Effects of vitamin D levels during pregnancy on prematurity: a systematic review protocol

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.

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.

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.

CRD42022303901.