Impact of vitamin D supplementation on preterm births and premature rupture of membranes: a study protocol for a double-blind randomised controlled trial

Introduction Preterm birth (PTB) is a major adverse outcome in developing countries, including India, with serious adverse consequences. Although the pathophysiology is not fully understood, maternal biomarkers influencing placental function may contribute to premature rupture of membranes and PTB. Vitamin D₃ supplementation during pregnancy may lower the risk of PTB, particularly in populations with a high prevalence of vitamin D deficiency (~90% in India). However, available biomarkers for PTB prediction are non-confirmatory, costly and not part of routine care. The aim of the study is to evaluate whether 2000 IU/day vitamin D₃ supplementation during pregnancy reduces PTB compared with 500 IU/day (standard care). Methods and analysis Singleton pregnant women ≤18 weeks gestation with deficiency of 25-hydroxyvitamin D (<20 ng/mL) attending antenatal clinics will be randomised (n=902) in a double-blind trial to receive either 500 IU or 2000 IU/day of vitamin D₃ with 1000 mg elemental calcium until delivery. Women with chronic illness, infections (HIV, positive Venereal Disease Research Laboratory test and positive hepatitis B surface antigen test), assisted conception or planned relocation before study completion will be excluded. Baseline details such as socio-demographic data, diet, anthropometry, ultrasound findings and blood pressure will be collected. Monthly antenatal visits will include clinical monitoring, supplement distribution and compliance checks. Blood samples at baseline and second and third trimesters and cord blood will be collected to assess vitamin D-related markers, pregnancy-associated plasma protein A and haemoglobin. Urinary calcium-to-creatinine ratio will be measured mid-study. Birth outcomes and infant anthropometry will be recorded. Baseline and endline characteristics will be compared between study arms using the χ2 test or Fisher’s exact test for categorical variables (depending on the expected frequencies) and the independent samples t-test or Mann–Whitney U test for continuous variables (following normality assessment). Ethics and dissemination The study has been funded by the Department of Biotechnology, India (Grant Ref: BT/PR41941/MED/97/548/2021). Ethical clearance has been obtained from Institutional Ethics Committee (Ref No. IEC/344/2021), St. John’s Medical College & Research Institute and Ethics Committee of Bangalore Medical College & Research Institute (BMCRI/EC/25/2024). Study data will be disseminated through peer-reviewed publications and conference presentations. Trial registration Trial has been prospectively registered with Clinical Trials Registry - India (CTRI/2021/10/037658).