Table 1.

Description of included studies.

Study	Design	Recruitment	Sample	Serum Vitamin D Measurement	Criteria for Vitamin D Deficiency and Reported Prevalence	Outcomes	Risk Factors
Ajmani et al., 2016 India [24]	Prospective cohort	Approached/screened: not reported Enrolled: not reported Complete data: n = 200 burka-clad women Setting: antenatal clinic and inpatients in antenatal ward at Kasturba Hospital, Delhi Season: all (year-long)	Age: mean 24.8 years Demographics: 64.5% multigravida, 44% dark complexion, 36% low SES, 64% illiterate, 2.5% graduate level educated Trimester: not reported Exclusion criteria: non-burka clad, age <18 years or >40 years, history of liver/renal disease, osteoporosis or rheumatoid arthritis, antitubercular or antiepileptic treatment in last 6 months, taking vitamin D supplements	ELISA	Criteria *: Deficiency < 50 nmol/L Inadequacy 50–75 nmol/L Adequate > 75 nmol/L Prevalence of deficiency: Deficient n = 75 (37.5%) Inadequate n = 78 (39%) Adequate n = 47 (23.5%)	Maternal: 7.5% women diagnosed with pre-eclampsia, significant correlation between VDD and pre-eclampsia (p = 0.001) Neonatal: 9.5% LBW babies, significant correlation between VDD and LBW (p = 0.0001) No correlation: VDD and GDM, LSCS, bony abnormality, Apgar score, premature birth or NICU admission	Dark skin complexion, limited outdoor activity, low dairy intake, low fish intake (p < 0.05)
Ates et al., 2016 Turkey [37]	Prospective cohort	Approached/screened: n = 286 Enrolled: n = 266 (93%) Complete data: n = 229 (86%) Setting: first antenatal appointment at outpatient clinic of Obstetrics and Gynecology, Bezmialem Vakif University, Istanbul Season: 48.9% summer (May–October), 51.1% winter (November–April)	Age: mean 29.5 years Demographics: 64.5% primigravida, mean BMI 25.3 kg/m2, 61.3% covered dress, 63.1% multivitamin use, 6.6% smoking, 46.9% ≥ 9 years education Trimester: first Exclusion criteria: thyroid, parathyroid or adrenal disease, hepatic or renal failure, metabolic bone disease, medication affecting vitamin D metabolism, multiple pregnancy, taking vitamin D supplements	LC-MS/MS	Criteria *: Severe deficiency < 25 nmol/L Mod deficiency 25–47.5 nmol/L Mild deficiency 50–72.5 nmol/L Adequate > 75 nmol/L Prevalence of deficiency: Severe n = 105 (45.9%) Mod n = 83 (36.2%) Mild n = 31 (13.5%) Adequate n = 10 (4.4%)	Maternal: 53.8% women with severe VDD had vaginal delivery, compared with 32.7% as primary caesarean section (p = 0.018) No correlation: VDD and GDM, pre-eclampsia, gestational hypertension, preterm birth, SGA, intrauterine fetal death, congenital malformation, birth weight or Apgar score	Covered dress, non-multivitamin use, winter (p < 0.05)
Aydogmus et al., 2014 Turkey [38]	Prospective cohort	Approached/screened: n = 180 Enrolled: n = 152 (84%) Complete data: n = 148 (97%) Setting: inpatients at Izmir Katip Celebi University Ataturk Training and Research Hospital, Izmir Season: not reported	Age: mean 24.4 years Demographics: not reported Trimester: third Exclusion criteria: taking vitamin D supplements, multiparity, disease affecting vitamin D and calcium metabolism, medications for chronic disease	ELISA	Criteria *: Deficient < 37.5 nmol/L Insufficient 37.5–72.5 nmol/L Sufficient > 75 nmol/L (grouped for analysis Deficient < 37.5 nmol/L; Other ≥ 37.5 nmol/L) Prevalence of deficiency: Deficient: n = 66 (44.6%) Other n = 82 (55.4%)	Maternal: 39.9% women with VDD had poor pregnancy outcomes compared with 23.2% of women without VDD (p = 0.001), VDD increased risk of perinatal complications (OR 4.30; 95% CI 1.85–9.99) Neonatal: 16.7% SGA neonates born to mothers with VDD compared with 4.9% neonates born to mothers without VDD (p = 0.007), VDD increased risk of SGA (OR 4.5; 95% CI 1.41–15.78); mean birthweight significantly lower for neonates born to mothers with VDD (3187.6 ± 495.5 g) compared with those born to mothers without VDD (3268.1 ± 477.1 g) (p = 0.02) No correlation: VDD and mode of delivery, post maturity, GDM, maternal anemia, hypertension, pre-eclampsia, cholestasis, oligohydraminos, fetal distress, still birth, preterm labor, PPROM, Apgar scores, prolonged hospitalization, mortality, NICU admission or macrosomia	No significant associations
Chen et al., 2015 China [35]	Prospective cohort	Approached/screened: n = 4358 (sub-sample of a population-based cohort study n = 16,766) Enrolled: n = 3658 (84%) Complete data: 3658 (100%) Setting: women recruited to the larger China-Anhui Birth Cohort study from six major cities of Anhui province Season: all (year-long), 36.7% spring, 22.5% summer, 20.6% autumn, 20.2% winter	Age: mean 27.5 years Demographics: 96.0% nulliparous, 45.2% low income, 75.3% healthy BMI, 16.5% multivitamin use Trimester: all, 35.1% first, 62.0% second, 2.9% third Exclusion criteria: multiple pregnancy, abortion	RIA	Criteria *: Deficient < 50 nmol/L Insufficient 50–74.75 nmol/L Sufficient ≥ 75 nmol/L Prevalence of deficiency: Deficiency n = 1405 (38.4%) Insufficiency n = 1289 (35.2%) Sufficient n = 964 (26.4%)	Maternal: not assessed Neonatal: 16.01% SGA neonates born to mothers with VDD compared with 5.59% born to mothers with vitamin D insufficiency and 2.80% with sufficient vitamin D (p < 0.001); compared to sufficiency, maternal VDD increased risk of SGA (RR 6.47; 95% CI 4.30–9.75) and insufficiency (RR 2.01; 95% CI 1.28–3.16) (p < 0.001); 4.98% LBW neonates born to mothers with VDD compared with 1.32% born to mothers with vitamin D insufficiency and 0.41% with sufficient vitamin D (p < 0.001); VDD increased risk of LBW (RR 12.31; 95% CI 4.47–33.89) (p < 0.001). Adjusted for: Pre-pregnancy maternal BMI, maternal age, season and gestational week	Not assessed
Farrant et al., 2009 India [34]	Prospective cohort	Approached/screened: n = 1539 Enrolled: n = 830 (54%) Complete data: n = 674 (81%) Setting: women attending antenatal clinic at Holdsworth Memorial Hospital, Mysore Season: all (year-long)	Age: mean 23.7 years Demographics: mean BMI 23.4 kg/m2, women supplemented at recruitment (n = 156) with vitamin D as part of routine management, no information available at 30 weeks Trimester: third Exclusion criteria: not reported	RIA	Criteria: Hypovitaminosis < 50 nmol/L Adequate > 50 nmol/L Prevalence of deficiency: Hypovitaminosis n = 372 (67%) Adequate: n = 187 (33%)	No correlation: VDD and GDM, birthweight, impaired fetal growth	Autumn/winter (p < 0.05)
Gbadegesin et al., 2016 Nigeria [27]	Prospective cohort	Approached/screened: not reported Enrolled: n = 461 Complete data: n = 461 (100%) Setting: maternity unit of the Lagos State University Teaching Hospital, Ikeja and women of mixed ethnicity, social class and religion Season: all (year-long)	Age: mean 31.3 years Demographics: mean parity 1.16 Trimester: all Exclusion criteria: multiple pregnancy, previous medical condition (hypertension, renal disease, diabetes), taking vitamin D supplements, elevated BP	HPLC	Criteria *: Deficiency < 50 nmol/L Insufficiency 52.5–75 nmol/L Adequate > 75 nmol/L Prevalence of deficiency: Deficiency n = 134 (29.0%) Insufficiency n = 48 (10.4%) Adequate n = 279 (60.6%)	No correlation: VDD and preeclampsia, SROM, anemia, GDM, preterm delivery, mode of delivery, Apgar score or stillbirth	No significant associations
Gur et al., 2014 Turkey [41]	Prospective cohort	Approached/screened: n = 687 Enrolled: n = 208 (30%) Complete data: n = 189 at 1/52 (91%); n = 184 at 6/52 (88%); n = 179 at 6/12 (86%) Setting: women attending routine antenatal reviews at Sifa University Bornova Health Research and Application Hospital, Izmir Season: summer/autumn	Age: mean 28.5 years Demographics: mean BMI 26.5 kg/m2, 7.6% women supplemented with vitamin D ≥ 3 days per week and 84.6% supplemented daily, all women Caucasian and native Turkish speaking Trimester: second Exclusion criteria: unmarried, unplanned pregnancy, BMI < 20 or >30 kg/m2, smoker, diagnosed psychiatric illness, pre-diagnosed medical condition, parity > 3, education level < 8 years, multiple birth, employed, annual income < US $450, fetal death, complex delivery, newborn with anomaly, postpartum bleeding or hysterectomy	ELISA	Criteria *: Severe deficiency < 25 nmol/L Mild deficiency 25 nmol/L–50 nmol/L Normal ≥ 50 nmol/L Prevalence of deficiency: Severe: n = 23 (11%) Mild n = 84 (40.3%) Normal n = 101 (48.5%)	Maternal: 21.1%, 23.2% and 23.7% women had PPD at week 1, 6 and 6 months respectively; significant negative correlation (r = −0.2, −0.2, −0.3) between vitamin D levels and Edinburgh Postnatal Depression Scale (EPDS) score at each of the three time points; mean vitamin D level was significantly different between women with and without PPD at each of the three time points (p = 0.003, p = 0.004 and p < 0.001 respectively) Neonatal: not assessed	Not assessed
Hossain et al., 2010 Pakistan [39]	Cross-sectional	Approached: not reported (all women admitted to the labor suite for delivery during the study period were deemed eligible) RR: not reported Complete data: n = 75 Setting: delivery at Dow University of Health Sciences and Civil Hospital, Karachi Season: spring	Age: mean 26.0 years Demographics: mean BMI 27 kg/m2, mean parity 2.2, 26% covering arms, hands heads, 76% covering face Trimester: third Exclusion criteria: not reported	CI	Criteria *: Severe deficiency < 25 nmol/L Mod deficiency 27.5–50 nmol/L Mild deficiency 52.5–60 nmol/L Adequate > 60 nmol/L Prevalence of deficiency: Severe: n = 34 (45%) Mod: n = 20 (27%) Mild: n = 13 (17%) Adequate: n = 8 (11%)	Maternal: compared with women in the highest tertile for vitamin D, women in the lowest tertile and mid-tertile were more likely to meet criteria for pre-eclampsia and gestational pre-hypertension (OR 2.28; 95% CI 0.35–23.28) and (OR 19.27; 95% CI 1.96–188.92 respectively); vitamin D levels were inversely correlated with maternal mean arterial pressure (r = 0.029) (p = 0.020) Neonatal: not assessed in relation to maternal vitamin D Adjusted for maternal age, level of exercise, maternal weight, birthweight and gestational age	Not assessed
Maghbooli et al., 2008 Iran [40]	Cross-sectional	Approached/screened: not reported Enrolled: n = 741 Complete data: n = 579 Setting: referral to five university hospital clinics of the Tehran University of Medical Sciences during the first half of pregnancy Season: not reported	Age: mean 27.4 years Demographics: mean BMI 26.4 kg/m2 Trimester: second Exclusion criteria: prenatal diabetes	RIA	Criteria: Severe deficiency < 12.5 nmol/L Mild deficiency 12.5–24.9 nmol/L Mod deficiency 25–34.9 nmol/L Sufficiency > 34.9 nmol/L Prevalence of deficiency: Severe n = 201 (27.1%) Mild n = 118 (15.9%) Mod n = 344 (46.4%) Sufficient n = 78 (10.5%)	Maternal: 52% women diagnosed with GDM, mean vitamin D significantly lower in women with GDM (16.49 ± 10.44 nmol/L) compared with non-GDM women (22.97 ± 18.25 nmol/L) (p = 0.009), prevalence of severe VDD was significantly higher in women with GDM (44.2%) compared with non-GDM women (23.5%) (p = 0.011) Neonatal: not assessed	Not assessed
Pirdehghan et al., 2016 Iran [25]	Cross-sectional	Approached/screened: not reported Enrolled: not reported Complete data: n = 200 Setting: admission to hospital delivery room for natural delivery, caesarean section or abortion at Shahid Sadoughi hospital Season: autumn/spring	Age: mean 26.7 years Demographics: all women nulliparous, 48.7% diploma/university educated, 97.5% housewives, 38.5% women taking multivitamins containing vitamin D during pregnancy, Trimester: not reported Exclusion criteria: pre-existing medical conditions (renal or bone disorders), medication influencing calcium or vitamin D metabolism. multiparity	ELISA	Criteria *: Severe deficiency < 25 nmol/L Moderate deficiency 25–50 nmol/L Mild deficiency 52.5–75 nmol/L Adequate 75–125 nmol/L Upper normal/toxic > 125 nmol/L Prevalence of deficiency: (figures reported in text) Severe 12.5% Deficiency 60%	Maternal: mean vitamin D significantly higher in natural or elective caesarean section women compared with abortion and emergency caesarean section women (p = 0.040); VDD associated with risk of abortion 3.1 (1.39–6.8) which was higher in severe deficiency women compared with VDD women (p = 0.045), mean vitamin D significantly lower in women with oligohydramnios or polyhydramnios complication (13.9 + 9.5 and 20.6 + 10.8 respectively) (p = 0.045) No correlation: VDD and preeclampsia, PROM, GDM, birth weight, birth length, head circumference or Apgar score	No significant associations
Song et al., 2012 China [26]	Cross-sectional	Approached/screened: not reported Enrolled: not reported Complete data: 70 Setting: delivery at 306 Hospital of PLA in Beijing from surrounding communities of the Beijing urban area Season: spring	Age: 29.9 (±0.3) years Demographics: Mean weight: 73.9 kg, pregravid range 0–3 Trimester: third Exclusion criteria: multiparity, taking calcium and/or vitamin D supplements, pre-existing medical conditions (hypertension, renal disease, pre-gestational diabetes)	ELISA	Criteria: Severe deficiency < 25 nmol/L Mild deficiency 25– < 50 nmol/L Insufficiency 50– < 75 nmol/L (21–29 ng/mL) Sufficiency ≥ 75 nmol/L (grouped for analysis Deficient < 25 nmol/L and other ≥ 25 nmol/L Prevalence of deficiency: Severe n = 38 (54.5%) Mild n = 25 (35.7%) Insufficient n = 7 (10.0%) Sufficient n = 0 (0%)	Maternal: not assessed Neonatal: significant correlation between maternal vitamin D and newborn length (r = 0.247) (p = 0.039); compared with women who had vitamin D ≥ 25 nmol/L, birth weight (3633.1 g) and length (51.0 cm) of newborns were significantly lower in women with vitamin D < 25 nmol/L (3386 g and 50.2 cm respectively) (p = 0.015, p = 0.037) No correlation: VDD or head circumference	Not assessed
Toko et al., 2016 Kenya [6]	Longitudinal	Approached/screened: n = 99 RR: not reported Complete data: n = 63 (baseline data used) (64%) Setting: women residing within a 10 km radius of Chulaimbo Sub-district hospital in Kisumu County Season: dry season	Age: mean 22.5 years Demographics: mean BMI 22.9 kg/m2 Trimester: second and third Exclusion criteria: more than 26 weeks gestation, HIV infected, residing >10 km from the hospital	ELISA	Criteria: Deficiency < 50 nmol/L Insufficiency 50–75 nmol/L Sufficiency > 75 nmol/L (grouped for analysis low < 50 nmol/L and adequate ≥ 50 nmol/L) Prevalence of deficiency: Deficient n = 13 (20.6%) Insufficient n = 32 (50.8%) Sufficient n = 19 (28.6%)	Maternal: not assessed Neonatal: newborns more likely to have stunted growth at birth when born to mothers with deficient vitamin D (RR 4.4 (CI 1.0–18.6) (p = 0.04) and more likely to be born preterm (<37 weeks) (RR 5.4 (CI 1.1, 25.3) (p = 0.03) Adjusted for: maternal age, gestational age at delivery and maternal BMI No correlation: VDD and wasting or BMI z-score	Not assessed
Xin et al., 2017 China [36]	Prospective cohort	Approached/screened: not reported Enrolled: n = 13,806 Complete data: n = 11,151 (81%) Setting: routine visit to antenatal care clinic and delivery at the Wuxi Maternity and Child Health Hospital Season: 28.4% winter, 18.5% spring, 22.7% autumn, 30.4% summer	Age: mean 27.3 years Demographics: 88.9% nulliparous, 9.2% BMI ≥ 25 kg/m2, 96% GA at delivery ≥ 37 weeks Trimester: second and third Exclusion criteria: taking calcium and/or vitamin D supplements, pre-existing medical conditions (hypertension, renal disease, pre-gestational diabetes), fetal anomalies	CI	Criteria: Deficiency < 50 nmol/L Sufficiency > 50 nmol/L Prevalence of deficiency: Deficient n = 8799 (78.9%) Sufficient n = 2352 (20.8%)	Maternal: 1.2% pre-eclampsia, significant difference in incidence of severe pre-eclampsia in pregnant women with VDD (<50 nmol/L) (n = 123; 1.4%) compared with sufficiency (≥50 nmol/L) (n = 16; 0.6%) (p = 0.002), women with VDD were more at risk of developing severe pre-eclampsia compared with women who were vitamin D sufficient (OR: 3.16; 95% CI: 1.77–5.65) (p = 0.000) Adjusted for: pre-pregnancy BMI, maternal age, parity and season of blood sampling Neonatal: not assessed	Age ≥ 35 years, pre-pregnancy BMI ≥ 25 kg/m2, nulliparity (p < 0.05)

* Vitamin D unit of measurement converted from ng/mL to nmol/L. Abbreviations: VDD—vitamin D deficiency; BMI—Body Mass Index; CI—automated chemiluminescence immunoassay; ELISA—enzyme-linked immunosorbent assay; GDM—gestational diabetes mellitus; LC-MS/MS—liquid chromatography tandem-mass spectrometer; LSCS—lower segment caesarean section; NICU—neonatal intensive care unit; OR—odds ratio; PPD—postpartum depression; RIA—radioimmunoassay; SGA—small for gestational age; PROM—premature rupture of membrane; SROM—spontaneous rupture of membranes.