Folic acid supplementation and risk for fetal abdominal wall defects in China: results from a large population-based intervention cohort study.

Folic acid (FA) can reduce the risk for selected birth defects other than neural tube defects. We examined whether FA has preventive effects against fetal abdominal wall defects (AWD) in a unique intervention cohort in China. Birth outcomes of 247 831 singleton births from a population-based cohort study with detailed pre-conceptional FA intake information were collected in China in 1993-1996. Information on births at 20 complete gestational weeks, including live births, stillbirths and pregnancy terminations, and all structural birth defects regardless of gestational week were recorded. The birth prevalence of omphalocele, gastroschisis and total fetal AWD was classified by maternal FA supplementation. The prevalence of total AWD was 4·30 per 10 000 births among women who took FA compared with 13·46 per 10 000 births among those who did not take FA in northern China and 6·28 and 5·18 per 10 000 births, respectively, in southern China. The prevalence of omphalocele was 0·54 per 10 000 births among women who took FA compared with 3·74 per 10 000 births among those who did not take FA in northern China and 1·79 and 1·44 per 10 000 births, respectively, in southern China. FA supplementation significantly prevented total AWD in multivariate analysis (relative risk 0·26, 95 % CI 0·11, 0·61) in northern China, although no preventive effect of FA on AWD was observed in southern China. FA supplementation successfully reduced the prevalence of AWD in northern China.