Association of season and vitamin D status with adverse pregnancy outcomes in Southern China: a retrospective cohort study

Objective: The association of season and vitamin D status with pregnancy outcomes remains poorly understood. This study aimed to investigate the correlation between vitamin D deficiency (VDD) during pregnancy in different seasons and adverse pregnancy outcomes (APOs). Methods: The risk of APOs, including preterm delivery, gestational diabetes mellitus, pre-eclampsia, small for gestational age, and large for gestational age, was retrospectively evaluated in 5234 women. The vitamin D status of all women was assessed during pregnancy, and VDD was considered at a maternal serum concentration of 25-hydroxyvitamin D concentration of <50.0 nmol/L. Binary logistic regression was used to assess the risk of APOs, which were expressed as adjusted odds ratios (a OR ) with 95% confidence intervals ( CI s). Results: VDD was observed in 4202 (80.28%) women. VDD during pregnancy was associated with preterm delivery (a OR = 2.10, 95% CI : 1.52–2.89) and small for gestational age (a OR = 1.40, 95% CI : 1.00–1.96). The association between VDD and preterm delivery was significant in spring (a OR = 1.97, 95% CI : 1.03–3.74), summer (a OR = 2.46, 95% CI : 1.32–4.58), and winter (a OR = 2.96, 95% CI : 1.55–5.66). Meanwhile, we did not observe an association of VDD with gestational diabetes mellitus, pre-eclampsia, or large for gestational age. Conclusions: Season and VDD are associated with APOs. Our findings suggest the optimal timing for vitamin D assessment and interventions to optimize pregnancy outcomes.