Associations of maternal circulating 25‐hydroxyvitamin D3 concentration with pregnancy and birth outcomes

Objective To investigate the association of maternal circulating 25‐hydroxyvitamin D3 [25( OH )D3] concentration with pregnancy and birth outcomes. Design Prospective cohort study. Setting Four geographical areas of Spain, 2003–2008. Population Of 2382 mother–child pairs participating in the IN fancia y Medio Ambiente ( INMA ) Project. Methods Maternal circulating 25( OH )D3 concentration was measured in pregnancy (mean [ SD ] 13.5 [2.2] weeks of gestation). We tested associations of maternal 25( OH )D3 concentration with pregnancy and birth outcomes. Main outcome measures Gestational diabetes mellitus ( GDM ), preterm delivery, caesarean section, fetal growth restriction ( FGR ) and small‐for‐gestational age ( SGA ), anthropometric birth outcomes including weight, length and head circumference ( HC ). Results Overall, 31.8% and 19.7% of women had vitamin D insufficiency [25( OH )D3 20–29.99 ng/ml] and deficiency [25( OH )D3 < 20 ng/ml], respectively. After adjustment, there was no association between maternal 25( OH )D3 concentration and risk of GDM or preterm delivery. Women with sufficient vitamin D [25( OH )D3 ≥ 30 ng/ml] had a decreased risk of caesarean section by obstructed labour compared with women with vitamin D deficiency [relative risk ( RR ) = 0.60, 95% CI 0.37, 0.97). Offspring of mothers with higher circulating 25( OH )D3 concentration tended to have smaller HC [coefficient ( SE ) per doubling concentration of 25( OH )D3, −0.10 (0.05), P  =   0.038]. No significant associations were found for other birth outcomes. Conclusion This study did not find any evidence of an association between vitamin D status in pregnancy and GDM , preterm delivery, FGR , SGA and anthropometric birth outcomes. Results suggest that sufficient circulating vitamin D concentration [25( OH )D3 ≥ 30 ng/ml] in pregnancy may reduce the risk of caesarean section by obstructed labour.