing 600 mg calcium (as calcium carbonate) plus 200 IU vitamin D (cholecalciferol‐D3) daily and group 2 (n = 41) received placebo (capsules of microcrystalline cellulose and corn starch; Quintessencia) daily. The protocol allowed pregnant women to continue with their iron and folate supplements, as part of their standard prenatal care. The use and composition of these supplements was not provided. Health worker cadre: capsules of calcium plus vitamin D or placebo were provided monthly to participants by a member of the research team during prenatal visits. Compliance was controlled by counting the remaining capsules at each visit and by telephone reminders. Calcium and vitamin D dietary intake was assessed by at least 3 x 24‐hour dietary recall questionnaires applied by a trained nutritionist. Standing height and body weight were measured using a stadiometer (Seca) and a calibrated electronic scale (Filizola), respectively. The same operator performed all scanning and calibration.
Outcomes	Maternal: 1 measurement at 5 and 20 weeks postpartum, serum 25(OH)D, PTH, insulin‐like growth factor (IGF‐I), lumbar spine PA, bone mineral content, serum prolactin, and oestradiol Laboratory method used for assessment of vitamin D concentrations: serum 25‐hydroxyvitamin D, intact PTH, and IGF‐I were analysed using a chemiluminescent enzyme‐labelled immunometric assay.
Notes	Total dose of supplementary vitamin D during pregnancy: 56,000 IU vitamin D or less Start of supplementation: 20 weeks of pregnancy or more; the supplementation started from 26 weeks of pregnancy (baseline) until parturition Pre‐gestational BMI (kg/m2): normal weight Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: the city lies on the Tropic of Capricorn Season at the start of pregnancy: all year round Source of funding: Conselho Nacional de Desenvolvimento Cientıfico e Tecnologico [grant 471872/2008‐3 (to CMD) and a doctoral fellowship (to MELD)] and the Fundacao Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (grant E‐26/102.759/2008; to CMD), Brazil. This study was funded by a combination of government programmes and non‐governmental organisations (NGOs). Dates of the study and location: September 2009 to June 2011, Rio de Janeiro, Brazil Declarations of interest among primary researchers: none of the authors had a conflict of interest.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Random assignment was done by a member of the research team in a 1:1 ratio within permuted blocks of size 10.
Allocation concealment (selection bias)	Unclear risk	The trial did not report the method of concealment.
Blinding of participants and personnel (performance bias) All outcomes	High risk	The trial reported that it was single‐blinded; only participants were blinded to the assigned groups. It is assumed that the assessment team was not blinded.
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Insufficient information to permit judgement
Incomplete outcome data (attrition bias) All outcomes	High risk	Out of 43 women in the intervention group, 13 did not complete the study. Out of 41 women in the placebo group, 14 did not complete the study.
Selective reporting (reporting bias)	Low risk	All pre‐specified outcomes reported
Other bias	Low risk	The study appears to be free of other sources of bias.

Grant 2013.

Study characteristics
Methods	Randomised, double‐blind, placebo‐controlled, multi‐arm, parallel study
Participants	260 pregnant women 26 to 30 weeks' gestation, with a singleton pregnancy attending a community‐based primary care maternity clinic in Auckland, New Zealand (latitude 36°S) from April 2010 to July 2011 and then their infants, from birth to age 6 months Women already taking vitamin D supplementation 200 IU per day, a history of renal stones or hypercalcaemia, or any serious pregnancy complication at enrolment were excluded from the study.
Interventions	Participants were randomly assigned to 1 of 3 mother/infant groups: group 1 (n = 87) women received placebo from 26 to 30 weeks of pregnancy until parturition and their infants also received placebo from 0 to 6 months of age; group 2 (n = 87) women received 1000 IU vitamin D (cholecalciferol‐D3) from 26 to 30 weeks of pregnancy until parturition and their infants received 400 IU vitamin D from 0 to 6 moths of age; group 3 (n = 86) women received 2000 IU vitamin D (cholecalciferol‐D3) from 26 to 30 weeks of pregnancy until parturition and their infants received 800 IU from birth to 6 months of age. Data from groups 2 and 3 were combined for our analysis. Health worker cadre: the study was conducted by the research team, but it is not reported who provided the supplements or measured the outcomes.
Outcomes	Maternal: serum 25(OH)D concentration Infant: serum 25(OH)D concentration Laboratory method used for assessment of vitamin D concentrations: serum 25‐hydroxyvitamin D concentration was measured using isotope‐dilution liquid chromatography‐tandem mass spectrometry in a Vitamin D External Quality Assurance Scheme‐certified laboratory.
Notes	Total dose of supplementary vitamin D during pregnancy: 56,000 IU vitamin D or less Start of supplementation: 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: between the Tropics of Cancer and Capricorn Season at the start of pregnancy: all year round Source of funding: Health Research Council of New Zealand, grant number 09/215R. Dr Mitchell is supported by Cure Kids. Study medicine was prepared by the Ddrops Company (Woodbridge, Ontario, Canada). This study was funded by a combination of government programmes and non‐governmental organisations (NGOs). Dates of the study and location: April 2010 to July 2011, Auckland, New Zealand Declarations of interest among primary researchers: the authors have indicated they have no potential conflicts of interest to disclose.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Trial reported computer‐generated randomisation list
Allocation concealment (selection bias)	Low risk	The allocation sequence was concealed from research staff involved in recruitment. Trial reported randomly allocated treatment to each participant and labelled identical study medicine bottles such that study staff and participants were unaware of the treatment status.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	The study statistician randomly allocated a treatment to each participant and labelled identical study medicine bottles such that study staff and participants were unaware of the treatment status.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	The study staff and participants were unaware of the treatment status.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Reported compliance did not differ between groups. In the placebo group, 6 did not complete the study; in the lower‐dose vitamin D group, 6 did not complete the study. In the higher‐dose vitamin D group, 6 did not complete the study.
Selective reporting (reporting bias)	Low risk	All pre‐specified outcomes reported
Other bias	Low risk	The study appears to be free of other sources of bias.

Harvey 2012.

Study characteristics
Methods	Randomised, double‐blind, placebo‐controlled trial
Participants	1200 pregnant women living in the UK, aged 18 years old and older, with a singleton pregnancy with less than 17 weeks' gestation at first assessment (based on last menstrual period and dating scan), aiming to give birth at local maternity hospital, and with serum 25‐hydroxyvitamin D is 25 to 100 nmol/L at nuchal fold/dating scan (10 to 17 weeks' gestation)
Interventions	Participants were randomly assigned to 1 of 2 groups: group 1 (n = 565) received 1000 IU cholecalciferol orally daily; group 2 (n = 569) received placebo, starting from 14 weeks' gestation until delivery Health worker cadre: the medication was blister packed in a single box for each woman for the duration of pregnancy. Study medication (active/placebo) was supplied to the local pharmacy pre‐randomised by the manufacturer (1:1, unstratified by centre) and sequentially numbered for storage and dispensing. Code break envelopes were supplied to the lead pharmacist, but were not available to the investigative team. Emergency code break access was available through the local principal investigator and on‐call pharmacist. A single pack for each participant was issued sequentially (containing all pills for the duration of the study). Each pack was individually prescribed for each participant. The trials pharmacist allocated a pack to that prescription, documenting both the pack number and the MAVIDOS participant ID; these were checked again by the research nurse on collection, and documented in the participant’s notes; the medication pack came with a tear‐off adhesive label, which was placed in the participant’s notes as an added safeguard against errors in pack allocation. The research nurse collected the medication pack for all participants attending to the clinic that day and issued to the participants directly.
Outcomes	Infant: whole body bone mineral content of the neonate adjusted for gestational age and age at neonatal DXA scan, whole body bone area, bone mineral density, and size corrected bone mineral density (BMC adjusted for BA, length and weight), body composition adjusted for gestational age and age at DXA scan Laboratory method used for assessment of vitamin D concentrations: a blood sample was taken and plasma was stored at ‐80°C for measurement of 25(OH)‐vitamin D, vitamin D binding protein (DBP), calcium, bone specific alkaline phosphatase and albumin centrally (MRC Human Nutrition Research, Cambridge, UK) at the end of the study.
Notes	Total dose of supplementary vitamin D during pregnancy: less than 200,000 IU Start of supplementation: less than 20 weeks of pregnancy Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: all year round Source of funding: Arthritis Research UK, Medical Research Council, Bupa Foundation, and National Institute for Health Research. Study was funded by a combination of research grants, government programmes, and non‐governmental organisations (NGOs). Dates of the study and location: October 2008 to February 2014, Southampton, Sheffield, Oxford, the UK Declarations of interest among primary researchers: CC reports personal fees, consultancy, lecture fees, and honoraria from Alliance for Better Bone Health, Amgen, Eli Lilly, GlaxoSmithKline, Medtronic, Merck, Novartis, Pfizer, Roche, Servier, and Takeda, outside the submitted work. NCH reports personal fees, consultancy, lecture fees, and honoraria from Alliance for Better Bone Health, AMGen, MSD, Eli Lilly, Servier, Shire, Consilient Healthcare, and Internis Pharma, outside the submitted work. NJB reports remuneration from Internis Pharmaceuticals, outside the submitted work. ATP reports grants from the Arthritis Research Council, during the conduct of the study. NKA has received honoraria, held advisory board positions (which involved receipt of fees), and received consortium research grants from Merck, grants from Roche, Bioiberica, and Novartis, personal fees from Smith & Nephew, Nicox, Flexion, Bioventus, and Freshfields, outside the submitted work. KMG reports reimbursement for speaking at Nestle Nutrition Institute conferences, and grants from Abbott Nutrition and Nestec, outside the submitted work. KMG also has a patent pending for phenotype prediction, a patent pending for predictive use of CpG methylation, and a patent pending for maternal nutrition composition, not directly related to this work. HMI reports grants from the Medical Research Council (MRC), Arthritis Research UK, and European Union’s Seventh Framework Programme, during the conduct of the study; and while not directly receiving funding from other bodies, members of her team have received funding from the following companies from other work: Danone, Nestec, and Abbott Nutrition. RE reports grants and personal fees from Amgen and Alexion; grants from the Department of Health, AstraZeneca, ARUK/MRC Centre for Excellence in Musculoskeletal Ageing Research, National Institute for Health Research, MRC/AZ Mechanisms of Diseases Call, and the MRC; grants, personal fees, and non‐financial support from Immunodiagnostic Systems; grants and membership of a clinical and scientific committee from the National Osteoporosis Society; grants, personal fees, and advisory board membership from Roche; personal fees from Otsuka, Novartis, Merck, Bayer, Johnson & Johnson, Fonterra Brands, Janssen Research, Ono Pharma, Alere (Unipath), Chronos, Teijin Pharma Limited, D‐STAR, and GSK Nutrition; personal fees and advisory board membership from Eli Lilly, and CL Biosystems; and advisory board membership from the European Calcifi ed Tissue Society, IOF CSA, and the American Society for Bone and Mineral Research, outside the submitted work. MKJ reports personal fees from Stirling Anglia, Consilient Health, and Internis, outside the submitted work. All other authors declare no competing interests.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Computer‐generated sequence in randomly permuted blocks of 10
Allocation concealment (selection bias)	Low risk	The treatments were blister packed in a single box for each woman for the duration of pregnancy and supplied to the local pharmacy pre‐randomised by the manufacturer (1:1, unstratified by centre) and sequentially numbered for storage and dispensing. The lead pharmacist was the only one with access to the code break envelopes.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Double‐blinded, matched pills, only lead pharmacist knew about pills
Blinding of outcome assessment (detection bias) All outcomes	Low risk	All assessments were double‐blinded.
Incomplete outcome data (attrition bias) All outcomes	Low risk	94% of sample had vitamin D assessment at term.
Selective reporting (reporting bias)	Unclear risk	There is insufficient information to permit judgement.
Other bias	High risk	Participants were allowed to continue taking their own multivitamin with 400 IU/d of vitamin D, but this was not recorded.

Roth 2010.

Study characteristics
Methods	Randomised, placebo‐controlled trial (AViDD‐2 trial)
Participants	160 pregnant women aged 18 < 35 years old, attending the International Centre for Diarrhoeal Disease Research, Dhaka, Bangladesh (latitude: 23.7000° N, 90.3750° E, north of the Tropic of Cancer). Inclusion criteria: women with residence in Dhaka, with plans to have the delivery performed at the Shimantik maternity centre, and to stay in Dhaka throughout the pregnancy and 1 month past the delivery, with gestational age of 26th to 29th week (inclusive), estimated based on the first day of the last menstrual period. Exclusion criteria: use of any dietary supplement containing more than 400 IU/day (10 μg/day) of vitamin D within the month prior to enrolment, or refusal to stop taking supplemental vitamin D at any dose after enrolment, current use of anti‐convulsant or anti‐mycobacterial (tuberculosis) medications, severe anaemia (haemoglobin concentration < 70 g/L), complicated medical or obstetric history: cardiovascular disease, uterine haemorrhage, placenta praevia, threatened abortion, hypertension, pre‐eclampsia, preterm labour, or multiple gestation), prior history of delivery of an infant with a major congenital anomaly, birth asphyxia, or perinatal death (stillbirth or death within first week of life)
Interventions	Participants were randomly assigned to 1 of 2 groups: group 1 (n = 80): women received vitamin D (cholecalciferol‐D3) 35,000 IU per week, started at 26 to 29 weeks' gestation, until delivery; group 2 (n = 80): women received placebo control administered weekly from 26 to 29 weeks' gestation until delivery. Health worker cadre: supplement doses were measured in disposable plastic syringes and orally administered by study personnel.
Outcomes	Maternal: serum 25‐hydroxyvitamin D concentration, serum calcium concentration, urine Ca:Cr ratio Infant: immune function, infant growth, postnatal vitamin D status, serum calcium Laboratory method used for assessment of vitamin D concentrations: serum 25‐hydroxyvitamin D was quantified by high‐performance liquid chromatography tandem mass spectroscopy (LCMS/MS) in the Department of Pathology and Laboratory Medicine at the Hospital for Sick Children.
Notes	Total dose of supplementary vitamin D during pregnancy: more than 200,000 IU Start of supplementation: 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: summer Source of funding: this study was funded by a non‐governmental organisation (NGO): the Thrasher Research Fund, Salt Lake City, USA. Dates of the study and location: August 2010 to January 2011, Dhaka, Bangladesh Declarations of interest among primary researchers: the authors declare that they have no competing interests.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Trial reported a computer‐generated randomisation list for the randomisation procedures.
Allocation concealment (selection bias)	Low risk	The allocation sequence was prepared by the International Centre for Diarrhoeal Disease Research, Dhaka, Bangladesh personnel not otherwise involved in the study, and was concealed from the investigators.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Trial reported that participants were blinded to allocation.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Trial reported that research staff (including lab personnel) were blinded to allocation.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Of the 160 participants recruited and randomly assigned to intervention or placebo, 13 were lost to follow‐up prior to delivery (6 in the placebo group and 7 in the vitamin D group), all because of having left the Dhaka area.
Selective reporting (reporting bias)	Low risk	All pre‐specified outcomes reported
Other bias	Low risk	The study appears to be free of other sources of bias.

Roth 2013.

Study characteristics
Methods	Randomised, double‐blind, placebo‐controlled trial (MDIG trial)
Participants	1300 generally healthy, pregnant women between 17 and 24 weeks of gestation
Interventions	Participants were randomly assigned at enrolment to 1 of 5 groups: group 1 (n = 260) received placebo throughout the prenatal period and for 26 weeks postpartum; group 2 (n = 260) received 4200 IU per week prenatally and no supplementation postpartum; group 3 (n = 260) received 16,800 IU per week prenatally and no supplementation postpartum; group 4 (n = 260) received 28,000 IU per week prenatally and no supplementation postpartum; and group 5 (n = 260) received 28,000 IU per week prenatally and during the postpartum for 26 weeks. Data from groups 2 to 5 were combined into the intervention group for this analysis. All participants received calcium (500 mg per day), iron (66 mg per day), and folic acid (350 μg per day) throughout the intervention phase. Health worker cadre: trial personnel contacted participants weekly from enrolment until 26 weeks postpartum, and infants were further assessed at 9 months and 12 months of age. Visits were conducted in the home or at a clinic and included the use of standardised questionnaires, point‐of‐care tests, anthropometric measurements, and specimen collection.
Outcomes	Maternal: maternal serum 25‐hydroxyvitamin D and calcium concentration, urinary calcium excretion, and maternal PTH concentrations Infant: length‐for‐age, birth outcomes, morbidity, and serum 25‐hydroxyvitamin D and calcium concentrations Laboratory method used for assessment of vitamin D concentrations: point‐of‐care tests
Notes	Total dose of supplementary vitamin D during pregnancy: more than 56,000 IU Start of supplementation: 17 to 24 weeks of pregnancy Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: weekly Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: unknown Source of funding: this study was funded by the Bill & Melinda Gates Foundation. Dates of the study and location: March 2014 to September 2015, Dhaka, Bangladesh Declarations of interest among primary researchers: no potential conflict of interest was reported.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomisation was conducted by a computer‐generated, simple randomisation scheme created independently by the trial statistician.
Allocation concealment (selection bias)	Low risk	Concealment of trial group assignments was ensured with the use of pre‐labelled and sequentially numbered but otherwise identical supplement vials.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	The trial reported that a master list linking participants to supplementation groups was not accessible to trial personnel until final group assignments were revealed.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	The trial reported that a master list linking participants to supplementation groups was not accessible to trial personnel until final group assignments were revealed.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Among 1164 infants assessed at 1 year of age (89.5% of 1300 pregnancies), < 5% of participants withdrew or were excluded after randomisation until birth.
Selective reporting (reporting bias)	Low risk	All outcomes pre‐specified in protocol reported.
Other bias	Low risk	There is no evidence of any other bias.

Sablok 2015.

Study characteristics
Methods	Randomised controlled trial with 2 arms, with randomisation at the individual level from 2010 to 2012
Participants	180 primigravidae women with singleton pregnancy at 14 to 20 weeks in the Department of Obstetrics and Gynaecology in Safdarjung Hospital, New Delhi, India (28°38′08″ N, 77°13′28″ E north of the Tropic of Cancer) Pregnant women with pre‐existing osteomalacia, known hyperparathyroidism, renal, liver dysfunction, tuberculosis, sarcoidosis, and women not willing to comply with the study protocol were excluded.
Interventions	Participants were randomly assigned to 1 of 2 groups: group 1 (n = 60) did not receive any supplementation of vitamin D; group 2 (n = 120) received vitamin D (cholecalciferol‐D3) supplementation in dosages depending upon the level of serum 25(OH)‐D levels estimated at entry into the study. Participants from this second group with sufficient levels of vitamin D (serum 25(OH)‐D levels > 50 nmol/L), received only 1 dose of 60,000 IU vitamin D (cholecalciferol‐D3) at 20 weeks; participants with insufficient levels of vitamin D (serum 25(OH)‐D levels 25 to 50 nmol/L) received 2 doses of 120,000 IU vitamin D (cholecalciferol‐D3) at 20 weeks and 24 weeks; and participants with deficient levels of vitamin D status (serum 25(OH)‐D levels < 25 nmol/L) received 4 doses of 120,000 IU vitamin D cholecalciferol‐D3 at 20, 24, 28, and 32 weeks. Independently of the dose, all participants in group 2 were grouped and compared to group 1 for this analysis. Health worker cadre: unclear what are the roles of the researchers and other workers in the health worker cadre
Outcomes	Maternal: preterm labour, pre‐eclampsia, gestational diabetes, serum 25(OH)‐D concentration, serum calcium, phosphorus, and serum ALP levels. Infants: Apgar score, birthweight, LBW, 25(OH)‐D concentration in cord blood, small‐for‐gestational age, appropriate for gestational age Laboratory method used for assessment of vitamin D concentrations: serum 25‐hydroxyvitamin D was quantified by sandwich ELISA
Notes	Total dose of supplementary vitamin D during pregnancy: more than 56,000 to 200,000 IU to more than 200,000 IU of vitamin D Start of supplementation: 20 weeks of pregnancy, or more Pre‐gestational BMI (kg/m2): healthy weight Supplementation scheme/regimen: single given at different weeks of gestation in the supplemented group Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: all year round. Authors report that sufficient levels of vitamin D were seen in 93.3% of women who had more than 4 hours of sun exposure every day, as compared to 18.5% in those with less than 1 hour of daily sun exposure Source of funding: self‐funded Dates of the study and location: 2010 to 2012, India Declarations of interest among primary researchers: all the authors have nothing to disclose Included in 2024 update because although trustworthiness red flags were discovered using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023), these concerns were dealt with satisfactorily in correspondence with the trial author. Trustworthiness red flags: no clinical trial registration; no information on ethics; no trial dates; no information on randomisation We reached out to the corresponding author, Dr Aanchal Sablok, who replied: "This study was conducted in 2010, at that time the Clinical trial registry was not very popular in my Country and hence we did not register our RCT in the Registry. The study was conducted in a Medical College and Hospital setting and the Ethical clearance was taken from the College Ethics board. But the copy of the clearance gets submitted in the College itself and I do not presently have the copy of the same. (Getting that copy might take a long time). The study was conducted from 1st Nov, 2010 to 30th April, 2012. Randomization was done using Random number table. I hope these answer all your queries. Please feel free to contact me for any further clarification."
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomisation was performed using computer‐generated random number tables.
Allocation concealment (selection bias)	Unclear risk	Allocation concealment method was not detailed in the trial report.
Blinding of participants and personnel (performance bias) All outcomes	High risk	The trial did not report if the study was blinded. It is assumed that it was not blinded to participants as one of the groups did not receive any supplementation and the other groups received different doses of vitamin D at different times.
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	At the time of delivery, both the groups underwent clinical evaluations and complete anthropometric assessment of the neonate, but it was not reported whether staff were blinded to the intervention groups.
Incomplete outcome data (attrition bias) All outcomes	High risk	The level of attrition was different in groups 1 and 2: 3/60 (5%) participants in group 1 and 12/120 (10%) participants in group 2 were lost to follow‐up.
Selective reporting (reporting bias)	Unclear risk	There is insufficient information to permit judgement.
Other bias	Low risk	The study appears to be free of other obvious sources of bias.

Vafaei 2019.

Study characteristics
Methods	Randomised controlled trial
Participants	Setting: early pregnant women under antenatal clinic care of Hafez hospital, the main centre for perinatology in Shiraz, in the south of Iran Pregnant women (n = 140) were referred to the obstetric clinic after 2 weeks of menstrual retardation as they were previously educated at preconception counselling. Inclusion criteria were 20‐ to 35‐year‐old healthy primigravida pregnant Iranian woman with normal body mass index (BMI), without any comorbidities such as diabetes mellitus, thyroid disease, liver disease, or mental illnesses. Exclusion criteria were smoking, drug abuse, alcohol consumption, multiple pregnancy, congenital anomaly or chromosomal abnormality, and cases that did not accept participation or did not sign the informed consent form. Moreover, during the survey, complicated pregnancies such as those with hypertension, pre‐eclampsia, premature rupture of membrane, severe vaginal bleeding, and threatened course of labour were excluded from the study.
Interventions	Total number randomised: n = 70 Experimental intervention: 1000 IU of vitamin D (Jalinous Pharmaceutical Company, Tehran, Iran) daily, starting 2 weeks after menstrual retardation. The pills were continued until the last sonography at 34 weeks of gestational age. Control/comparison intervention: control group received placebo (same colour and shape capsules containing starch)
Outcomes	Crown‐rump length (CRL) and femur length (FL) during the first trimester and humerus and femur lengths as well as their proximal metaphyseal diameter (PMD), midshaft diameter (MSD), and distal metaphyseal diameter (DMD) in the second and third trimester were measured using ultrasonography technique. No outcomes were relevant to this review.
Notes	Study dates: June 2017 to September 2017 Study funding sources: the project was financed by the Vice Chancellor for Research of the Shiraz University of Medical Science, Shiraz, Iran (Grant No. 94-01-01-9357). Study authors’ declarations of interest: none declared Ethical approval obtained? All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Included in 2024 update because although trustworthiness red flags were discovered using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023), these concerns were dealt with satisfactorily in correspondence with the trial author. Trustworthiness red flags: unclear randomisation We reached out to the corresponding author, Niloofar Namazi, who replied: "In our study, the permuted block randomization (35 blocks, size of each block=4) was performed to divide participants into two groups of 70. The randomization list was generated in the random allocation (RA) software and a code was generated for each participant. When the eligible participants entered the study and received the code, they met a trained secretary that documented the related code of each participant and got her the related box of drug (vitamin D versus placebo) that was previously designed by her. The only person aware of the drug was the secretary who had no conflict of interest. Performing sonography and laboratory examination were done by documenting the participants' code that no one was aware of the group of the patient. After finishing the study, the codes were unfolded by the secretory and analysis was done."
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Block randomisation (size of each block = 4) was performed to divide participants into 2 groups of 70 each, using random allocation software.
Allocation concealment (selection bias)	Unclear risk	Not described
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Placebo identical to intervention
Blinding of outcome assessment (detection bias) All outcomes	Low risk	The lab technicians were blinded to the group allocation.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Ten excluded: 8 in placebo, 2 in treatment. Removed for obstetric complications.
Selective reporting (reporting bias)	Unclear risk	Could not find protocol; all outcomes reported are from the methods.
Other bias	Unclear risk	Baseline data appear even across groups. Very short trial dates, would not have fitted third trimester into dates ‐ this could be dates of recruitment but not clear in the report.

Yu 2008.

Study characteristics
Methods	Randomised controlled trial; 4 x 3 block design with randomisation at individual level
Participants	180 pregnant women (45 Indian Asians, 45 Middle Eastern, 45 Black, and 45 Caucasian) women at 27 weeks' gestation attending the routine antenatal clinic at St Mary’s Hospital, London, the UK (latitude: 51°30'N north of tropic of Cancer) Exclusion criteria: pre‐existing sarcoidosis, osteomalacia, renal dysfunction, and tuberculosis Pre‐gestational BMI and skin pigmentation (in addition to ethnicity) not reported. The study took place between April 2007 and November 2007. In addition, there was a follow‐up trial of the infants of these trial participants. All the offspring of the 180 mothers recruited in this trial were eligible and were invited to participate in a follow‐up study when their children were 3 years of age.
Interventions	Participants were randomised in blocks of 15 within each of the 4 ethnic groups to 3 groups: Group 1 (n = 60) received a daily dose of vitamin D (ergocalciferol D2) at 800 IU; Group 2 (n = 60) received a one dose of 200,000 IU of calciferol; Group 3 (n = 60) received no treatment and served as controls. All groups received the intervention for 13 weeks, from start of supplementation to term. Data from groups 1 and 2 were collapsed for this analysis. Health worker cadre: each woman collected her tablets directly from the hospital pharmacy department or her local pharmacy.
Outcomes	Maternal: maternal and cord 25‐hydroxyvitamin D levels at delivery, maternal PTH, corrected calcium levels at delivery, adverse events. Infant: small‐for‐gestational age was defined as birthweight less than the 10th percentile after adjustments for gestation at delivery, infant sex, maternal ethnicity, parity, height, and weight. Wheezing episode in the first 3 years of life, measured at 36 to 48 months, use of inhaled bronchodilators in the last 12 months, doctor‐diagnosed rhinitis, any wheezing episode in the preceding 12 months, doctor‐diagnosed asthma, doctor‐diagnosed eczema, doctor‐diagnosed food allergy, positive skin prick test responses, 25‐hydroxyvitamin D levels, bronchodilator responsiveness, exhaled nitric oxide level (in parts per billion), nasal secretions for inflammatory mediators, pulmonary airflow resistance and reactance at a range of frequencies using impulse oscillometry, total number of all wheezing episodes since birth and total number of upper and lower respiratory tract infections since birth, at 36 to 48 months Laboratory method used for assessment of vitamin D concentrations: not specified
Notes	Women who did not speak English were only included if a health advocate was able to interpret and a leaflet was provided in their language. Total dose of supplementary vitamin D during pregnancy: more than 56,000 to 200,000 IU Start of supplementation: 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: single and daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: April to November 2007; summer Source of funding: this study was funded by a research grant from the Institute of Obstetrics and Gynaecology Trust, Wolfson and Weston Research Centre for Family Health, Imperial College, Du Cane Road, Hammersmith Hospital, London W12 0NN, UK. Dates of the study and location: April 2007 to November 2007, London, UK Declarations of interest among primary researchers: none declared
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Computer‐generated random number lists were drawn up by an independent researcher, with randomisation in blocks of 15.
Allocation concealment (selection bias)	Low risk	The research staff allocating participants used the next available number on entry to the trial, and each woman collected her tablets directly from the hospital pharmacy department or her local pharmacy.
Blinding of participants and personnel (performance bias) All outcomes	High risk	Study personnel and participants were not blinded to treatment assignment.
Blinding of outcome assessment (detection bias) All outcomes	High risk	Study personnel were not blinded to treatment assignment.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Only 1 loss to follow‐up from group 3.
Selective reporting (reporting bias)	Unclear risk	There is insufficient information to permit judgement.
Other bias	Unclear risk	Women were randomised within each ethnic group. It is not clear if the ethnicity can be clearly established as it was self‐reported. Women who did not speak English were included only if a health advocate was able to interpret and a leaflet was provided in their language (English, Arabic, Bengali, and Farsi) although the ability to read was not clearly established.

25(OH)D: 25‐hydroxycholecalciferol
β‐hCG: beta human chorionic gonadotropin
ALP: alkaline phosphatase
BA: bone area
BMC: bone mineral content
BMI: body mass index
CHW: community health workers
DXA: dual‐energy X‐ray absorptiometry
ELISA: enzyme‐linked immunosorbent assay
GDM: gestational diabetes mellitus
GSH: glutathione
HDL: high‐density lipoprotein
IGF‐I: insulin‐like growth factor
IU: international units
IUFD: intrauterine fetal death
LBW: low birthweight
LDL: low‐density lipoprotein
μg: microgram
PA: physical activity
PTH: parathyroid hormone

Characteristics of excluded studies [ordered by study ID]

Study	Reason for exclusion
ACTRN12612001145897 (first received 2012)	This is a study of different regimens of vitamin D (no placebo group). This type of intervention is outside the scope of this review.
Adegboye 2020	This trial is on women with periodontitis. This type of participant is outside the scope of this review.
Ala‐Houhala 1986	49 healthy, well‐nourished mothers (non‐pregnant) exclusively breastfeeding their infants, were divided in succession into 3 groups: group 1 (n = 17): mothers were given 2000 IU vitamin D3 a day, infants not supplemented; group 2 (n = 16): mothers were given 1000 IU vitamin D3 a day, infants not supplemented; group 3 (n = 16): mothers were not supplemented, and their breastfed infants were given 400 IU of vitamin D2 a day. This type of participant is outside the scope of this review.
Asemi 2013a	54 pregnant women aged 18 to 40 years diagnosed with gestational diabetes were randomly assigned to 1 of 2 groups: group 1 (n = 27), women received capsules containing 50,000 IU vitamin D (cholecalciferol‐D3) (D‐Vitin 50000; Zahravi Pharm Co) 2 times during the study (at baseline and at day 21 of the intervention): group 2 (n = 27), women received 2 placebos (Barij Essence Co) at the same times. All pregnant women in the study had a diagnosis of gestational diabetes. This type of participant is outside the scope of this review. Also, there are two published articles for this study. One has been retracted by the publisher: Effects of vitamin D supplementation on glucose metabolism, lipid concentrations, inflammation, and oxidative stress in gestational diabetes: a double‐blind randomized controlled clinical trial. Am J Clin Nutr 2013;98(6):1425–32. https://www.sciencedirect.com/science/article/pii/S0002916522007134?via%3Dihub The other article has a note of concern from the publisher, who states: "Favorable Effects of Vitamin D Supplementation on Pregnancy Outcomes in Gestational Diabetes: A Double Blind Randomized Controlled Clinical Trial" Asemi Z et al. Horm Metab Res 2015; 47 DOI:10.1055/s-0034-1394414.Since publication of this article,serious concernshave been raised about the integrity of the reported methods, results and analysis. Responses by the leading author and ethics committees have been unsatisfactory and inconclusive; we advise readers to interpret the information presented in the article with due caution. https://www.thieme‐connect.de/products/ejournals/abstract/10.1055/s‐1736‐8751
Asemi 2013b	This article has been retracted by the publisher. The Editor‐in‐Chief is retracting this article due to concerns about the validity of participant data in the study. Vitamin D Supplementation Affects Serum High‐Sensitivity C‐Reactive Protein, Insulin Resistance, and Biomarkers of Oxidative Stress in Pregnant Women. J Nutr 2013;143(9):1432–38. https://jn.nutrition.org/article/S0022‐3166(22)00172‐9/pdf
Asemi 2014	56 pregnant women 18 to 40 years of age with gestational diabetes and 24 to 28 weeks' gestation attending prenatal care at maternity clinics affiliated to Kashan University of Medical Sciences, Kashan, Iran were randomly assigned to 1 of 2 groups: group 1 (n = 28) received 1000 mg calcium per day and a 50,000 U vitamin D (cholecalciferol‐D3) pearl twice during the study (at study baseline and on day 21 of the intervention); group 2 (n = 28) received 2 placebos at the same times. Participants were pregnant women with a diagnosis of gestational diabetes. This type of participant is outside the scope of this review. Also, articles by Asemi and colleagues are under investigation https://retractionwatch.com/2020/11/10/journals‐flag‐concerns‐in‐three‐dozen‐papers‐by‐nutrition‐researchers/
Asemi 2015	46 women at risk for pre‐eclampsia at 27 weeks' gestation with positive roll‐over test were randomly assigned to receive either the multi mineral‐vitamin D supplements (n = 23) or the placebo (n = 23) for 9 weeks. Study was conducted in Kashan, Iran, during November 2013 to May 2014. Multi mineral‐vitamin D supplements contained 800 mg calcium, 200 mg magnesium, 8 mg zinc, and 400 IU vitamin D3. Fasting blood samples were taken at baseline and after 9‐week intervention to measure related factors. Newborn's outcomes were determined. This type of intervention is outside the scope of this review. Also, articles by Asemi and colleagues are under investigation https://retractionwatch.com/2020/11/10/journals‐flag‐concerns‐in‐three‐dozen‐papers‐by‐nutrition‐researchers/
Atkinson 2010	120 African American or Caucasian primigravidae women 19 to 40 years of age in their first trimester of pregnancy in Children’s Hospital & Research Center Oakland, California, USA were included in this study. Women who were smokers, had a pre‐pregnancy BMI higher than 30, had a medical condition that affected bone or were taking a medication that affected bone were excluded. Participants were randomly assigned to 1 of 3 groups: group 1: 1000 mg of calcium; group 2: 2000 IU vitamin D; group 3: placebo. The intervention started at week 16 of pregnancy until delivery. This type of intervention is outside the scope of this review.
Azami 2017	90 pregnant women with least 1 of the risk factors for pre‐eclampsia were randomly divided into 3 groups according to randomised selection: Group A received 1 ferrous sulphate tablet + 1 Claci‐care multimineral‐vitamin D tablet with 800 mg Ca, 200 mg Mg, 8 mg Zn and 400 IU vitamin D3)]per day; Group B received 1 ferrous sulphate tablet + 250 mg vitamin C and 55 mg vitamin E; control group only received ferrous sulphate daily. This type of intervention is outside the scope of this review.
Baidya 2022	Participants had gestational diabetes. This type of participant is outside the scope of this review.
Baqui 2009	28 pregnant women from Bangladesh, aged 18 to 34 years, at 27 to 30 weeks of pregnancy were randomly assigned to 1 of 2 groups: Group 1 (N = 14) received a single dose of vitamin D3 70,000 IU (1.75 mg, where 1 mg = 40,000 IU) on day 0 followed by vitamin D3 35,000 IU (0.875 mg) per week starting on day 7 and continuing until delivery; Group 2 (N = 14) received vitamin D3 14,000 IU (0.350 mg) per week starting on day 0 and continuing until delivery. All participants received vitamin D supplementation in different regimens. This type of intervention is outside the scope of this review.
Bhatia 2010	150 consecutive pregnant women during their second trimester from 6 villages of a poor socio‐economic region in north India were initially randomised to receive either no dose or 1 dose of 60,000 IU cholecalciferol under observation in the 5th gestational month. This was not a randomised trial and the comparisons are outside the scope of this review.
Bhatia 2012	299 pregnant women with 12 and 24 weeks of gestation of lower‐middle and middle socio‐economic groups in India were randomly assigned to 1 of 2 groups, group 1: received 1500 μg cholecalciferol at induction into the study; group 2: 3000 μg cholecalciferol at induction as well as at 28 weeks of gestation. All were prescribed 1 g of elemental calcium daily as calcium carbonate without vitamin D. This type of intervention is outside the scope of our review.
Bhavya 2020	The participants had gestational diabetes. This type of participant is outside the scope of this review.
Bimson 2017	A randomised, double‐blind, placebo‐controlled study was conducted among pregnant women with a gestational age < 20 weeks and with vitamin D deficiency (25(OH)D level < 25 ng/mL). Women were randomised to 50,000 IU vitamin D3 or placebo once a week for 8 weeks in addition to a prenatal vitamin with 400 IU vitamin D3. No placebo group. This type of intervention is outside the scope of this review.
Bisgaard 2009	623 women were recruited at 24 weeks of pregnancy and randomised 1:1 to a daily dose of 2400 IU vitamin D3 supplementation or matching placebo tablets from pregnancy week 24 to 1 week postpartum. In addition, all women were instructed to continue supplementation of 400 IU of vitamin D3 during pregnancy. Thus, the study was a dose comparison of 2800 IU/day vs 400 IU/day of vitamin D3 supplementation. This type of intervention is outside the scope of this review.
Camarena 2022	The participants had gestational diabetes. This type of participant is outside the scope of this review.
Chandy 2016	230 infants and mothers giving birth in 2 maternity units who intended to continue exclusive breastfeeding until the first 6 months and coming to the hospital of birth for immunisation were randomly assigned at birth to 1 of 3 treatment regimens of vitamin D for 9 months. The intervention was only at postpartum. This type of intervention is outside the scope of our review.
ChiCTR‐TRC‐14005235 (first received 2014)	The participants had gestational diabetes. This type of participant is outside the scope of this review.
ChiCTR1900027679 (first received 2019)	Both arms received vitamin D. This type of intervention is outside the scope of our review.
Cockburn 1980	1139 pregnant women were assigned to 1 of 2 groups: group 1 (n = 506) received a daily dietary supplement of 400 IU of vitamin D2 from about the 12th week of pregnancy until delivery; group 2 (n = 633) received a placebo containing no vitamin D. This is not a randomised trial.
CTRI/2013/10/004056 (first received 2013)	Both arms received vitamin D. This type of intervention is outside the scope of our review.
CTRI/2014/12/005343 (first received 2014)	Participants had gestational diabetes. This type of participant is outside the scope of this review.
CTRI/2019/01/017185 (first received 2019)	The participants had gestational diabetes. This type of participant is outside the scope of this review.
CTRI/2022/01/039091 (first received 2022)	Both groups received vitamin D. This type of intervention is outside the scope of this review.
Cullers 2019	Calcium intervention. This type of intervention is outside the scope of this review.
Czech‐Kowalska 2013	174 healthy postpartum women who had delivered babies at term were randomised to 1 of 2 groups: group 1 (n = 70) received 1200 IU/day vitamin D (cholecalciferol‐D3 as 800 IU/day alone + 400 IU/day from a multiple micronutrient supplements); group 2 (n = 67) received 400 IU/day vitamin D (cholecalciferol‐D3 as placebo + 400 IU/day from multiple micronutrient supplements) during 6 months of lactation. Participants from both groups received vitamin D supplements. The participants were postpartum women. The type of participant and the type of interventions are outside the scope of this review.
Das 2009	The trial switched to non‐randomised regimens part of the way through the trial. This type of intervention is outside the scope of this review.
Das 2010	200 pregnant women from Queen Mary Hospital in CSMMU were randomly allocated to 1 of 3 groups: (1) Intervention group 1 vitamin D 120,000 IU in 3 doses each 8 weeks apart + calcium carbonate containing 500 mg elemental calcium with 250 IU vitamin D twice a day, daily throughout pregnancy; (2) Intervention group 2 vitamin D 60,000 IU in 3 doses each 8 weeks apart + calcium carbonate containing 500 mg elemental calcium with 250 IU vitamin D twice a day, daily throughout pregnancy; (3) Comparator agent group 3 calcium carbonate containing 500 mg elemental calcium with 250 IU vitamin D twice a day, daily throughout pregnancy. This is a registry for an ongoing study (open to recruitment). This type of intervention is outside the scope of this review.
Dawodu 2013	192 Arab women between 12 and 16 weeks of gestation who had a singleton pregnancy were randomised into different regimens of vitamin D supplementation. This type of intervention is outside the scope of this review.
de Menibus 1984	77 mother‐child couples were divided into 3 groups, according to whether women were not receiving vitamin D (29 couples) during the last 3 months of pregnancy ending in winter or taking 1000 units of vitamin D in the form of Uvesterol (21 pairs) or a single dose of 200,000 units of vitamin D at 7 months (27 pairs). There was no placebo group. This type of intervention is outside the scope of this review.
DRKS00005421 (first received 2013)	This trial was not randomised. This type of trial is outside the scope of our review.
Enkhmaa 2018 Dec	This trial is of different regimens of vitamin D. This type of intervention is outside the scope of this review.
Etemadifar 2015	45 pregnant women with confirmed multiple sclerosis from Iran aged 20 to 40 years with low serum 25(OH)D levels were randomly allocated to 2 groups in an open‐label randomised, controlled clinical phase I/II pilot study. Groups received 50,000 IU/week vitamin D3 (n = 21) or routine care (n = 22) from 12 to 16 weeks of gestation until delivery. This type of participant is outside the scope of this review.
Fang 2019	This intervention was serum 25(OH)D measured at ~12 weeks or not. This type of intervention is outside the scope of this review.
Gerais 2015	88 women were recruited at different gestational ages. A single daily dose ranging from 1000 IU to 2000 IU according to the level of serum vitamin D was given to the patient for 6 weeks. There was no placebo group. This type of intervention is outside the scope of this review.
Gunasegaran 2020	All participants had gestational diabetes. This type of participant is outside the scope of our review.
Gupta 2018	This trial was of different regimens of vitamin D (no placebo). This type of intervention is outside the scope of our review.
Gurkan 2022	The study has been retracted by the author. The publisher states: "The article “Vitamin D supplementation during pregnancy inhibits the activation of fetal membrane NF‐κB pathway, by N. Gurkan, published in Eur Rev Med Pharmacol Sci 2022; 26 (16): 5926‐5931–10.26355/eurrev_202208_29532–PMID: 36066168” has been retracted by the author as she stated that Vitamin D was used in the study as supplement and not a drug without the permission of the Ministry. Although Vitamin D is a commonly used supplement during pregnancy, the Local Ethics Committee accepted it as a drug and asked for it to be approved by the Ministry of Health. Therefore, the manuscript has been withdrawn." https://www.europeanreview.org/article/30349
Hajhashemi 2017	This trial was an intervention comparing vitamin D supplements versus sunlight. This type of intervention is outside the scope of this review.
Hanieh 2014	This was a prospective study, not a randomised controlled trial. This type of study is outside the scope of this review.
Hashemipour 2014	160 pregnant women (24 to 26 weeks of gestation) were randomised to the control group (multivitamin containing 400 IU vitamin D3 plus 200 mg elemental calcium each day until delivery) or to the intervention group (weekly dose of 50,000 IU oral vitamin D3 for 8 weeks from 26 to 28 weeks of pregnancy as well as a multivitamin and elemental calcium). Both groups received vitamin D and calcium. This type of intervention is outside the scope of our review.
Hollis 2011	This trial was on different regimens of vitamin D (no placebo group). This type of intervention is outside the scope of this review.
Hossain 2012	200 pregnant women aged 18 to 40 years were randomised into group 1 (n = 100) who received ferrous sulphate and 4000 IU of vitamin D3 or group 2 (n = 100) who received ferrous sulphate + calcium. Both groups received medications from 20 weeks of pregnancy until delivery. This type of intervention is outside the scope of our review.
Hosseinzadeh 2020	All participants had gestational diabetes. This type of participant is outside the scope of our review.
Huang 2021	All participants had gestational diabetes. This type of participant is outside the scope of our review.
IRCT20100102002954N11 (first received 2018)	All participants had gestational diabetes. This type of participant is outside the scope of this review.
IRCT20120718010324N59 (first received 2020)	All participants had gestational diabetes. This type of participant is outside the scope of this review.
IRCT20120718010324N61 (first received 2021)	All participants had sleep disorder. This type of participant is outside the scope of this review.
IRCT2012101611144N1 (first received 2012)	All participants had gestational diabetes. This type of participant is outside the scope of this review.
IRCT20130616013678N29 (first received 2019)	All participants had gestational diabetes. This type of participant is outside the scope of this review.
IRCT201306253140N11 (first received 2013)	All participants had gestational diabetes. This type of participant is outside the scope of this review.
IRCT2015022714275N2 (first received 2015)	All participants had gestational diabetes. This type of participant is outside the scope of this review.
IRCT20150607022585N3 (first received 2018)	All participants had gestational diabetes. This type of participant is outside the scope of this review.
IRCT2015122725725N1 (first received 2016)	All participants had gestational diabetes. This type of participant is outside the scope of this review.
Ito 1994	876 singleton pregnant women with blood pressure lower than 140/90 mmHg at 20 weeks’ gestation, and no evidence of proteinuria were divided into 2 groups: group 1 (n = 666) women received conventional antenatal care; group 2 (n = 210 women) were managed under a protocol for the prediction of pre‐eclampsia with an angiotensin sensitivity test and prevention of the condition by calcium supplementation. This is not a randomised trial and this type of intervention is outside the scope of this review.
Jamilian 2016	60 participants with gestational diabetes were divided into 2 groups of either 1000 IU vitamin D3 and 1000 mg evening primrose oil or placebo for 6 weeks. At the beginning and end of the study, fasting blood samples were obtained from the participants to measure related variables. This type of participant is outside the scope of this review.
Jamilian 2017a	140 participants with gestational diabetes. Participants were randomly divided into 4 groups to receive: (1) 1000 mg omega‐3 fatty acids containing 360 mg eicosapentaenoic acid and 240 mg docosahexaenoic acid (DHA) twice a day + vitamin D placebo (n = 35); (2) 50,000 IU vitamin D every 2 weeks + omega‐3 fatty acid placebo (n = 35); (3) 50,000 IU vitamin D every 2 weeks + 1000 mg omega‐3 fatty acids twice a day (n = 35), and (4) vitamin D placebo + omega‐3 fatty acid placebo (n = 35) for 6 weeks. This type of participant is outside the scope of this review.
Jamilian 2017b	All participants had gestational diabetes. This type of participant is outside the scope of this review.
Jamilian 2019	All participants had gestational diabetes. This type of participant is outside the scope of this review.
Jefferson 2019	This trial is of different regimens of vitamin D. This type of intervention is outside the scope of this review.
Judkins 2010	Both arms received vitamin D. This type of intervention is outside the scope of this review.
Kachhawa 2014	243 pregnant patients aged 18 to 40 years old and with gestational age 12 to 16 weeks were randomised into 4 groups in a ratio of 1:1:1:1. Group 1 active control group received 600 units of vitamin D per day, group 2: 1000 units/day, group 3: 2000 units/day and group 4: 4000 units per day. No placebo group. This type of intervention is outside the scope of our review.
Karamali 2014	60 women with gestational diabetes were divided into 2 groups to receive Ca + vitamin D supplements or placebo. Individuals in the Ca + vitamin D group (n = 30) received 1000 mg Ca/day and 2 pearls containing 1250 μg (50,000 IU) of cholecalciferol (vitamin D3) during the intervention (one at study baseline and another at day 21 of the intervention); those in the placebo group (n = 30) received 2 placebos of vitamin D at the mentioned times and placebos of Ca every day for 6 weeks. This type of participant is outside the scope of this review. Also, articles by Asemi and colleagues are under investigation https://retractionwatch.com/2020/11/10/journals‐flag‐concerns‐in‐three‐dozen‐papers‐by‐nutrition‐researchers/
Karamali 2015	60 pregnant women at risk for pre‐eclampsia according to abnormal uterine artery Doppler waveform were randomly divided into 2 groups to receive 50,000 IU vitamin D supplements (n = 30) or placebo (n = 30) every 2 weeks from 20 to 32 weeks of gestation. All pregnant women were also taking 400 μg/day folic acid from the start of pregnancy, 60 mg/day ferrous sulphate from the second trimester, and a multivitamin mineral capsule (containing 400 IU vitamin D) from the second half of pregnancy. This type of participant is outside the scope of this review. Also, articles by Asemi and colleagues are under investigation https://retractionwatch.com/2020/11/10/journals‐flag‐concerns‐in‐three‐dozen‐papers‐by‐nutrition‐researchers/
Kermack 2017	111 couples were recruited for a 6‐week intervention prior to oocyte retrieval consisted of a daily drink, containing 2 g of DHA plus EPA and 10 μg of vitamin D, and olive oil and olive oil spreads, all in unmarked containers. The control group received a placebo drink and sunflower oil and spreads, again in unmarked containers. 55 couples were randomised to the treatment group and 56 to placebo. Following IVF, embryos were cultured in an embryoscope and validated morphokinetic markers of embryo quality were recorded; day 3 and 5 KIDScores (Known Implantation Data Score) were calculated for individual embryos. This type of intervention is outside the scope of our review.
Khatiwada 2020	Both groups received vitamin D (no placebo). This type of intervention is outside the scope of this review.
Kiely 2015	144 pregnant women aged older than 18 years of age, with no more than 18 weeks' gestation, in good general health, with low‐risk pregnancy and not consuming > 10 μg/day vitamin D from supplements were randomised in a 3‐arm, parallel, double‐blind, placebo‐controlled dose‐response intervention study with vitamin D. Group 1 received 10 μg (400 IU) vitamin D3 once daily taken from baseline visit (approximately 15 weeks' gestation) until endpoint (delivery). Group 2 received 20 μg (800 IU) vitamin D3 once daily from baseline visit (approximately 15 weeks' gestation) until endpoint (delivery). Group 3 (placebo) received a placebo capsule containing 0 μg (0 IU) of vitamin D3 taken from baseline visit (approximately 15 weeks' gestation) until endpoint (delivery). The primary outcome was serum 25‐hydroxyvitamin D in pregnant women and cord blood. Women were permitted to continue with self‐administration of antenatal supplements containing ≤ 10 μg vitamin D per day. This type of intervention is outside the scope of our review.
Lalooha 2012	Participants were randomly assigned to 1 of 2 groups: group 1: vitamin D capsule 50,000 U weekly for 8 weeks from 28 gestational age and multivitamin tablet including 400 IU vitamin D daily until termination; group 2: multivitamin tab including 400 IU vitamin D daily until termination. No placebo group. This type of intervention is outside the scope of our review.
Li 2016	The study included 103 pregnant gestational diabetes women. All participants in their second trimester were randomly assigned to consume 2 servings (100 g per serving) of either plain yogurt (PY) drink (‘PY’ without any vitamin D3 supplement, from Mengniu Dairy, Hohhot, China) or VDY drink (‘PY’ supplemented with 500 IU vitamin D3, from Mengniu Dairy, Hohhot, China), with 1 serving at breakfast and the other 1 at dinner, on a daily basis for a period of 16 weeks. This type of participant is outside the scope of this review.
Lindqvist 2010	We emailed the authors on 7 March 2023 and they replied that the randomisation had been destroyed by the IT department so they had not been able to produce any results. Therefore, this study will not be able to provide results.
MacDonald 1986	This trial was registered in 1986 on the Oxford Database of Perinatal Trials and reports the recruitment and follow‐up completed in 1979. The registration form reports a randomised controlled trial to assess the efficacy of calcium and vitamin D supplementation versus placebo in the prevention of maternal and fetal hypocalcaemia. The reports indicate that the sample size was 55 Asian women with morbidity and laboratory results as primary outcomes, but no further information is available. No further information has been available since 1986.
March 2010	226 healthy pregnant women from Greater Vancouver, British Columbia, Canada, from 13 to 24 weeks of gestation were randomly allocated to 10, 25, or 50 mg vitamin D/day from 13 to 24 weeks of gestation until 8 weeks postpartum, with no infant supplementation. Mother and infant blood was collected at 8 weeks postpartum (n = 76, n = 76, and n = 74, respectively). The overall study retention rate from beginning to end was 76% (n = 172). There was no placebo group. This type of intervention is outside the scope of this review.
Marya 1981	45 Hindu pregnant women were randomly assigned to 1 of 2 groups: group 1 (n = 25) received tablets containing 1200 IU vitamin D and 375 mg calcium daily throughout the 3rd trimester; group 2 (n = 20) received an oral single dose of 600,000 IU vitamin D2 once during the 7th month and 8th month (total 2 doses). This group was compared with group 3 (n = 75) who had not received vitamin D supplements during pregnancy. The results were also compared with data from 25 non‐pregnant, non‐lactating healthy women. The randomised study compared 2 doses of vitamin D supplementation. This type of study, the type of participants, and these types of interventions are outside the scope of this review.
McLean 2012	Pregnant women, aged 18 years or more, with less than 20 weeks’ gestation at recruitment. Participants were randomly assigned to 1 of 2 groups: group 1: received high‐dose vitamin D supplementation (5000 IU/day); group 2: standard dose pregnancy vitamin supplementation (400 IU vitamin D daily), administered as an oral capsule, from the time of the first antenatal clinic visit (around 12 weeks’ gestation) until delivery. This type of intervention is outside the scope of this review.
Mirzaei‐Azandaryani 2022	Both groups received vitamin D from 16 weeks onwards. This type of intervention is outside the scope of this review.
Moghaddam 2012	Participants all had gestational diabetes. This type of participant is outside the scope of this review.
Mojibian 2015	500 women with gestational age 12 to 16 weeks and serum 25 hydroxy vitamin D (25(OH)D) less than 30 ng/mL were randomly categorised into 2 groups: group A received 400 IU vitamin D daily and group B 50,000 IU vitamin D every 2 weeks orally until delivery. Maternal and neonatal outcomes were assessed in 2 groups. No placebo group. This type of intervention is outside the scope of this review.
Mosalanejad 2016	Both arms received vitamin D. This type of intervention is outside the scope of this review.
Mozzafari 2010	Women between 20 and 45 years old with gestational diabetes at their recent pregnancy, from the list of GDM Diabetes Research Center of Yazd University, and without thyroid disease, kidney disease, bone disease, PCO, liver disease, and not using anti‐epilepsy drugs, glucocorticoids, and statins. Exclusion criteria: risk of any illness that requires medication and lack of any willingness to co‐operate. Women were randomly assigned to 1 of 2 groups: group 1: intramuscular injection of vitamin D 300,000 IU dose; group 2: control: did not receive any intervention. This type of participant is outside the scope of this review.
Mutlu 2013	91 pregnant women aged 16 to 42 years were admitted to Kocaeli Maternity and Children Hospital between April 2011 and April 2012. The participants were randomly divided into 3 groups: 600 IU/day (control group; n = 31); 1200 IU/day (n = 31), and 2000 IU/day (n = 32) of vitamin D. No placebo group. This type of intervention is outside the scope of our review.
Nausheen 2014	315 pregnant women aged 15 to 45 years with less than 16 weeks of gestation in a hospital in Pakistan. Pregnant women with pre‐existing type 1 or type II diabetes, pre‐existing hypertension, multiple fetuses, babies (twins, triplets), or with a diagnosis of pregnancy with a fetal anomaly on scan were excluded. Participants were randomly assigned to 1 of 3 groups: group 1 received a dose of 400 IU/day until the time of delivery; group 2 received 2000 IU/day until the time of delivery; group 3 received 4000 IU/day until the time of delivery. No placebo group. This type of intervention is outside the scope of this review.
NCT02272387 (first received 2014)	Both groups received vitamin D in prenatal vitamins (no placebo group). This type of intervention is outside the scope of this review.
NCT02706158 (first received 2016 March 11)	Participants were randomised to vitamin C and E supplementation. This type of intervention is outside the scope of this review.
NCT02713009 (first received 2016 Jan 14)	Both groups received some vitamin D (no placebo group). This type of intervention is outside the scope of this review.
NCT03645109 (first received 2018 Aug 24)	All participants had gestational diabetes. This type of participant is outside the scope of this review.
NCT04825093 (first received 2021 April 01)	Both groups received vitamin D (no placebo group). This type of intervention is outside the scope of this review.
Niramitmahapanya 2017	76 Thai lactating mothers and their breastfed infants were studied with low maternal 25(OH)D levels (10 to 30 ng/mL); 1 group received vitamin D3 1800 IU/day supplementation for 6 weeks, and members of the other group were given a placebo. This type of participant is outside the scope of this review.
Pandey 2015	20 women with 25(OH)D < 20 ng/mL and Hb = 8 to 10 g/dL were randomised into groups using a computerised program (8 patients in iron alone group and 12 pregnant mothers in iron + vitamin D group). Recruited pregnant mothers received group iron + vitamin D: tablets containing a fixed‐dose combination of vitamin D (1000 IU) + ferrous ascorbate (100 mg of elemental iron) + folic acid (1 mg) + vitamin B12 (7.5 μg) (1 tablet/day) for 12 weeks. Group iron alone: tablets containing a fixed‐dose combination of ferrous ascorbate (100 mg of elemental iron) + folic acid (1.1 mg) (1 tablet/day) for 12 weeks. This type of intervention is outside the scope of this review.
Qian 2015	This study recruited 60 pregnant women at risk for pre‐eclampsia, experiencing their first pregnancy, aged between 20 and 32 years, between 18 and 20 weeks' gestation, and pregnant with a single fetus. Each pregnant woman selected for the study showed the following abnormalities on uterine artery Doppler, qualifying them as high‐risk: average resistance index (RI) > 0.67, pulsatility index (PI) > 1.65, and incisura at early diastole phase. Pregnant women were randomised into 2 groups to take either cholecalciferol supplements (n = 30) or placebo (n = 30). This study has been retracted by the publisher, who states: "Qian L, Wang HY, Wu FH, Li M, Chen W, Lv LZ. International Journal of Clinical and Experimental Medicine. 2015;8:18041‐18049. It was discovered by an investigator who was performing a meta‐analysis that a large portion of the data in this article was identical or very similar to another paper published in another journal within similar time frame, which was subsequently confirmed by the Editorial Office. Authors have failed to respond to the editorial request for explanation why this had happened before the deadline. Therefore, the entire article has been retracted in accordance with this journal’s policy and Editorial decision." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6188103/
Rasmussen 2009	Participants were recruited and randomised pre‐pregnancy. This type of participant is outside the scope of this review.
Razavi 2017	120 women with gestational diabetes were randomly divided into 4 groups to receive: 1) 1000 mg omega‐3 fatty acids containing 180 mg eicosapentaenoic acid (EPA) and 120 mg docosahexaenoic acid (DHA) twice a day + vitamin D placebo (n = 30); 2) 50,000 IU vitamin D every 2 weeks + omega‐3 fatty acid placebo (n = 30); 3) 50,000 IU vitamin D every 2 weeks + 1000 mg omega‐3 fatty acids twice a day (n = 30) and 4) vitamin D placebo + omega‐3 fatty acid placebo (n = 30) for 6 weeks. This type of participant is outside the scope of this review.
Rostami 2018	1600 and 900 first‐trimester pregnant women, aged 18 to 40 years, with gestational age < 14 weeks, with singleton pregnancy in Masjed‐Soleiman, Khuzestan province, Iran were randomised by levels of vitamin D in serum to: Moderate deficiency I1: 50,000 IU of oral D3 weekly for a total duration of 6 weeks I2: 50,000 IU of oral D3 weekly for a total duration of 6 weeks and then a monthly maintenance dose of 50,000 IU of D3 until delivery I3: a single dose of intramuscular administration of 300,000 IU of D3 I4: a single dose of intramuscular administration of 300,000 IU of D3 and then a monthly maintenance dose of 50,000 IU of D3 until delivery Severe deficiency I5: 50,000 IU of oral D3 weekly for a total duration of 12 weeks I6: 50,000 IU of oral D3 weekly for a total duration of 12 weeks and then a monthly maintenance dose of 50,000 IU of D3 until delivery I7: intramuscular administration of 300,000 IU of D3; 2 doses for 6 weeks I8: intramuscular administration of 300,000 IU of D3; 2 doses for 6 weeks, followed by a monthly maintenance dose of 50,000 IU of D3 until delivery Pregnant women with normal vitamin D status were the controls. All women were allowed to consume multivitamins containing no more than 400 IU per day of vitamin D3. This type of intervention is outside the scope of this review.
Rostami 2020	This trial is on regimens of vitamin D (no placebo group). This type of intervention is outside the scope of this review.
Shakiba 2013	51 healthy pregnant women from the beginning of their second trimester of pregnancy during the autumn and winter of 2009 recruited from 2 primary care clinics in Yazd, Iran. Participants were distributed in 3 groups according to their serum 25(OH)D at the beginning of the second trimester of pregnancy. Participants with low concentrations (25(OH)D levels < 20 ng/mL) (n = 17) were treated with 200,000 IU (50,000 IU/week for 4 weeks) of vitamin D (as cholecalciferol‐D3), followed by supplementation with 50,000 IU/month vitamin D (cholecalciferol‐D3). The other 34 participants were randomly assigned to 1 of 2 groups: group 1 received 50,000 IU/month vitamin D (cholecalciferol‐D3); group 2 received 100,000 IU/month (50,000 IU every 2 weeks) of vitamin D (cholecalciferol‐D3) supplementation. All participants received vitamin D supplements. This type of study design and this type of intervention are outside the scope of this review.
Shi 2017	602 women with singleton pregnancy who were diagnosed with pre‐eclampsia in Cangzhou Central Hospital participated in the trial and were divided into 2 groups using stratified permuted‐block randomisation method with diastolic blood pressure as a factor: (1) nifedipine + vitamin D group (n = 298), given 1 capsule containing nifedipine (10 mg per capsule) and vitamin D (200 IU per capsule) every 15 min orally, up to 4 doses, until blood pressure was equal to or below 150/100 mmHg; (2) nifedipine + placebo group (n = 304), given 1 capsule containing nifedipine (10 mg per capsule) plus glucose (20 mg per capsule) as placebo every 15 minutes orally, up to 4 doses, until blood pressure was equal to or below 150/100 mmHg. This type of participant is outside the scope of this review.
Simsek 2011	Women with gestational diabetes were randomly assigned to 1 of 2 groups: group 1: cholecalciferol 15,000 IU once a week during pregnancy; group 2: placebo. This type of participant is outside the scope of this review.
Singh 2021	This trial is of oral vs intramuscular vitamin D. This type of intervention is outside the scope of this review.
SLCTR/2018/020 (first received 2018)	Participants all had gestational diabetes. This type of participant is outside the scope of this review.
Soheilykhah 2013	120 women with gestational age less than 12 weeks without gestational diabetes, history of PCO, BMI less than 30 kg/m2 before pregnancy, no vitamin D supplementation in the past 6 months were randomised into 2 groups: supplementation with 50,000 IU of vitamin D monthly (2000 IU daily) or 50,000 IU every 2 weeks (4000 IU daily). Maternal and neonatal outcomes were assessed in 2 groups. No placebo group. This type of intervention is outside the scope of this review.
Stephensen 2011	Pregnant women less than 20 weeks' gestation and over 18 years of age with no use of medications known to affect vitamin D metabolism, diagnosis of type 1 diabetes, history of thyroid, renal, or liver disease, problems with digestion or absorption participated in the study at USDA Western Human Nutrition Research Center and clinicians at UC Davis Medical Center. They were distributed into 2 groups, receiving either 400 IU or 2000 IU of vitamin D per day for the duration of their pregnancy. No placebo group. This type of intervention is outside the scope of our review.
Sudfeld 2017	2300 HIV‐infected pregnant women receiving triple‐drug ART under Option B+ in Dar es Salaam Tanzania. HIV‐infected pregnant women of 12 to 27 weeks' gestation were randomised to either: 1) 3000 IU vitamin D3 taken daily from randomisation in pregnancy until trial discharge at 12 months postpartum; or 2) a matching placebo regimen. Maternal participants were followed up at monthly clinic visits during pregnancy, at delivery, and then with their children at monthly postpartum clinic visits. This type of participant is outside the scope of this review.
Taheri 2014	229 women (non‐pregnant) 18 to 35 years old, who were confirmed to be vitamin D deficient (vitamin D < 75 nmol/L), were randomised into the intervention (2000 IU/day oral vitamin D) and placebo. The study was conducted among reproductive women in a high‐risk population for vitamin D deficiency. This type of participant is outside the scope of this review.
Thiele 2014	16 pregnant women at 24 to 28 weeks' gestation were randomised to the control group (n = 8), who received a prenatal vitamin containing 400 IU vitamin D daily, plus a placebo capsule or the experimental group (n = 8), who received the same prenatal vitamin with an additional capsule containing 3400 IU vitamin D, for a total of 3800 IU daily. No placebo group without vitamin D. This type of intervention is outside the scope of this review.
Trivedi 2020	The trial was on postnatal women. This type of participant is outside the scope of this review.
Valizadeh 2016	96 women with gestational diabetes at weeks 12 to 32 of gestation, age > 16 years, singleton pregnancy were randomly assigned to either the intervention (n = 48) or control group (n = 48). Patients were referred from primary health centres affiliated with Zanjan University of Medical Sciences, as well as private obstetric clinics throughout the city. This type of participant is outside the scope of this review.
von Hurst 2009	235 South Asian women, aged 23 to 68 years, living in Auckland, New Zealand were recruited for the study and those who were insulin resistant ‐ homeostasis model assessment 1 (HOMA1) > 1.93 and had serum 25‐hydroxyvitamin D concentration < 50 nmol/L were randomly assigned to 1 of 2 groups: group 1 (n = 42) received 100 μg (4000 IU) vitamin D3; group 2 (n = 39) received a placebo daily for 6 months. The study participants were non‐pregnant women. This type of participant is outside the scope of this review.
Wagner 2006	494 apparently healthy pregnant women (16 to 45 years of age) with 12 to 16 weeks' gestation of singletons attending prenatal care at the Medical University of South Carolina, United States were randomised into 1 of 3 groups stratified by race: group 1 received 400 IU vitamin D (cholecalciferol‐D3)/day; group 2 received 2000 IU vitamin D (cholecalciferol‐D3)/day; and group 3 received 4000 IU vitamin D (cholecalciferol‐D3)/day until delivery. All women received daily multiple micronutrient supplements. All women received vitamin D supplementation at different doses. This type of intervention is outside the scope of this review.
Wagner 2013	258 women (non‐pregnant), exclusively breastfeeding (n = 201) and formula‐feeding (n = 57) women participating in a prospective randomised controlled trial of vitamin D supplementation were compared at baseline 1 month postpartum (V1), at 4 months (V4), and 7 months postpartum (V7) on the basis of vitamin D status (measured by total circulating 25(OH)D concentration) and BMI. This type of intervention is outside the scope of this review.
Wagner 2020	This trial is on regimens of vitamin D for postnatal women. This type of intervention is outside the scope of this review.
Weiss 2009	881 pregnant women with either a personal history of asthma or allergies or a similar history in the spouse or partner, between 18 and 40 years of age and at an estimated gestational age between 10 and 18 weeks, were recruited at a scheduled obstetrical prenatal visit at 3 clinical centres in the US. Participants were randomised to either vitamin D (cholecalciferol, 4000 IU/day; equivalent to 100 μg/day) or placebo. All pregnant mother participants received prenatal vitamins containing 400 IU (10 μg/day) of cholecalciferol; thus, the vitamin D arm received a total of 4400 IU/day (110 μg/day) and the placebo arm received 400 IU/day (10 μg/day). Both groups received vitamin D supplements. This type of intervention is outside the scope of our review.
Wheeler 2017	The participants were all breastfeeding. This type of participant is outside the scope of this review.
Xiaomang 2021	The trial is of different regimens of vitamin D with no placebo group. This type of intervention is outside the scope of this review.
Yap 2014	179 pregnant women 18 years of age or older, with singleton pregnancy, with plasma 25(OH)D levels lower than 32 ng/mL, less than 20 weeks of gestation were randomly assigned to 1 of 2 groups: group 1 (n = 89) received 5000 IU/day of vitamin D (cholecalciferol‐D3) until delivery; group 2 (n = 90) received 400 IU/day of vitamin D (cholecalciferol‐D3) until delivery. All participants received vitamin D supplements at different doses. This type of intervention is outside the scope of this review.
Yazdchi 2016	76 pregnant women without a prior diagnosis of glucose intolerance in the first trimester were asked to participate in a 75‐g OGTT at 24 to 28 weeks of gestation. Diagnosis of gestational diabetes was based on the International Association of Diabetes and Pregnancy Study Groups criteria. Patients were recruited from Al‐Zahra Hospital, the academic outpatient centre of the Tabriz University of Medical Sciences, Tabriz, Iran. Participants in the vitamin D group received oral capsules containing 50,000 IU vitamin D3 once every 2 weeks for 2 months, for a total of 4 capsules. Those in the placebo group received capsules composed of paraffin oil using the same schedule, for a total of 4 placebos. This type of participant is outside the scope of this review.
Zhang 2016	133 pregnant women with gestational diabetes during weeks 24 to 28 of pregnancy. The patients were randomly divided into 4 groups. The control group (n = 20) received a placebo (sucrose; 1 granule/day), the low‐dosage group (n = 38) received the daily recommended intake of 200 IU vitamin D (calciferol) daily, the medium‐dosage group (n = 38) received 50,000 IU monthly (2000 IU daily for 25 days) and the high‐dosage group (n = 37) received 50,000 IU every 2 weeks (4000 IU daily for 12.5 days). This type of participant is outside the scope of this review.
Zhao 2019	This trial is of different regimens of vitamin D (no placebo group). This type of intervention is outside the scope of this review.

25(OH)D: 25‐hydroxycholecalciferol
BMI: body mass index
Ca: calcium
DHA: docosahexaenoic
DRI: dietary reference intakes
EPA: eicosapentaenoic acid
FPG: fasting plasma glucose
GDM: gestational diabetes mellitus
Hb: haemoglobin
IU: international units
IVF: in vitro fertilisation
μg: microgram
Mg: magnesium
OGTT: oral glucose tolerance test
PCO: polycystic ovary syndrome
PTH: parathyroid hormone
Zn: zinc

Characteristics of studies awaiting classification [ordered by study ID]

Asemi 2012.

Methods	Randomised, single‐blinded, controlled trial with 2 arms: vitamin D plus calcium and placebo
Participants	54 pregnant women at risk for pre‐eclampsia, primigravida, aged 18 to 35 years old, carrying singleton pregnancy at their third trimester attending maternity clinics affiliated to Kashan University of Medical Sciences, Kashan, Islamic Republic of Iran (latitude: 33.9889° N, 51.4772° E) Exclusion criteria: maternal severe pre‐eclampsia, IUFD, placenta abortion, preterm delivery and GDM
Interventions	Participants were randomly allocated to 1 of 2 groups: group 1 (n = 27): women received 500 mg of carbonate calcium plus 200 IU of vitamin D (cholecalciferol‐D3) daily for 9 weeks; group 2 (n = 27): women received placebo. The intervention lasted 9 weeks overall, starting at 25 weeks of pregnancy until week 34. Participants were asked not to alter their routine PA or usual diets and not to consume any supplement other than the one provided to them by the investigators. Health worker cadre: the trial was carried out in maternity clinics affiliated to Kashan University of Medical Sciences, Kashan, Islamic Republic of Iran and the investigators provided the supplements to the participants.
Outcomes	Maternal: body weight and height, BMI, fasting plasma glucose levels, serum total cholesterol, triglycerol concentrations, serum HDL‐cholesterol, serum LDL‐cholesterol levels, dietary intakes, total HDL: cholesterol ratio, gestational diabetes, severe pre‐eclampsia, preterm delivery Laboratory method used for assessment of vitamin D concentrations: serum 25‐hydroxyvitamin D concentrations were measured using a commercial ELISA kit (Immuno Diagnostic Systems). The inter‐ and intra‐assay coefficient of variation for serum 25(OH)D assays ranged from 5% to 7.5%.
Notes	Total dose of supplementary vitamin D during pregnancy: 56,000 IU vitamin D or less Start of supplementation: 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): overweight Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: mixed/unknown Source of funding: study was funded by a research grant from the Vice‐Chancellor for research, KUMS, and Iran. Dates of the study and location: April 2011 to February 2012, Iran Declarations of interest among primary researchers: none declared Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no registration, no participant dropouts, no information given on the randomisation method, trial ended February 2012, paper received by publishers May 2012. Also, Dr Asemi is under investigation for suspicious findings in 172 trials; some papers have already been retracted. Please see Retraction Watch references; at the time of writing this was the latest reference to Asemi https://retractionwatch.com/2021/05/14/yep‐pretty‐slow‐nutrition‐researchers‐lose‐six‐papers/. We reached out to the authors and Kashan university authorities, asking for clarification, twice over a three‐month period, but have had no response.

Basutkar 2020.

Methods	Randomised controlled trial
Participants	Inclusion criteria: Maternal age of 18 years or greater at the time of consent The ability to provide written informed consent Confirmed singleton pregnancy on or before 13 weeks of gestation at the time of consent as defined by their last menstrual period or determined by ultrasound Planned to receive ongoing prenatal care Iron deficiency anaemia with average haemoglobin ranging from 10 to 10.9 and 7 to 9.9g/dL at the onset of study and serum ferritin < 12 µg/L No history of blood transfusion 120 days prior to first visit Pregnant women with or with the condition of anaemia and vitamin D deficiency Pregnant women with condition of vitamin D less than 30 nmol/L or 12 ng/mlL Pregnant women already on routine medication prescribed by the treating physician (folic acid 0.5 mg + FeSo4 333 mg (100 mg of elemental iron) OD in the morning; calcium 300 mg OD in the noon; vitamin B complex tablet (vitamin B1: 2mg, vitamin B2 2 mg, vitamin B6 0.5mg, niacinamide 25mg, calcium pantothenate 1mg) OD in the morning) Exclusion criteria: Severe anaemia (Hb < 7g/L) Pregnant women with pre‐existing calcium or who required chronic diuretic or cardiac medication therapy, including calcium channel blockers, or who suffered chronic hypertension Pregnant women with active thyroid disease (e.g. Graves disease, Hashimoto disease, or thyroiditis), but mothers on thyroid supplement with normal serologic parameters could participate in the study if they were without any other endocrine dysfunction Patient with seizure disorders History of anaemia due to other causes or chronic blood loss, haemolytic anaemia or thalassaemia History of hepatic and renal abnormality Known peptic ulcer, oesophagitis, gastritis, or hiatus hernia Family history of thalassaemia, sickle cell anaemia, or malabsorption syndrome History of high‐risk pregnancy including abruption placenta, placenta previa, gestational diabetes, coagulation disorders, thrombocytopenia, or history of drug abuse Chronic haematological disease, hereditary defects of red cells or haemoglobin Patient with multiple pregnancy Allergy to study medications Elimination criteria: Blood haemoglobin levels become less than 7 g/dL during the course of treatment Any other acute illness during the course of treatment Subject not willing to continue the treatment Any severe untoward effect
Interventions	Intervention: 1000 IU vitamin D Control: placebo
Outcomes	Primary outcomes: change in vitamin D, ferritin, haemoglobin concentration, and serum iron and transferrin saturation from baseline visit 1 to visit 4 Secondary outcomes: RBC count, haematocrit, serum iron, transferrin saturation, total iron binding capacity, mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), PTH, calcium and creatinine, symptoms of anaemia, weight gain, blood transfusion
Notes	Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: the study report states that the trial was carried out between July 2017 and August 2018, but trial registration states that enrolment was from August 2017 and that the trial was completed 31 January 2018. Retrospective registration April 2018. The report also states that the trial was a single‐blind study, but online trial registration states that it was a double‐blind study. The paper states 60 in each group: 6 dropouts in each group and extreme outliers were removed; the tables show 54/52 in each group. Standard deviation results are implausible. We reached out to the authors, asking for clarification. The authors replied with information, but too late for it to be assessed for inclusion in this review.

Brooke 1980.

Methods	Randomised, double‐blind, controlled trial; 2‐arm design with individual randomisation
Participants	126 Asian pregnant women 28 to 32 weeks of gestation attending the antenatal clinic at St George's Hospital, London, UK (latitude: 51°30'N, north of the Tropic of Cancer). All pregnant women were first‐generation immigrants, mostly from India, Pakistan, Bangladesh, Sri Lanka, Mauritius, and East Africa. Exclusion and elimination criteria: preterm deliveries, congenital malformations, and maternal illnesses likely to affect fetal growth (such as diabetes) although these data are not presented
Interventions	Participants were randomly allocated to 1 of 2 groups: group 1 (n = 59) received daily 1000 IU vitamin D (ergocalciferol‐D2) daily until term (estimated total dose: 56,000 to 84,000 IU); group 2 (n = 67) received a placebo until term. Start of supplementation: 28 to 32 weeks gestation Length of the intervention/follow‐up: 8 to 12 weeks from supplementation to term Health worker cadre: St George's Hospital Medical School, London, UK. Medical doctors that were part of the team conducted the measurements and provided the supplements.
Outcomes	Maternal: maternal weight gain, dietary vitamin D intake, 25‐hydroxyvitamin D (25‐OHD) concentrations in cord blood and at term. Plasma calcium (adjusted for albumin concentration), inorganic phosphate, bilirubin, albumin concentrations and total alkaline phosphatase activity, alanine transaminase and ʏ‐glutamyl transferase activities, vitamin D binding globulin concentration, compliance Infant: weight, crown‐heel length, crown‐rump length, rump‐heel length, occipitofrontal head circumference, forearm length, lower leg length, triceps and subscapular skinfold thickness, fontanelle area, plasma cholecalciferol at day 3 and day 6, weight, length, and head circumference at 3, 6, 9, and 12 months. Laboratory method used for assessment of vitamin D concentrations: serum 25‐hydroxyvitamin D was measured by competitive protein binding assay after chromatographic purification of lipid extracts of serum.
Notes	Total dose of supplementary vitamin D during pregnancy: more than 56,000 to 200,000 IU Start of supplementation: 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: authors report that to avoid distortion of the results due to seasonal variation in sunlight hours, the trial was carried out during autumn and winter 1977, the whole of 1978, and spring and summer 1979. Source of funding: the pathological research fund, St George's Hospital Medical School, and the South‐west Thames Regional Health Authority. This study was funded by a combination of a research grant and non‐governmental organisations. Dates of the study and location: autumn and winter 1977, the whole of 1978, and spring and summer 1979, London, UK Declarations of interest among primary researchers: none declared Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no mention of written consent, no trial dates given, no mention of method of randomisation, and no participant dropouts reported. We were unable to find any author contact details to ask authors for more details or possible explanations.

Chen 2022.

Methods	Randomised controlled trial; method of randomisation unclear
Participants	60 pregnant women
Interventions	Standard pregnancy care versus standard pregnancy care plus calcium carbonate‐vitamin D3 administration
Outcomes	Diastolic blood pressure Systolic blood pressure Blood calcium levels
Notes	Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no mention of trial registration, no participant flow chart/dropouts (the trial shows exactly 30 women in each group), and randomisation method not clear. We reached out to the authors, asking for clarification, twice over a three‐month period; but have had no response.

ChiCTR‐IOQ‐16009309 (first received 2016).

Methods	Quasi‐randomised, controlled interventional study
Participants	Inclusion criteria: Pregnant women diagnosed with pre‐eclampsia Gestational age 20 to 24 weeks Vitamin D deficient Exclusion criteria: pre‐existing hypertension, diabetes, smoker, complications other than pre‐eclampsia, multiple pregnancy, fetal anomalies, gestational diabetes, pre‐existing renal disease Age: 25 to 35 Gender: female
Interventions	Intervention 200,000 IU of vitamin D versus placebo
Outcomes	Renin, aldosterone, proteinuria
Notes	Unable to locate this reference

ChiCTR1900024080 (first received 2019).

Methods	Randomised controlled trial
Participants	Inclusion criteria: Aged 18 to 45 years Pregnancy or early pregnancy There are no serious basic diseases that interfere with the absorption and metabolism of vitamin D, such as severe skin allergy, autoimmune diseases such as SLE and scleroderma, liver and kidney diseases, gastrointestinal or biliary diseases, etc. Exclusion criteria: Cannot co‐operate to complete the relevant investigation Unable to complete perinatal examination and delivery in the research institution Cannot sign informed consent
Interventions	Control group 1: vitamin D is not tested Control group 2: test for vitamin D without intervention Experimental group 1: self‐intervention Experimental group 2: vitamin D 400 IU/d Experimental group 3: vitamin D 800 IU/d Experimental group 4: vitamin D 2000 IU/d
Outcomes	Primary outcome: vitamin D Secondary outcome: pre‐eclampsia/eclampsia; pregnancy‐related bone metabolism abnormalities; infants smaller than gestational age
Notes	Unable to locate this reference

CTRI/2015/07/006039 (first received 2015).

Methods	Randomised controlled trial
Participants	Pregnant women
Interventions	Weekly 60,000 IU of cholecalciferol orally for 6 weeks and then 1‐monthly for 4 months versus placebo
Outcomes	Pre‐eclampsia Gestational hypertension Preterm birth
Notes	Cannot find published report of this trial, though it began in 2012. Emailed authors 7 March 2023 (pankilamittal@yahoo.in)

CTRI/2017/12/010850 (first received 2017).

Methods	Randomised controlled trial
Participants	Pregnant women
Interventions	Vitamin D versus placebo
Outcomes	Infant rickets and vitamin D levels
Notes	Unable to find published report with results. Emailed authors 7 March 2023 (prashant15prabhakar@gmail.com).

Dabbaghmanesh 2019.

Methods	Randomised experimental study; the vitamin D group was presented with two 1000 IU vitamin D3 pills (2000 IU) daily from 26th to 28th week of gestation until birth; the control group received placebo. Inter‐ and intra‐group comparisons were performed in terms of maternal serum level of 25‐hydroxyvitamin D, thyroid‐stimulating hormone (TSH), free thyroxine (FT4), and thyroid peroxidase (TPO), and 4th week postnatal depression score.
Participants	Nulliparous as well as multiparous females experiencing antenatal care in a teaching hospital in Shiraz, Iran. The inclusion criteria were: age ≥ 18 years, no history of medical problems, no dependence on any types of narcotic substances or alcohol, no gestational complications, a singleton pregnancy with live fetus, and being at week 26 to 28 of gestation. Less than eight weeks' consumption of vitamin D3 supplement led to participant exclusion.
Interventions	Intervention group: 2000 IU of vitamin D3 from 26 to 28 weeks gestation daily until delivery Control group: placebo from 26 to 28 weeks gestation daily until delivery
Outcomes	Maternal serum level of 25‐hydroxyvitamin D, thyroid‐stimulating hormone (TSH), free thyroxine (FT4), thyroid peroxidase (TPO), and 4th week postnatal depression score
Notes	Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no mention of consent, no flow chart, and participant numbers in tables are implausible. Also, the trial from Vaziri 2016 was not mentioned, although they were the same authors and had the same subject. We reached out to the authors, asking for clarification, twice over a three‐month period, but have had no response.

Delvin 1986.

Methods	Randomised trial; 2‐arm design with individual randomisation
Participants	40 pregnant women attending their compulsory visit during the third month of pregnancy at the Obstetrical Unit of the Hopital Edouard Herriot, Lyon, France (latitude: 45° 45' 0" N north of the Tropic of Cancer) Inclusion criteria: singleton pregnancy at term and uneventful vaginal deliveries. Pre‐gestational BMI and skin pigmentation not reported.
Interventions	Participants were randomly assigned to 1 of 2 groups at the time of the compulsory visit: group 1 (n = 20): women received daily 1000 IU vitamin D (cholecalciferol‐D3) (estimated total dose: 55,000 IU) and group 2 (n = 20): women received no supplement, during the last trimester of pregnancy for 12 weeks from start of supplementation to term. Health worker cadre: compliance was verified by a weekly visit by a midwife.
Outcomes	Maternal: serum (during last trimester of pregnancy) and cord blood immunoreactive PTH, serum 25(OH)D levels, 1‐alfa,25‐dihydroxyvitamin D (1,25(OH)2D), total calcium, ionised calcium, magnesium, inorganic phosphate Infant: immunoreactive PTH, serum 25(OH)D levels, 1‐alfa,25‐dihydroxyvitamin D (1,25(OH)2D), total calcium, ionised calcium, magnesium, inorganic phosphate at 4 days of age
Notes	Total dose of supplementary vitamin D during pregnancy: 56,000 IU vitamin D or less Start of supplementation: 20 weeks of pregnancy, or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: winter‐spring. All selections were performed in December, and all deliveries occurred in June. Source of funding: Shriners of North America, the France‐Quebec Exchange Program, and INSERM Grant 121023. This study was funded by a combination of research grant and non‐governmental organisations. Dates of the study and location: dates not reported, Lyon, France Declarations of interest among primary researchers: none declared. Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no information on ethics, one of the authors (Salle) has had another publication retracted for duplication of text, no information on participant characteristics, no trial dates, and randomisation method not clear. We attempted to reach out to the authors to clarify these issues but were unable to contact them.

IRCT20140317017034N6 (first received 2018).

Methods	Randomised controlled trial
Participants	Inclusion criteria: Iranian race; healthy appearance; no history of chronic disease; normal body mass; non‐smoker Exclusion criteria: abortion; intrauterine fetal death; multiple pregnancy; pregnancy with abnormal fetus Age 20 to 34 years Gender: female
Interventions	Intervention group: vitamin D 1000 units per day until delivery Control group: routine pregnancy care including calcium
Outcomes	Femur length: time point: 12th, 19th, and 34th weeks of pregnancy; method of measurement: ultrasound machine PMD, MSD, HL, fetal weight: time point: 12th, 19th, and 34th weeks of pregnancy; method of measurement: ultrasound machine CRL: time point: 12th, 19th, and 34th weeks of pregnancy; method of measurement: ultrasound machine
Notes	Unable to find corresponding papers. Copy of trial registration in Word document (Homeira Vafaei Cisakht). Email address vafaeih@sums.ac.ir: emailed authors on 7 March 2023.

Jelsma 2013.

Methods	Randomised controlled trial with a factorial design
Participants	Pregnant women with gestational age at recruitment < 12 weeks, more than 18 years of age Inclusion criteria: pre‐pregnancy BMI (self‐reported weight, measured height) is ≥ 29 kg/m2), sufficiently fluent in major language of the country of recruitment, being able to be moderately physically active, giving written informed consent, agree to give birth in 1 of the participating hospitals Exclusion criteria: pre‐existing diabetes, diagnosed with (gestational) diabetes mellitus before randomisation, defined as fasting glucose ≥ 5.1 mmol/L and/or 1‐hour glucose ≥ 10 mmol/L and/or 2‐hour glucose ≥ 8.5 mmol/L at baseline, not able to walk at least 100 metres safely, requirement for complex diets, advanced chronic conditions (e.g. valvular heart disease), significant psychiatric disease, unable to speak major language of the country of recruitment fluently, known abnormal calcium metabolism (hypo/hyperparathyroidism, nephrolithiasis, hypercalciuria) or hypercalciuria detected at screening (0.6 mmol/mmol creatinine in spot morning urine) and twin pregnancy
Interventions	The design is that of 2 trials with a factorial design: PA, diet, PA and diet, control, vitamin D, PA and diet and placebo, vitamin D and PA and diet, placebo; to compare the impact of increased PA, enhanced nutrition, and vitamin D supplementation either alone or in combination on maternal glucose tolerance, maternal weight gain, and insulin sensitivity. The doses of vitamin D that will be tested in the dosing study are 500, 1000, and 1500 IU/day. One of these doses will be used in the trial.
Outcomes	Maternal: weight gain during pregnancy, fasting plasma glucose, HbA1c, fasting C peptide, leptin, triglycerides, free fatty acids, high density lipoprotein cholesterol (HDL‐C) and low density lipoprotein cholesterol (LDL‐C), adiponectin 2. 3 beta‐hydroxybutyrate, blood pressure, C‐reactive protein Infant: neonatal growth, adiposity, adipo‐insular axis, glucose‐insulin axis, electrolyte concentrations, clinical outcomes and hypoxia exposure at birth, biparietal diameter, head circumference, abdominal circumference, femur length and determinants of fetal body composition variables (lean body mass and fat body mass, C‐peptide, glucose, leptin, triglycerides, 3‐ beta‐hydroxybutyric acid, pH and erythropoietin, jaundice, hypocalcaemia, hypomagnesaemia)
Notes	Sponsor: European Union (EU) (Belgium) ‐ Seventh Framework Programme (FP7) Moved to awaiting classification from ongoing in last update. Contacted authors in March 2023 to ascertain interventions in trial. Unclear from reporting if both groups received vitamin D. Awaiting response.

Kaur 1991.

Methods	Randomised controlled trial
Participants	50 pregnant women with similar socioeconomic conditions in India
Interventions	Participants were randomised into 2 groups: group 1 (n = 25) received orally 2 pharmacological doses of vitamin D (60,000 IU each) in 6th and 7th month of pregnancy; group 2 (n = 25) did not receive any vitamin supplement and served as controls. Health worker cadre: not specified
Outcomes	Infant: mean birthweight, placental weight and DNA content, total protein and RNA, protein/DNA and RNA/DNA ratios Laboratory method used for assessment of vitamin D concentrations: not applicable
Notes	Total dose of supplementary vitamin D during pregnancy: more than 56,000 to 200,000 IU Start of supplementation: 24 and 28 weeks of pregnancy Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: monthly Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: unknown Source of funding: unknown/unreported Dates of the study and location: dates not reported, India Declarations of interest among primary researchers: not reported Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no information on ethics, no trial dates, does not say how the trial was randomised, exactly the same number in each group with no dropouts, P values recalculated. We reached out to the authors, asking for clarification, twice over a three‐month period. The second time Dr Kaur responded, saying that she was trying to find the information requested and would respond very soon. We have not heard anything else yet from Dr Kaur.

Klar 2020.

Methods	A multicentre randomised controlled trial that examined the influence of a complete nutritional programme on women with risk factors for PE, implemented from the 8th gestational week to delivery
Participants	46 pregnant women with risk factors for PE
Interventions	Intervention group (N = 26) received 8 CNP sessions, nutritional supplementation of calcium and vitamin D, and dietary weight control counselling. The control group (N = 20) received 5 follow‐up sessions with no nutritional counselling.
Outcomes	The collected data included anthropometric measurements, blood pressure, 24‐hour dietary recall, and neonatal outcomes.
Notes	Cannot find full text: queried with Cochrane search team 10 February 2023

Li 2000a.

Methods	Clinical controlled trial with 3 arms
Participants	88 pregnant women with a predisposition to pregnancy‐induced hypertension, at 20 to 24 weeks' gestation, a BMI index of lower than 24, and an arterial pressure of < 11.3 kPa attending an outpatient clinic and labour ward of the First Affiliated Hospital of Xi’an Medical University, Xi’an, China
Interventions	Participants were divided into 3 groups: group 1 (n = 29) received a daily dose of a tablet containing 600 mg of calcium and 200 IU of vitamin D (Caltrate‐D) daily from 20 to 24 weeks until delivery; group 2 (n = 29) received 1200 mg of calcium and 400 IU vitamin D (Caltrate‐D) daily from 20 to 24 weeks until delivery; group 3 (n = 30) received no intervention from 20 to 24 weeks until delivery. Health worker cadre: not specified
Outcomes	Maternal: blood pressure, ionised calcium and platelet intracellular calcium, incidence rates of pregnancy‐induced hypertension Laboratory method used for assessment of vitamin D concentrations: not applicable
Notes	Total dose of supplementary vitamin D during pregnancy: 56,000 IU vitamin D or less Start of supplementation: 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: all year round Source of funding: unknown/unreported Dates of the study and location: August 1996 to December 1998, China Declarations of interest among primary researchers: none declared Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no information on ethics, no information on participant characteristics, does not say how the trial was randomised, no dropouts, no trial dates. We reached out to the authors, asking for clarification, twice over a three‐month period, but have had no response.

Mallet 1986.

Methods	Randomised controlled trial; 3‐arm design with individual randomisation
Participants	77 white pregnant women 18 to 36 years of age in the last trimester of pregnancy living in the Northwest of France (latitude: 49° 26' 0" N north of the Tropic of Cancer). Pre‐gestational BMI not reported.
Interventions	Participants were randomly assigned to 1 of 3 groups: group 1 (n = 21) women received daily 1000 IU of vitamin D (ergocalciferol‐D2) for the last 3 months of pregnancy (estimated total dose throughout pregnancy: 90,000 IU); group 2 (n = 27) women received a single dose of 200,000 IU (5 mg) vitamin D at the 7th month of pregnancy; group 3 (n = 29) women received no supplement and served as controls. Length of the intervention/follow‐up: 12 weeks from start of supplementation to term Health worker cadre: the study was conducted by the maternity research team at Balvedere, Rouen, France but the roles are not described. It is unclear who provided the supplements and measured the outcomes.
Outcomes	Maternal: 24‐hour urinary calcium excretion after 6 weeks supplementation, calcium, 25‐hydroxyvitamin D (25‐OHD) and 1‐alfa,25‐dihydroxyvitamin D (1,25(OH)2D) metabolites of vitamin D from serum and cord during labour and delivery Infant: serum calcium levels at days 2 and 6 of life, birthweight Laboratory method used for assessment of vitamin D concentrations: for 25‐hydroxyvitamin D and 1,25‐dihydroxyvitamin D determinations the following techniques were used: extraction with chloroform‐methanol‐water according to Preece, double step purification, first on a Sephadex LH 20 column with chloroform hexan 45 to 55 vol/vol as solvent, then on a high‐pressure liquid pression system according to Shepard. Plasma metabolites were measured by competitive assay using rat protein for 25 OHD and chicken intestine cytosol for 1,25 (OH)2 D according to Jongen. Assay sensitivity for 1,25 (OH)2 D was 5 pmol/tube and for 25 OHD was 25 pmol/tube.
Notes	Total dose of supplementary vitamin D during pregnancy: more than 56,000 to 200,000 IU Start of supplementation: 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: single/daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): unknown/mixed Latitude: north of the Tropic of Cancer Season at the start of pregnancy: winter pregnancy; infants born during February and March Source of funding: unknown/unreported Dates of the study and location: January 1979 to December 1982, France Declarations of interest among primary researchers: none declared Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no mention of written consent, no information on trial dates, and no information on participant dropouts We reached out to the authors concerning these red flags. But the authors were deceased/long‐term retired, and so unable to answer our queries. Prof Mallet replied, "Prof De Menibus is dead and I am retired until 10 years …So I am unable to answer whith accuracy" (sic).

Marya 1987.

Methods	Randomised controlled trial; 2‐arm design with randomisation at individual level
Participants	400 pregnant women 20 to 35 years of age, attending the antenatal clinic of Medical College Hospital in Rohtak, India (latitude: 76° 34' 0' north of the Tropic of Cancer). Pre‐gestational BMI and skin pigmentation not reported.
Interventions	Participants were allocated to 1 of 2 groups: group 1 (n = 200) received a daily supplement containing 1200 IU vitamin D and 375 mg calcium (estimated total dose from week 20 to 24 of gestation to term: 134,400 to 168,000 IU); group 2 (n = 200) received no supplement from 20 to 24 weeks of pregnancy until delivery and served as controls. Length of the intervention/follow‐up: 20 to 24 weeks from start of supplementation to term Health worker cadre: not specified
Outcomes	Maternal: pre‐eclampsia (defined as blood pressure of 140 mmHg or higher systolic and/or 90 mmHg diastolic along with proteinuria higher than 300 mg/24 hours); systolic and diastolic blood pressure at 24, 28, 32, and 36 weeks of gestation. Serum calcium and creatinine. Laboratory method used for assessment of vitamin D concentrations: not applicable
Notes	Biochemical analyses were made for those who developed pre‐eclampsia (n = 12) and also in a group of women with no pre‐eclampsia (n = 25) and a control group of non‐pregnant women. The results of the stratified analysis are not reported in this review. Total dose of supplementary vitamin D during pregnancy: more than 56,000 to 200,000 IU Start of supplementation: 20 weeks of pregnancy, or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: mixed/unknown Source of funding: unknown/unreported Dates of the study and location: dates not reported, India Declarations of interest among primary researchers: none declared Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no information on ethics, no information on how the trial was randomised, no participant characteristics given, no dropouts reported, no trial dates We reached out to the author, but she was unable to send us clarification on all these issues as she was long‐term retired and the trial took place so long ago. Professor Marya, however, provided very helpful information (that quasi‐randomisation was reported per protocol, rather than intention‐to‐treat, and that incomplete data were used). She wrote: "In late 1980, our team of research workers published a series of reports on the effect of vitamin D supplementation during pregnancy on the mother and the fetus. Since these reports were published more than 30 years ago; I have no documents with me, not even reprints of the said publications. At present, I am over 80 years old. However, I can let you know whatever I can recall. The research projects were cleared by the ethical committee of the medical college hospital. The vitamin supplements are administered by the obstetrician of the team to alternate subject attending the antenatal clinic. The researcher examining the newborn was not aware of supplementation/non‐supplementation of the mother, nor was the one involved in biochemical studies. Only the cases who completed the study were reported in the publications."

Marya 1988.

Methods	Randomised clinical trial; 2‐arm design with individual randomisation
Participants	200 pregnant women, aged 22 to 35 years old, attending the antenatal clinic of the Medical College Hospital, Rohtak, India (latitude: 76° 34' 0' north of the Tropic of Cancer) Inclusion criterion: uncomplicated single pregnancy Exclusion criteria: pre‐eclampsia, antepartum haemorrhage, premature delivery. Pre‐gestational BMI and skin pigmentation not reported.
Interventions	Participants were allocated to 1 of the following groups: group 1 (n = 100) women received 2 doses of 600,000 IU (each dose at 7th and 8th month of pregnancy (estimated total dose: 1,200,000 IU); group 2 (n = 100) women received no intervention and served as controls. Length of the intervention/follow‐up: 12 weeks from start of supplementation to term Health worker cadre: not specified
Outcomes	Maternal: venous and cord serum calcium, serum proteins, inorganic phosphate, alkaline phosphatase, weight. Radiological examination on women with abnormal biochemistry or osteomalacia symptomatology. Side effects: back age, leg‐pains, general weakness, cramps. Infant: birthweight, LBW, crown‐heel length, head circumference, mid‐arm circumference within 24 hours after birth. Skinfold thickness (triceps and infrascapular). Laboratory method used for assessment of vitamin D concentrations: not applicable
Notes	Total dose of supplementary vitamin D during pregnancy: more than 200,000 IU of vitamin D Start of supplementation: 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: 2 single doses were provided at 7th and 8th months of pregnancy Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: mixed/unknown Source of funding: unknown/unreported Dates of the study and location: dates not reported, India Declarations of interest among primary researchers: none declared Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no information on ethics, no information on how the trial was randomised, no dropouts reported, no trial dates, recalculated P value data was problematic. We reached out to the author, but she was unable to send us clarification on all these issues as she was long‐term retired and the trial took place so long ago. Professor Marya, however, provided very helpful information (that quasi‐randomisation was reported per protocol, rather than intention to treat, and that incomplete data was used). She wrote: "In late 1980, our team of research workers published a series of reports on the effect of vitamin D supplementation during pregnancy on the mother and the fetus. Since these reports were published more than 30 years ago; I have no documents with me, not even reprints of the said publications. At present, I am over 80 years old. However, I can let you know whatever I can recall. The research projects were cleared by the ethical committee of the medical college hospital. The vitamin supplements are administered by the obstetrician of the team to alternate subject attending the antenatal clinic. The researcher examining the newborn was not aware of supplementation/non‐supplementation of the mother, nor was the one involved in biochemical studies. Only the cases who completed the study were reported in the publications."

Mazurkevich 2013.

Methods	Randomised controlled trial
Participants	72 pregnant women with physiological pregnancy aged 18 to 35 years with low alimentary consumption of calcium (< 600 mg/day) who attended Moscow State University of Medicine and Dentistry, Department of Obstetrics and Gynaecology (latitude: 55.7500° N, 37.6167° E)
Interventions	Participants were randomly assigned to 1 of 2 groups: group 1 (n = 43) received 1250 mg of calcium carbonate and 200 IU of vitamin D (cholecalciferol‐D3) from the second pregnancy trimester until term, in 2 takes a day; group 2 (n = 29) did not receive any treatment and served as controls. Health worker cadre: not specified
Outcomes	Maternal: resistance of uterine arteries, resistance of umbilical arteries, uterine‐placental circulation Infant: fetal‐placental circulation, intrauterine growth retardation, assessed by dopplerometry Laboratory method used for assessment of vitamin D concentrations: not applicable
Notes	Total dose of supplementary vitamin D during pregnancy: 56,000 IU or less IU Start of supplementation: 20 weeks of pregnancy, or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: mixed/unknown Source of funding: unknown/unreported Dates of the study and location: dates not reported, Moscow, Russia Declarations of interest among primary researchers: none declared Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: the university is flagged for problematic research on Dissernet; https://biblio.dissernet.org/organization/moskovskiy_gosudarstvennyy_mediko_stomatologicheskiy_universitet_im_a_i_yevdokimova?key=13; no information on baseline participant characteristics; no information on randomisation, dropouts, or trial dates; poster only. We attempted to reach out to the authors to clarify these issues but could not find a contact address.

Mirghafourvand 2013.

Methods	Triple‐blind, randomised, controlled clinical trial
Participants	126 pregnant women, aged 18 to 39 years with gestational age of 25 to 30 weeks referring to Tabriz health centres, Iran in 2013 to 2014
Interventions	Participants were allocated to 3 groups using a randomised block design with block sizes of 3 and 6 with the allocation ratio 1:1:1: group 1 (n = 40) calcium‐vitamin D group (300 mg carbonate calcium plus 1000 units of vitamin D supplements); group 2 (n = 42) vitamin D group (1000 units of vitamin D supplements); and group 3 (n = 42) received placebo. To hide the allocation, each participant received 2 small envelopes, each with enough medicine for 3 weeks, inside a large matte‐coloured envelope of the same shape that was serially numbered. Each participant received 1 pill every day for 42 days. All pills were of the same shape, size, and weight. Health worker cadre: not specified
Outcomes	Maternal: gestational age, mode of delivery based on gestational age, food consumption, in terms of calcium and vitamin D content, pre‐pregnancy BMI, BMI during pregnancy Infant: birthweight, height, head circumference Laboratory method used for assessment of vitamin D concentrations: not specified
Notes	Total dose of supplementary vitamin D during pregnancy: 56,000 to 200,000 IU Start of supplementation: 25 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): most were overweight Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: all year round Source of funding: this study was funded by a research grant from Tabriz University of Medical Sciences (Project number: 388). Dates of the study and location: July 2013 to April 2014, Iran Declarations of interest among primary researchers: no conflicts of interest Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no information on ethical supervisory body, unclear whether this is a write‐up of just one trial, or more than one. We reached out to the authors, asking for clarification, twice over a three‐month period, but have had no response.

Naghshineh 2016.

Methods	Double‐blind randomised controlled trial
Participants	140 nulliparous pregnant women who had been referred to “Shahid Beheshti” hospital in Isfahan, Iran. Pregnant women at less than 16 weeks' gestation from outpatient clinics at “Shahid Beheshti” hospital were eligible if they did not have any sign of vitamin D deficiency, did not use aspirin and had no diagnosis of chronic hypertension, gestational diabetes, renal disease, or systemic lupus erythematous.
Interventions	Subjects were randomly divided into 2 groups: group 1 (n = 70) received supplementation with 600 IU daily of vitamin D at 16 weeks' gestation until labour; group 2 (n = 70) received daily supplementation free of vitamin D and followed until labour (placebo group). Women were unaware of the treatment allocation. Health worker cadre: women were followed up monthly by a doctor who was blinded to the study groups
Outcomes	Maternal: age and gestational age at delivery, pre‐eclampsia Infant: birthweight Laboratory method used for assessment of vitamin D concentrations: not applicable
Notes	Total dose of supplementary vitamin D during pregnancy: 56,000 IU or less Start of supplementation: less than 20 weeks of pregnancy Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: From May 2012 until January 2012 Source of funding: this study was funded by a research grant. Financial support was provided by the Isfahan University of Medical Sciences (Grant 392004); Isfahan, Iran. Dates of the study and location: May 2012 to January 2012, Iran Declarations of interest among primary researchers: none declared Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flag: no information on clinical trial registration. We attempted to reach out to the authors to clarify this issue but were unable to contact them.

NCT03743922 (first received 2018 Nov 16).

Methods	Randomised controlled trial
Participants	Target: 60 pregnant women Inclusion criteria: Women who are pregnant and ANC at Rajavithi hospital Aged at least 18 years Thai nationality Gestational age under 20 weeks Exclusion criteria: Do not plan to be delivered at Rajavithi hospital Complications of pregnancy Delivered before gestational age 37 weeks Abnormal kidney function Used anticonvulsant drug or others form of vitamin D Vitamin D allergy
Interventions	Intervention: participants will receive oral calciferol 20,000 IU per week during pregnancy until delivered Control: participants will receive oral placebo 1 tablet per week during pregnancy until delivered
Outcomes	Unclear
Notes	No published report apparent. No contact details.

NCT04591847 (first received 2020 Oct 19).

Methods	Randomised controlled trial
Participants	Target: 60 pregnant women
Interventions	Intervention: vitamin D2 20,000 IU 1 tablet oral weekly, start at gestational age 18 to 22 weeks until delivery Control: placebo drug (appearance same as vitamin D2) 1 tablet oral weekly, start at gestational age 18 to 22 weeks until delivery
Outcomes	Serum vitamin D levels Pre‐eclampsia Preterm birth APGAR scores Birthweight Infant length
Notes	No published report apparent. No contact details.

Persad 2019.

Methods	Randomised clinical trial
Participants	Pregnant women with 12 to 16 weeks gestation not taking vitamin D3 supplements, without malabsorption disorder, carrying anomalous fetus, or with diabetes mellitus
Interventions	Women were assigned to receive daily vitamin D3 3000 IU or no supplement in addition to prenatal vitamins.
Outcomes	Maternal serum vitamin D (25(OH)D) levels and incidence of gestational diabetes
Notes	Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no information on clinical trial registration, results from posters only. We reached out to the authors, asking for clarification, twice over a three‐month period, but have had no response.

Sabet 2012.

Methods	Randomised, double‐blind trial
Participants	50 pregnant women, in their third trimester, who were scheduled to deliver at Mahdieh Hospital in Tehran
Interventions	Participants were randomly allocated to 1 of 2 groups: group 1 (n = 25) received oral vitamin D3 100,000 IU monthly, 3 times; group 2 (n = 25) control (placebo); until term Health worker cadre: not specified
Outcomes	Maternal: the final maternal 25(OH) serum concentrations at delivery, cord 25(OH) vitamin D concentration serum 25(OH), maternal serum iPTH and cord blood iPTH concentration mean PTH concentration Infant: serum vitamin D lower than 30 ng/mL in newborn infants Laboratory method used for assessment of vitamin D concentrations: serum 25 (OH) D concentrations were measured by EIA using the 25(OH) Vit D kit (Immune Diagnostic System Ltd, Bolden, UK)
Notes	Total dose of supplementary vitamin D during pregnancy: more than 200,000 IU Start of supplementation: 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: monthly Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: unknown Source of funding: this study was funded by a research grant from the Research Institute of Endocrine Sciences, Shahid Beheshti University of Medical Sciences. Dates of the study and location: 2009 to 2010, Tehran, Iran Declarations of interest among primary researchers: no conflict of interest declared Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: retrospective registration, exactly the same number in each group with no dropouts, and randomisation method not clear. We reached out to the authors, asking for clarification, twice over a three‐month period, but have had no response.

Samimi 2016.

Methods	Prospective, double‐blind, placebo‐controlled trial
Participants	60 primigravida pregnant women, aged 18 to 40 years old, who were at risk for pre‐eclampsia, and lived approximately 20 km or less from the clinic and hospital. Women ‘at‐risk’ for pre‐eclampsia were recognised by laboratory tests including free β‐human chorionic gonadotrophin, inhibin a dimeric, unconjugated oestriol and maternal serum α‐foetoprotein, and haemodynamic assessment of uterine artery Doppler wave form at 16 to 20 weeks of gestation.
Interventions	Participants were randomly allocated into 2 groups: group 1 (n = 30) received 50,000 IU vitamin D3 every 2 weeks plus 1000 mg day 1 calcium supplements (as calcium carbonate); group 2 (n = 30) received placebos at the same times; from 20 to 32 weeks of gestation. Health worker cadre: an investigator with no clinical involvement in the present study packed cholecalciferol, calcium supplements, and placebos into numbered bottles based on the random list. Anthropometric measurements of pregnant women at the maternity clinic were measured by a trained midwife at baseline and then after 12 weeks of intervention.
Outcomes	Maternal: serum 25(OH)D concentrations, fasting plasma glucose, serum insulin concentrations, homeostasis model assessment (HOMA)‐B, inQUICKI score, serum HDL‐cholesterol, plasma GSH concentrations, systolic blood pressure, diastolic blood pressure, lipid profiles and inflammatory markers, pre‐eclampsia Infant: LBW (< 2500 g), newborn’s birth size (newborn’s weight, length, and head circumference) and prevalence of preterm delivery (< 37 weeks) Laboratory method used for assessment of vitamin D concentrations: serum 25‐hydroxyvitamin D concentrations was determined using a commercial enzyme‐linked immunosorbent assay (ELISA) kit (IDS, Boldon, UK) with inter‐ and intra‐assay coefficients of variation (CVs) of 4.5–7.0%, respectively.
Notes	Total dose of supplementary vitamin D during pregnancy: more than 200,000 IU of vitamin D Start of supplementation: 20 weeks of pregnancy, or more Pre‐gestational BMI (kg/m2): mixed Supplementation scheme/regimen: bi‐weekly Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: unknown Source of funding: the study was supported by a research grant from Kashan University of Medical Sciences. Dates of the study and location: September 2014 to February 2015, Kashan, Iran Declarations of interest among primary researchers: the authors declare that there are no conflicts of interest. Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: one of the co‐authors of this paper is Dr Zatollah Asemi, whose work is under investigation for trustworthiness concerns. https://retractionwatch.com/2020/11/10/journals‐flag‐concerns‐in‐three‐dozen‐papers‐by‐nutrition‐researchers/. Dr Samimi has had two publishers' expressions of concern over papers for which he is the lead author http://retractiondatabase.org/RetractionSearch.aspx#?auth%3dSamimi%252c%2bM. We reached out to the authors and to Dr Samimi's university, asking for clarification, twice over a three‐month period, but have had no response.

Samimi 2017.

Methods	Double‐blind, randomised, controlled clinical trial
Participants	80 women aged 18 to 35 years were examined from November 2013 to March 2015 at the Shabihkhani Maternity Hospital in Kashan, Iran.
Interventions	Participants were randomised into 2 groups: group 1 (n = 40) received a pill of vitamin D3 400 IU/day and Group 2 (n = 40) received a placebo tablet that was similar to vitamin D3, with no active ingredient. Both groups received standard treatment with vaginal progesterone (Behvarzan, Iran) at a dose of 400 mg per day. The serum levels of vitamin D3 were evaluated in the 10th and 20th weeks to prevent any possible poisoning. If so, the patient was excluded from the study. Health worker cadre: all participants in the study received antenatal care and were given folic acid and ferrous sulphate at least 1 month prior to pregnancy, under the supervision of a gynaecologist. They were checked by monitoring serum β‐hCG level levels and abdominal ultrasound until the confirmation of pregnancy, after which the mothers were divided into 2 groups of intervention and control using permuted block randomisation with 20 blocks of size 4. Only the person responsible for the distribution of drugs knew how the women were allocated to the treatment groups.
Outcomes	Maternal: the serum level of vitamin D3, serum level of IL‐23, serum levels of vitamin D3 and IL‐23, spontaneous abortion Laboratory method used for assessment of vitamin D concentrations: not specified
Notes	Total dose of supplementary vitamin D during pregnancy: more than 200,000 IU of vitamin D Start of supplementation: less than 20 weeks of pregnancy Pre‐gestational BMI (kg/m2): unknown Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: unknown Source of funding: this study was funded by a research grant from the Kashan University of Medical Sciences. Dates of the study and location: November 2013 to March 2015, Kashan, Iran Declarations of interest among primary researchers: the authors declare that there is no conflict of interests regarding the publication of this paper. Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: Dr Samimi has had two publishers' expressions of concern over papers for which he is the lead author, http://retractiondatabase.org/RetractionSearch.aspx#?auth%3dSamimi%252c%2bM. We reached out to the authors and to Dr Samimi's university, asking for clarification, twice over a three‐month period, but have no response.

Sasan 2017.

Methods	Randomised, controlled clinical trial.
Participants	142 women who were referred to the obstetrical clinic in Besat Hospital of Sanandaj City, Kurdistan Province, Iran, who were receiving prenatal care and had a history of pre‐eclampsia in previous pregnancies
Interventions	The participants were randomly placed into 2 groups: group 1 (n = 70) received 50,000 IU pearl vitamin D3 once every 2 weeks; group 2 (n = 72) received placebo. Vitamin D or placebo was given until the 36th week of pregnancy. Health worker cadre: not specified
Outcomes	Maternal: level of vitamin D, pre‐eclampsia Laboratory method used for assessment of vitamin D concentrations: level of vitamin D was determined through Liebermann–Burchard method
Notes	Total dose of supplementary vitamin D during pregnancy: more than 56,000 and less than 20,0000 IU of vitamin D Start of supplementation: 10 weeks of pregnancy, or more Pre‐gestational BMI (kg/m2): mixed Supplementation scheme/regimen: once every 2 weeks Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: unknown Source of funding: unknown/unreported Dates of the study and location: dates not reported, Iran Declarations of interest among primary researchers: the authors announce that there are no conflicts of interest between different individuals and organisations involved in the study. Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: the trial registration date overlaps with when the paper was submitted to the publishing journal, no information on the supervisory ethical body, no information on randomisation, any participant dropouts, or trial dates. We were unable to reach out to the authors to ask for clarification as neither we nor the local Cochrane office could find a contact address.

Shahgheibi 2016.

Methods	Randomised, double‐blind, placebo‐controlled trial
Participants	90 pregnant women with at least 1 risk factor for gestational diabetes including BMI (kg/m2) more than 25, history of macrosomic neonate, positive family history for diabetes and gestational diabetes, history of gestational diabetes in previous pregnancies, and glycosuria
Interventions	Participants were randomised into 1 of 2 groups: group 1 (n = 46) received 5000 units of vitamin D weekly; group 2 (n = 44) received placebo. Both groups were treated until the 26th week of pregnancy. Then the glucose challenge test (GCT) and the glucose tolerance test (GTT) were performed to evaluate GDM. Health worker cadre: not specified
Outcomes	Maternal: vitamin D levels and GCT, incidence of diabetes Laboratory method used for assessment of vitamin D concentrations: vitamin D level was determined in a laboratory by the Liebermann–Burchard method, in which the patient should fast for 12 hours and not have a fatty dinner.
Notes	Total dose of supplementary vitamin D during pregnancy: less than 56,000 IU Start of supplementation: enrolled until 26 weeks of pregnancy Pre‐gestational BMI (kg/m2): > 25 Supplementation scheme/regimen: weekly Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: North of the Tropic of Cancer Season at the start of pregnancy: unknown Source of funding: no funding Dates of the study and location: 2013, Iran Declarations of interest among primary researchers: the authors have no conflicts of interest. Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flag: we could find neither the registration number given in the paper (ICRT 14140742), nor the trial itself, on any trial registration database. We reached out to the authors, asking for clarification, twice over a three‐month period, but have had no response.

Singh 2015.

Methods	Randomised controlled trial
Participants	100 healthy, pregnant women, primigravida with a singleton pregnancy, gestational age 12 to 16 weeks in Sawangi, Meghe, Wardha
Interventions	Participants were randomised into 2 groups: group 1 (n = 50): received 2000 IU of vitamin D3 per day from 12 to 16 weeks gestation of pregnancy; group 2 (n = 50) received no supplementation and served as controls. 25‐hydroxyvitamin D (25(OH)D) in maternal blood was measured by chemiluminescence immunoassay, at recruitment and at the time of delivery, and a serum 25(OH)D level lower than 30 nmol/L was defined as deficiency. Health worker cadre: not specified
Outcomes	Maternal: deficiency of vitamin D, mean gestational age, preterm birth Laboratory method used for assessment of vitamin D concentrations: mean serum 25(OH)D levels were measured by Roche diagnostic ELECSYS (electrochemiluminescence immunoassay) 2010 Cobase E 411 Analyser Immunoassay System Germany.
Notes	Total dose of supplementary vitamin D during pregnancy: 56,000 IU Start of supplementation: 12 to 16 weeks of pregnancy Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: all year, from October 2012 to September 2014 Source of funding: no funding sources reported Dates of the study and location: October 2012 to September 2014, India Declarations of interest among primary researchers: none declared Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no clinical trial registration; no mention of consent, no participant dropouts (mentioned, but no flowchart; exactly 50 women in each group recruited, and no information on the numbers at the end), randomisation method not clear, trial finished September 2014, but paper submitted for publication November 2014. We reached out to the authors, asking for clarification, twice over a three‐month period, but have had no response.

Sircar 2021.

Methods	Randomised, double‐blind, interventional study
Participants	178 antenatal women with biochemically diagnosed vitamin D deficiency having at least one risk factor for gestational diabetes mellitus (BMI > 25 kg/m2, history of macrosome neonate, positive family history for diabetes, history of GDM in previous pregnancies, glycosuria in morning urine sample) with gestational age 13 to 24 weeks
Interventions	Control group (n = 92): multi‐vitamin and mineral supplement (iron, calcium, and vitamin B, C) supplement from second trimester until delivery Intervention group (n = 86): oral cholecalciferol sachet 60,000 IU per week until 26th week of gestation + multi‐vitamin and mineral supplement from second trimester until delivery
Outcomes	Incidence of gestational diabetes mellitus
Notes	Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: abstract only. We were unable to reach out to the authors to ask for full information on the trial, as neither we nor the local Cochrane office could find a contact address.

Taherian 2002.

Methods	Randomised, controlled study with 3 arms
Participants	990 nulliparous women attending antenatal outpatient clinics of Isfahan Health Centers (32.6333° N, 51.6500° E north of the Tropic of Cancer) between April 1998 and March 2001, with singleton pregnancies, first prenatal visit before 20 weeks of gestation, systolic/diastolic blood pressure lower than 130/80 mmHg, and no proteinuria detectable by a dipstick. Women with a history of cardiovascular, renal, or endocrinologic problems, medical or obstetric complications, and those with known hazardous conditions (multifetal gestation, hydatidiform mole) were excluded.
Interventions	Participants were randomly assigned to 1 of 3 groups: group 1 (n = 330) received 75 mg aspirin each day from 20th week of gestation until delivery; group 2 (n = 330) received a tablet containing 500 mg calcium carbonate + 200 IU vitamin D (cholecalciferol‐D3) daily from 20th week of gestation until delivery; and group 3 received no intervention (n = 330). All cases received standard prenatal care. Health worker cadre: the women were examined by trained staff every 4 weeks through the 28 weeks of gestation, and every 2 weeks through the 36th week and weekly thereafter. Blood pressure was measured by a certified examiner.
Outcomes	Maternal: blood pressure, bodyweight, BMI, maternal height, urine protein measurements, maternal weight gain, duration of gestation Infant: neonatal weight at birth, the presence of respiratory distress syndrome, sepsis, jaundice and intrauterine growth retardation, fetal or neonatal death Laboratory method used for assessment of vitamin D concentrations: not applicable
Notes	Total dose of supplementary vitamin D during pregnancy: less than 56,000 IU Start of supplementation: 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): normal weight (18.5 to 24.9) Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: April 1998 to March 2001 Source of funding: this study was funded by a research grant of the Research Deputy of Isfahan University of Medical Sciences (No: 76085). Dates of the study and location: April 1998 to March 2001, Iran Declarations of interest among primary researchers: none declared Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: no mention of ethical supervisory body or consent, exactly the same number (330) in each group with no dropouts, the sampling method was 'non‐probablility convenience'. We reached out to the authors, asking for clarification, twice over a three‐month period, but have had no response.

Tehrani 2014.

Methods	Single‐arm study, not blinded
Participants	210 pregnant women referring to the obstetric clinic of Shahid Beheshti and Alzahra Hospital in Esfahan city in 2012 Inclusion criteria: patient satisfaction; normal BMI; gestational age below 16 weeks; no history of diabetes mellitus type 2 or GDM; no family history of diabetes mellitus type 1 in first degree relatives Exclusion criteria: patient dissatisfaction; incorrect consumption of vitamin D supplementation; follow‐up discontinuation
Interventions	Participants individually randomised to 1 of 2 groups: group 1 (n = 70) received vitamin D supplementation with dose of 50,000 unit every 2 weeks for 10 weeks; group 2 received a placebo. Pregnant women with levels of above 25 nmol/L were selected as the normal healthy control group and were the ones who received placebo. Health worker cadre: not specified
Outcomes	Maternal: gestational blood sugar level, serum vitamin D level Laboratory method used for assessment of vitamin D concentrations: not specified
Notes	Total dose of supplementary vitamin D during pregnancy: more than 200,000 IU Start of supplementation: 20 weeks of pregnancy or more Pre‐gestational BMI (kg/m2): normal BMI Supplementation scheme/regimen: bi‐weekly Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: north of the Tropic of Cancer Season at the start of pregnancy: unknown Source of funding: this study was funded by a non‐governmental organisation; Sponsor: Isfahan University of Medical Sciences Dates of the study and location: January 2013 to January 2014, Iran Declarations of interest among primary researchers: there are no conflicts of interest. Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: retrospective registration, no information on ethical oversight or consent, no information on baseline participant characteristics, and exactly the same number in each group with no dropouts. We attempted to reach out to the authors to clarify these issues but were unable to contact them.

Vaziri 2016.

Methods	Randomised clinical trial
Participants	169 pregnant women, both nulliparous and multiparous, aged 18 years or older, no history of mental illness and internal diseases such as hyper/hypothyroidism, no addiction to any kind of narcotic drugs or alcohol, not divorced or widowed, no pregnancy complications such as pre‐eclampsia, gestational diabetes, ruptured membranes and suspicion of preterm birth, no previous caesarean sections, with a live fetus singleton pregnancy, and gestational age of 26 to 28 weeks based on ultrasound results, who were under prenatal care in Hafez teaching hospital in Shiraz, Iran
Interventions	Participants were assigned to 2 groups through block randomisation design: group 1 (n = 78) received 2000 IU of vitamin D3 per day; group 2 (n = 75) received placebo. Both groups received their assigned treatments from 26 to 28 weeks of gestation until childbirth. Maternal serum 25‐hydroxyvitamin D concentrations were measured at baseline and childbirth. In addition, depression scores were evaluated 4 times: at 26 to 28 and 38 to 40 weeks of gestation, and finally at 4 and 8 weeks after birth. Participants were allowed to use prescribed supplementation outside this study’s protocol. Usually, pregnant women in Iran are prescribed iron and folic acid, which may be consumed as part of the multivitamin supplementation with 200 to 400 IU of vitamin D. They reported that 55 participants in the intervention and 69 in the control group used other supplements, but they did not provide the composition of the supplements. Health worker cadre: at first, a research team member who was responsible for data collection visited the prenatal care clinic of the hospital daily and, based on the inclusion criteria, invited the mothers to participate in the study. The consumption of pills was assessed in later prenatal care visits and over the phone.
Outcomes	Maternal: baseline 25‐hydroxy vitamin D concentrations and at childbirth, depression score Infant: vitamin D concentrations, anthropometric measurements of their infants at birth, 4th and 8th weeks of birth Laboratory method used for assessment of vitamin D concentrations: serum 25‐hydroxyvitamin D was measured with the chemiluminescence immunoassay (CLIA) method
Notes	Total dose of supplementary vitamin D during pregnancy: less than 56,000 IU Start of supplementation: 26 to 28 weeks of pregnancy Pre‐gestational BMI (kg/m2): unknown/mixed Supplementation scheme/regimen: daily Skin pigmentation based on Fitzpatrick skin tone chart (Fitzpatrick 1988): mixed/unknown Latitude: North of the Tropic of Cancer Season at the start of pregnancy: all year, from November 2014 to October 2015 Source of funding: the study was financially supported by a research grant from the Research Vice‐chancellor of Shiraz University of Medical Sciences. Dates of the study and location: November 2014 to October 2015, Iran Declarations of interest among primary researchers: the authors declare that they have no competing interests. Not included in 2024 update because of trustworthiness red flags discovered by using the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool (Weeks 2023). Trustworthiness red flags: the two papers each give different trial registration numbers, protocols appear different, and neither paper mentions the other, both retrospectively registered, no mention of ethical oversight or consent, no trial dates given, no participant dropouts, and no information on randomisation. We reached out to the authors, asking for clarification, twice over a three‐month period, but have had no response.

25(OH)D: 25‐hydroxycholecalciferol
β‐hCG: beta human chorionic gonadotropin
ANC: antenatal care
BMI: body mass index
CNP: complete nutritional programme
CRL: crown‐rump length 
d: day
ELISA: enzyme‐linked immunosorbent assay
GCT: glucose challenge test
GDM: gestational diabetes mellitus
HDL: high‐density lipoprotein
HL: humeral length
iPTH: intact parathyroid hormone
IU: international units
IUFD: intrauterine fetal death
LBW: low birthweight 
LDL: low‐density lipoprotein
MSD: midshaft diameter 
PA: physical activity
PE: pre‐eclampsia
PG: placebo group
PMD: proximal metaphyseal diameter 
PTH: parathyroid hormone
RBC: red blood cell
SLE: systemic lupus erythematosus
SMS: distal metaphyseal diameter
TG: treatment group
VD: vitamin D

Characteristics of ongoing studies [ordered by study ID]

Baird 2016.

Study name	Southampton PRegnancy Intervention for the Next Generation (SPRING): protocol for a randomised controlled trial
Methods	Randomised controlled trial that uses a 2‐by‐2 factorial design
Participants	600 women, with less than 17 weeks' gestation at recruitment based on LMP, aged over 18 years, with a singleton pregnancy and aiming to give birth at local maternity (Princess Anne) hospital
Interventions	Healthy conversation skills support plus vitamin D supplementation (1000 IU cholecalciferol) (n = 150); healthy conversation skills support plus placebo (n = 150); usual care plus vitamin D supplementation (n = 150); usual care plus placebo (n = 150)
Outcomes	This trial is evaluating 2 approaches to improving maternal diet: a behaviour change intervention and vitamin D supplementation. The factorial design of this trial has the advantage of enabling each intervention be tested separately as well as allowing exploration of the synergistic effect of both interventions on women’s diets and vitamin D levels.
Starting date	Registered on 13 September 2013
Contact information	Janis Baird MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton SO16 6YD, UK jb@mrc.soton.ac.uk
Notes	Sponsor: Medical Research Council, NIHR Southampton Nutrition Biomedical Research Centre and Danone Nutricia Early Life Nutrition Contacted authors March 2023. Awaiting publication.

ChiCTR2000032488 (first received 2020).

Study name	Yunxian 2020
Methods	RCT
Participants	Target size: 1600 women Inclusion criteria: First visit in gestational week 8 to 14 Age: 18 to 45 years Pregnant women who are at high risk of gestational diabetes mellitus (pregnant women who meet any of the following conditions defined as having high risk of gestational diabetes): Pre‐pregnancy or early pregnancy BMI ≥ 24 kg/m2 Family history of diabetes or gestational diabetes Gestational diabetes diagnosed in previous pregnancies A history of childbirth of macrosomia (≥ 4000 g) Glycosylated haemoglobin 5.7% to 6.4% or fasting blood glucose 6.1 to 7.0 mmol/L Agree to participate in the programme and sign the informed consent form Exclusion criteria: Patients with a history of diabetes or diagnosis of diabetes (including type 1 and type 2 diabetes) Glycosylated haemoglobin ≥ 6.5% or fasting blood glucose > 7.0 mmol/L accompanied by obvious symptoms of diabetes Daily vitamin D intake > 800 IU Severe chronic disease (heart, lung, liver, kidney insufficiency, etc.) or mental illness Present or past abnormalities in calcium metabolism (hypoparathyroidism/hyperthyroidism, kidney stones), hyperuria or hypercalcaemia Abortion history (natural/artificial), multiple births or artificial reproduction Require a complex diet Unable to use Mandarin fluently Participating in other clinical trials or researchers believe that other reasons are not suitable for clinical trials Cannot walk 100 metres safely
Interventions	Intervention group: 800 IU vitamin D3, 2 pills per day Control group: placebo 2 pills per day
Outcomes	GDM Birthweight Birth length
Starting date	2020
Contact information	Yu Yunxian C205 Research Building, School of Public Health, Zhejiang University, 388 Yu‐Hang‐Tang Road, Xihu District, Hanghou, Zhejiang, China 310058 yunxianyu@zju.edu.cn
Notes	—

IRCT20190814044529N1 (first received 2020).

Study name	Effect of vitamin D intake on maternal of delivery on retinopathy of prematurity in infants
Methods	A concealed, randomised, blinded, controlled clinical trial with a parallel‐group design
Participants	93 women below 34 weeks gestation
Interventions	The intervention group received 50,000 units of vitamin D injections 72 hours before birth The control group received no medication
Outcomes	Retinopathy of prematurity
Starting date	May 2022
Contact information	Hassan Boskabadi Mashhad University of Medical Sciences Email address: boskabadih@mums.ac.ir
Notes	—

ISRCTN46539495 (first received 2020).

Study name	Prevention of pre‐eclampsia through vitamin D supplementation: a single‐blinded randomized clinical trial without placebo
Methods	Randomised controlled trial
Participants	1232 primiparous women
Interventions	The supplement group takes vitamin D by mouth as one dose per month from the 3rd to the 8th month of pregnancy. The control group will be followed in the antenatal clinic and will receive an iron supplement and anti‐parasitic treatment.
Outcomes	Pre‐eclampsia
Starting date	May 2017
Contact information	Dr Richard Kabuyanga Kabuseba Email: docrica@hotmail.com; richard.kabuseba@unigom.ac.cd University of Lubumbashi (Congo, Democratic Republic)
Notes	Awaiting publication

ISRCTN87262826 (first received 2022).

Study name	Differences in the effect of sun exposure and vitamin D supplementation on blood pressure of pregnant women and anthropometric status of newborn babies
Methods	Randomised controlled trial
Participants	Pregnant women below 20 weeks' gestation Target: 108
Interventions	Group 1 receives standard therapy (calcium, Fe) plus sun exposure 3 times every week for 15 to 30 minutes Group 2 receives standard therapy (calcium, Fe) plus vitamin D supplementation 1000 IU/day Group 3 (control group) receives standard therapy (calcium, Fe)
Outcomes	Primary outcome measure: 25(OH)D measured using ELFA method before and after intervention Secondary outcome measures: Blood pressure measured using a tensimeter before intervention and every 3 weeks Anthropometry of the newborn measured using a weighing scale and microtoise at delivery
Starting date	March 2021
Contact information	Rita Sunarno Public Health Diponegoro University, Prof. Soedarto Street, No. 1269, Tembalang, Semarang, 50275, Indonesia +62 8155614633 sunarno_rita@yahoo.co.id
Notes	—

NCT05208827 (first received 2022).

Study name	A multicenter randomized controlled study of vitamin D supplementation in pregnant women for the prevention of gestational diabetes
Methods	Randomised controlled trial
Participants	Pregnant women who are 8 to 14 weeks pregnant; 18 to 45 years old; at high risk of GDM (meeting any of the following conditions): 1) BMI ≥ 24 kg/m2 before or during pregnancy; 2) family history of diabetes or GDM; 3) previous pregnancy diagnosed as GDM; 4) history of macrosomia (≥ 4000 g); 5) history of abnormal glucose tolerance: 5.7% to 6.4% HbA1c or 6.1 to 7.0 mmol/L fasting blood glucose
Interventions	Intervention group: 2 tablets once a day. Each tablet contains 800 units of vitamin D3, for a total of 1600 units taken orally daily. Control group: the same packaged, similar‐looking, similar‐tasting placebo from the same manufacturer, containing starch, peanut oil (no pharmaceutical value)
Outcomes	Gestational diabetes Newborn birth weight Gestational age Weight gain during pregnancy
Starting date	January 2022
Contact information	Luyang Han, doctor Women's Hospital School of Medicine Zhejiang University Email: 524519232@163.com
Notes	—

NCT05329428 (first received 2022).

Study name	PREDIN: Pregnancy and Vitamin D Intervention Study (PREDIN)
Methods	Randomised clinical trial
Participants	102 women below 15 weeks gestation
Interventions	Intervention 1: vitamin D supplementation 20 µg/day Dietary supplements containing 20 µg of vitamin D per day will be provided to study participants. Intervention 2: Vitamin D supplementation 40 µg/day Dietary supplements containing 40 µg of vitamin D per day will be provided to study participants. Control: usual antenatal care Women randomised to usual antenatal care will receive advice about vitamin D supplementation according to usual antenatal care routines (< 15 weeks).
Outcomes	Vitamin D levels
Starting date	December 2023
Contact information	Hanna Augustin, Assistant Prof Göteborg University
Notes	—

BMI: body mass index
DRI: dietary reference intakes
GDM: gestational diabetes mellitus
HTN: hypertension
IU: international units
LMP: last menstrual period
μg: microgram
PA: physical activity
PCO: polycystic ovary

Differences between protocol and review

In comparison with the protocol, this update as well as the previous updates had the following differences:

• Types of participants: pregnant women with pre‐existing conditions were excluded.

• Use of the Cochrane Pregnancy and Childbirth Trustworthiness Screening Tool to assess the scientific integrity and trustworthiness of the studies (Weeks 2023).

• Types of outcome measures: we moved “maternal vitamin D concentrations at the end of pregnancy” and "Adverse events (e.g. hypercalcaemia, kidney stones, etc.)" from secondary to primary outcomes.

• Unit of analysis issues: we had originally intended to include randomised cross‐over trials (their first period), but we decided not to include them as this type of study design is considered inappropriate for the topic under investigation.

• Subgroup analysis and investigation of heterogeneity: we had originally sought to conduct subgroup analysis by total dose of supplementary vitamin D during pregnancy but due to the large variability in regimens (dose, frequency, start), we removed this.

• New authors: Jo Weeks and Anna Cuthbert were added as new authors in this revised update.

Contributions of authors

Lia L Kostiuk, Anna Cuthbert, and Jo Weeks assessed the eligibility of the new trials and extracted the data in duplicate. Any differences were discussed and resolved with Cristina Palacios.

All authors contributed to the preparation of the updated review.

Sources of support

Internal sources

• None, Other

  None

External sources

• Children's Investment Foundation Fund (CIFF) Project, UK

  The Children's Investment Foundation Fund (CIFF) provided funding to the University of Liverpool. This funding provided support for Jo Weeks and Anna Cuthbert to work on this review update.

Declarations of interest

Cristina Palacios: none known.

Lia L Kostiuk: none known

Anna Cuthbert was previously employed by Cochrane Pregnancy and Childbirth as a research associate. She was then employed by the University of Liverpool for a project that included the work on this review. The project was funded by the Children’s Investment Foundation Fund (CIFF); however, the views expressed are those of the authors and not of CIFF. Anna Cuthbert has no known conflicts of interest to declare.

Jo Weeks was employed on a consultancy basis by the University of Liverpool for a project that included the work on this review. The project was funded by the Children’s Investment Foundation Fund (CIFF); however, the views expressed are those of the authors and not of CIFF. Jo Weeks has no known conflicts of interest to declare.

New search for studies and content updated (conclusions changed)