Abstract
Introduction
Prematurity is an urgent public health problem worldwide. Recent studies associate maternal hypovitaminosis D during pregnancy with an increased risk of prematurity. However, the evidence on this association remains inconclusive, and there is lack of consensus in the literature. The exact mechanism by which low vitamin D levels may increase the risk of preterm birth is not yet fully understood. Nevertheless, it is known that vitamin D may play a role in maintaining a healthy pregnancy by regulating inflammation and immunomodulation by acting on the maternal and fetal immune systems. Inflammation and immune dysregulation are both associated with preterm birth, and low vitamin D levels may exacerbate these processes. The results of this review may have important implications for clinical practice and public health policy, particularly regarding vitamin D supplementation during pregnancy.
Methods and analysis
A systematic review of the literature will be conducted. The search will be performed in electronic databases: CINAHL; MEDLINE; Cochrane Central Register of Controlled Trials; Cochrane Library; Academic Search Complete; Information Science and Technology Abstracts; MedicLatina; SCOPUS; PubMed; and Google Scholar, with the chronological range of January 2018 to November 2022. The search strategy will include the following Medical Subject Headings or similar terms: ‘Vitamin D’; ‘25-hydroxyvitamin D’; ‘Hypovitaminosis D’; ‘Pregnancy’; ‘Pregnant women’; ‘Expectant mother’; ‘Prematurity’; ‘Premature birth’; ‘Premature delivery’; ‘Preterm birth’; and ‘Preterm labour’. This review will include quantitative primary studies, both experimental (clinical trials) and observational (cohort, cross-sectional, and case–control). The quality of each selected study and the results obtained will be assessed by two reviewers separately, using the Cochrane risk of bias tool for evaluating randomised clinical trials or the Newcastle Ottawa Scale for non-randomised studies, following the respective checklist. In case of disagreement, a third reviewer will be consulted.
Ethics and dissemination
This study does not involve human subjects and therefore does not require ethics approval. The results will be disseminated through publication in a peer-reviewed scientific journal and through conference presentations. All changes made to the protocol will be registered in PROSPERO, with information on the nature and justification for the changes made.
PROSPERO registration number
CRD42022303901.
Keywords: OBSTETRICS, NEONATOLOGY, Fetal medicine, Maternal medicine, Prenatal diagnosis
Strengths and limitations of this study.
This protocol provides a clear and comprehensive framework for a systematic review on the effects of maternal vitamin D levels during pregnancy on prematurity, based on recent evidence. The review will be carried out in a rigorous and transparent manner.
This review will include the most recent quantitative primary studies, both experimental (clinical trials) and observational (cohort, cross-sectional, and case–control). If available, eligible clinical trials studies will be included with a control group that was not exposed to supplements and/or used different vitamin D dosages or a placebo.
Publications in languages other than English, Portuguese, and Spanish will not be included due to language barriers, potentially introducing language bias.
There may be some heterogeneity between studies, such as differences in gestational age cut-offs (eg, <37 weeks, <34 weeks) or preterm birth ethology (eg, spontaneous preterm birth or medically induced due to maternal and/or fetal complications).
Introduction
Prematurity (birth before 37 completed weeks of gestation) is an urgent public health problem worldwide, occurring for 15 million births annually. It has a prevalence of approximately 10% of all births worldwide, with potential consequences affecting a significant part of the society. Prematurity is responsible for 35% of all neonatal deaths and contributes to 16% of deaths in children under 5 years of age (about 1 million per year) due to complications from preterm birth.1 Technological advances, combined with scientific knowledge, have allowed these premature babies to survive, although with a risk of developing multimorbidity’s throughout their life cycle, including cognitive and learning difficulties, behavioural problems, paralysis, motor deficiencies, lung dysplasia, chronic respiratory infections, visual and hearing problems, including retinopathy and deafness. Prematurity as a cause of neonatal and infant morbidity and mortality is considered a general indicator of the health, quality of life, and development of a population in several countries.2
As one of the determinants of health, proper nutrition during pregnancy is crucial for achieving good maternal and child outcomes. Nutrition is considered the main environmental factor involved in modulating gene expression, as we are exposed to this factor throughout our lives, from the intrauterine phase.3 4 Among the nutrients, the importance of vitamin D stands out as one of the determinants that most influence the health-disease process, from fetal development.
Vitamin D is a fat-soluble vitamin, a prohormone that regulates the body’s defences, being related to natural and adaptive immunity. It is considered essential for its classic action in regulating the calcium/phosphorus metabolism for bone health, fall and fracture prevention. Currently, its deficiency and insufficiency have been associated with non-classical actions, acting as a genetic modulator and influencing the risk of developing non-communicable chronic diseases, such as hypertension, diabetes, obesity, metabolic syndromes, carcinomas, cardiovascular, infectious, and autoimmune diseases.4
The serum level of 25-hydroxyvitamin D [25(OH) D] is considered the most reliable indicator of vitamin D storage in the human body. The Canadian Endocrine Society classifies the definition of vitamin D adequacy according to the following cut-off levels of 25(OH) D: deficiency<20 ng/mL (50 nmol/L), insufficiency 20–29 ng/mL (50–75 nmol/L), and sufficiency≥30 ng/mL (75 nmol/L) considered normal.5 In available epidemiological data, deficiency has a high prevalence, especially in risk groups identified as pregnant women and infants.6 The prevalence of vitamin D deficiency during pregnancy can reach 96% and insufficiency 99.4%.7 There is a high prevalence of hypovitaminosis D during pregnancy with the adopted supplementation of 400–600 IU/d, which induct to be considered an under-dose.8 9 The main risk factors associated with vitamin D deficiency during pregnancy include winter months, ethnicity, skin pigmentation, liberal use of sunscreen, extensive skin coverage, high body mass index, low dietary vitamin D intake, smoking, and high latitudes.9–11
An adequate maternal level of this micronutrient is essential for good health outcomes on the mother and on the fetus, with a potential ability to modulate maternal and fetal gene expression. Fetal and neonatal vitamin D levels are dependent on maternal levels, as well as their relationship with epigenetic mechanisms in early fetal life, which can explain their non-classical benefits.12–14 The fetus, depending entirely on maternal 25(OH) D levels, reinforces its role in fetal development. The 25(OH)D easily crosses the placenta and is activated into 1,25-dihydroxyvitamin D [1,25(OH)2 D] by fetal kidneys. Alternatively, if maternal levels are sufficient, 1,25(OH)2 D can also be synthesised inside the placenta to regulate placental metabolism through the vitamin D receptor constituting an important source of this micronutrient. Vitamin D deficiency will affect the process of placental formation and development itself, negatively affecting maternal–fetal metabolism, which is directly related to gestational duration and birth weight.5 11 13
Recent studies associate low maternal levels of vitamin D during pregnancy with an increased risk of pre-eclampsia, hypertension, and gestational diabetes as major causes of prematurity.9 10 15 16 However, the evidence regarding this association remains inconclusive, with a great lack of consensus in the literature. The exact mechanism by which low levels of vitamin D may increase the risk of preterm birth is still not fully understood. Nevertheless, it is known that vitamin D may play a role in maintaining a healthy pregnancy by regulating inflammation and immunomodulation by acting on the maternal and fetal immune systems. Inflammation and immune dysregulation are both associated with preterm birth, and low levels of vitamin D may exacerbate these processes.17 18
Therefore, it has become necessary to conduct a systematic review that synthesises the available evidence on the subject in order to provide a clearer evaluation and understanding of this possible association. The results of this review may have important implications for clinical practice and public health policies, particularly regarding vitamin D supplementation during pregnancy.
Objectives
The main objective of this study is to identify and evaluate the association between maternal levels of vitamin D during pregnancy and prematurity. As a secondary objective, we aim to identify the recommended dosage of vitamin D supplementation during pregnancy with effective action in reducing the risk of prematurity.
Review questions
This review will be undertaken in order to answer the following questions: What are the effects on pregnant women and on newborns of vitamin D levels related to prematurity? What is the recommended dosage of vitamin D supplementation during pregnancy with effective action in reducing the risk of prematurity? The review will include observational studies with vitamin D supplementation or different forms of vitamin D exposure during pregnancy, as well as studies comparing vitamin D supplementation with placebo or no intervention.
Methods and analysis
Authors will adhere to the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA).19 This protocol has followed the PRISMA Protocols 2015 guidelines.20
Randomised controlled trials, cohort studies, case–control studies, and cross-sectional studies will be included. Pregnant women of any age and gestational age will be included. Maternal vitamin D levels during pregnancy (serum 25-hydroxyvitamin D or 1,25-dihydroxyvitamin D levels) will be the exposure of interest.
Patient and public involvement
This study will not involve the participation of patients or the general public.
Eligibility criteria
Population
Pregnant women of any age and gestational age will be included as well as newborns.
Intervention/exposure
Maternal vitamin D levels during pregnancy (serum 25-hydroxyvitamin D or 1,25-dihydroxyvitamin D levels) will be the exposure of interest. This review will integrate studies reporting the impact of interventions and/or associations between vitamin D levels during pregnancy and preterm birth.
Comparison
This review will include studies with or without comparative group.
Primary outcomes
The primary outcome of interest is prematurity. All studies to be included will need to assess the association between vitamin D levels during pregnancy and prematurity.
Prematurity-related adverse effects such as gestational age, small for gestational age, birth weight, admission to the neonatal intensive care unit, Apgar scores, length, head circumference, congenital malformation, mortality, morbidity, and infection will also be researched.
Study design
Studies meeting the following criteria will be included: quantitative studies with scientific evidence; primary studies—randomised clinical trials, cohort, cross-sectional, and case–control studies; studies with the exposure of interest (maternal Vitamin D level) prior to the outcome with or without supplementation, including dietary intake of vitamin D; studies where the outcome was the risk of adverse maternal–infant outcomes, with a mandatory focus on prematurity; studies integrating association/effect measures, risk estimation (relative risk, risk difference, or OR), with 95% CI. If available, eligible randomised controlled trials with a control group without exposure to supplements, or exposure to different dosages of vitamin D, or through the use of a placebo, will be included in this review.
Studies will be excluded according to the following criteria: no full text available; qualitative studies; case-reports; letters; animal studies; reviews; meta-analyses; studies without reporting positive or negative association between maternal vitamin D levels and prematurity; studies whose results do not fit the review objectives; studies presented in a language other than English, Portuguese, or Spanish.
Search strategy
Data sources
The systematic review will be conducted in electronic databases: CINAHL; MEDLINE; Cochrane Central Register of Controlled Trials; Cochrane Library; Academic Search Complete; Information Science and Technology Abstracts; MedicLatina; SCOPUS, PubMed, and Google Scholar. The search will be conducted with the timeline from January 2018 to November 2022. The selection of this timeline is related to the relevance of recent studies to the research question. Despite the existence of some reviews on this topic, which included studies before the year 2018, several limitations had been identified by previous studies such heterogeneity between studies including the dosage of vitamin D used, the timing of supplementation during pregnancy, the duration of supplementation, the method of exposure assessment, and the cut-off point used for vitamin D.
Additionally, the research field of vitamin D, particularly its effects during pregnancy and its impact on maternal and infant outcomes, is constantly evolving. New studies can provide new insights, improved methodologies, refined study designs, revised understandings, address gaps, or present new perspectives. Given that prematurity is a complex and multifactorial outcome influenced by multiple factors, our intention is to conduct a detailed analysis of the most recent studies to gain a better understanding of the current association between vitamin D levels and prematurity. This ensures that the review incorporates the most up-to-date evidence available. Furthermore, current recommendations may have implications for policy decisions regarding vitamin D supplementation during pregnancy, with potential impact on clinical practice.
Search terms
The search will include the combination of three key concepts according to Medical Subject Headings terms, in the following sequence:[(Vitamin D OR 25-hydroxyvitamin D OR 25 (OH) D OR Hypovitaminosis D OR Vitamin D deficiency OR Vitamin D insufficiency) AND (Pregnancy OR Pregnant women OR Prenatal OR Perinatal OR Expectant mother OR Maternal-fetal) AND (Prematurity OR Premature birth OR Premature infant OR Premature delivery OR Preterm delivery OR Preterm birth OR Preterm labour OR Preterm infant OR Neonates OR Fetal development OR Birth outcomes OR Pregnancy outcomes OR Pregnancy complications)].
Data collection and analysis
Selection of studies
After the article searches in the selected databases, the results will be exported to EndNote Web and duplicates will be eliminated. In order to reduce bias, two of the reviewers (OB and CP) will independently perform the screening, eligibility, and inclusion of the studies identified in the searches. After reading the titles and abstracts, articles that do not fit the purpose of the review will be excluded. Subsequently, studies that, after reading the full texts, do not meet the inclusion and quality criteria will be excluded. The relevance of the selected articles will be discussed between the two reviewers. If there is no agreement or if there are doubts about any article, the third reviewer will be consulted. All studies should assess the relationship between the intervention or exposure and the outcome, that is, the association between vitamin D levels during pregnancy and prematurity. The effects of this association should be explicitly reported in the data analysis. A PRISMA flowchart19 will be presented in the review with the identification, screening, and selection of articles.
Data extraction
Two reviewers, OB and CP, will independently screen, assess eligibility, and select studies. The EndNote Web reference management programme will be used during the study selection phase. In cases of disagreement, a third reviewer will be consulted. A standardised data extraction form will be utilised to collect information on study characteristics (author, year, country, objective, study design, and sample size), participant characteristics (age, ethnicity, gestational week, and vitamin D levels), exposure (exposure assessment method, gestational week for vitamin D measurement, and categories of vitamin D level), outcome (outcome measures and key results). The results will be compared and agreed on through consensus. The data synthesis matrix will be analysed to determine whether the results of different studies can be grouped and subjected to meta-analysis using statistical techniques. If two or more articles have comparable results, a quantitative synthesis will be conducted.
Quality appraisal
Two reviewers will independently assess the quality of each selected study and its results using a standardised data extraction form. The risk of bias in the included studies will be evaluated using the Cochrane Risk of Bias tool for randomised clinical trials21 and the Newcastle-Ottawa Scale22 for non-randomised studies, such as cohort, case–control, and cross-sectional studies. The respective checklists will be followed. Taking into account the objectives of the review, if available, will be included eligible randomised clinical trials, due to their importance and impact on clinical decision-making. If there is any disagreement a third reviewer will be consulted.
Strategy for data synthesis and reporting
The first step in the process is to perform a descriptive assessment of each study using a data extraction sheet. Cochrane Review Manager software (RevMan Web) will then be used for analysis. There may be some heterogeneity between studies due to various factors, including differences in the definition and measurement of prematurity, such as gestational age cut-offs (eg, <37 weeks, <34 weeks) or causes for preterm birth (eg, spontaneous preterm birth or medically induced due to maternal and/or fetal complications).
These differences can affect the results of the studies and make comparison challenging. Therefore, factors that may introduce heterogeneity in the selected studies will be carefully considered when synthesising the evidence.
A meta-analysis will be conducted if there is sufficient homogeneity among the included studies. Random-effects models will be used to calculate the pooled effect size, and heterogeneity will be assessed using the I-squared statistic. Subgroup analyses will be conducted based on study design, population characteristics, vitamin D supplementation status, and timing of vitamin D assessment during pregnancy. Publication bias will be assessed using funnel plots and Egger’s test. The systematic review will be reported according to the PRISMA19 guidelines.
Assessment of the quality of the evidence produced by the review
To assess the quality of evidence produced by the systematic review, the GRADE23 24 (Grading of Recommendations Assessment, Development, and Evaluation) protocol will be utilised. This involves a structured approach to evaluating the quality of evidence for each outcome, taking into account study design, risk of bias, inconsistency, indirectness, and imprecision. The following steps will be taken: (1) Clearly define the relevant results for the review. (2) Evaluate the quality of evidence for each outcome by classifying it as high, moderate, low, or very low. (3) Synthesise the evidence for each outcome, considering the quality of evidence, direction and magnitude of the effect. (4) Make recommendations based on available evidence, taking into account the balance of benefits and harms, patient values and preferences, and other relevant factors. In addition to the GRADE guidelines, other methodological considerations in study design will be incorporated to assess the quality of the systematic review’s evidence synthesis.
Supplementary Material
Footnotes
Contributors: The authors will contribute to the study as follows: OB developed the study protocol design; OB and CP perform the screening, assess eligibility, and select the studies identified in the searches; OB wrote the protocol; MS-S, MPS, and DC reviewed the protocol. All authors contributed to refining the review protocol and approving the final manuscript.
Funding: This work is funded by national funds through the Foundation for Science and Technology, under the project UIDP/04923/2020.
Competing interests: None declared.
Patient and public involvement: Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Provenance and peer review: Not commissioned; externally peer reviewed.
Ethics statements
Patient consent for publication
Not required.
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