The impact of folic acid supplementation in early to mid-pregnancy on the risk of hypertensive disorders of pregnancy: The Japan Environment and Children’s Study

Background Hypertensive disorders of pregnancy (HDP) are potentially life-threatening conditions for both the pregnant woman and fetus with long-term effects on maternal health, and while folic acid supplementation has been suggested to reduce the risk of HDP, current epidemiological evidence remains inconclusive. This study evaluated the impact of folic acid supplementation on the risk of HDP and postpartum hypertension in a large-scale prospective cohort study within the Japanese population. Methods Data were obtained from the national birth cohort of the Japan Environment and Children's Study (JECS), collected between 2011 and 2014. Crude and adjusted binomial logistic regression analyses were performed to assess the association between maternal folic acid supplementation during pregnancy and HDP as well as postpartum hypertension. HDP was further classified according to onset timing as early-onset (Eo) or late-onset (Lo), and multinomial logistic regression was applied to examine these subtypes. Results A total of 89,683 pregnant women were included in the analysis. Compared with the non-supplementation group, participants who took folic acid supplements from early to mid-pregnancy exhibited a significantly lower incidence of HDP (adjusted odds ratio [aOR]: 0.87, 95% confidence interval [CI]: 0.79–0.96). Furthermore, the incidence of postpartum hypertension was significantly lower in the supplementation group (aOR: 0.86, 95% CI: 0.80–0.93). In additional analysis where HDP was classified into Eo and Lo, a reduced risk was observed for Lo HDP (aOR: 0.88, 95% CI: 0.80–0.96), whereas no significant association was identified for Eo HDP (aOR: 0.89, 95% CI: 0.76–1.05). Conclusions Folic acid supplementation during pregnancy was inversely associated with the incidence of HDP, particularly Lo HDP. Furthermore, it may contribute to the prevention of postpartum hypertension. In addition to its established role in preventing neural tube defects, continuous folic acid supplementation before conception may be beneficial for maintaining maternal health throughout pregnancy and the postpartum period.