The Association Between Vitamin D Supplementation Dose in Pregnant Women and Birth Weight

Background: Vitamin D supports maternal calcium metabolism and fetal skeletal development, but evidence on whether supplementation dose is associated with birth weight remains inconsistent. Low birth weight (LBW), defined as birth weight below 2,500 g, remains an important neonatal health concern. Objective: This study examined the association between maternal vitamin D supplementation dose and birth weight and assessed potential maternal confounders. Methods: A quantitative cross-sectional study was conducted among 170 mothers who delivered live-born infants between 2023 and 2026 and reported taking vitamin D-containing supplements during pregnancy. Data were collected using a self-administered online questionnaire. Supplementation dose was categorized as <400, 400–600, 601–4,000, >4,000 IU/day, or unknown; birth weight was classified as normal (2,500–3,999 g) or LBW (<2,500 g). Univariate and bivariate analyses were performed, followed by binary logistic regression for variables with bivariate p-values <0.25. Results: Vitamin D supplementation dose was not associated with LBW in bivariate analysis (p=0.942) and was not retained in the adjusted model. Gestational age, parity, hypertension/preeclampsia history, previous LBW, secondhand smoke exposure, and maternal vitamin D knowledge were associated with birth weight in bivariate analyses. After adjustment, gestational age <37 weeks was the only variable significantly associated with LBW (B=2.577; p=0.015; OR=13.155; 95% CI 1.653–104.693). The wide confidence interval indicates substantial uncertainty in the effect estimate and warrants cautious interpretation. The remaining variables were not statistically significant in the multivariable model. Conclusion: In this sample, supplementation dose was not independently associated with birth weight, while gestational age was the only significant adjusted correlate of LBW. These cross-sectional findings indicate statistical associations and do not establish causal effects. Antenatal care should encourage adequate vitamin D intake and monitor gestational duration as part of efforts to reduce LBW.