Sex differences in the prevalence of neural tube defects and preventive effects of folic acid (FA) supplementation among five counties in northern China: results from a population-based birth defect surveillance programme.

Objectives Sex differences in prevalence of neural tube defects (NTDs) have previously been recognised; however, the different susceptibility of men and women have not been examined in relation to the effects of folic acid (FA) supplementation. We hypothesised that FA may have a disproportionate effect that alters the sex-specific prevalence of NTDs. Setting Data from two time points, before (2003-2004) and after (2011-2016) the start of the supplementation programme, were obtained from a population-based birth defect surveillance programme among five counties in northern China. All live births (28 or more complete gestational weeks), all stillbirths of at least 20 weeks' gestational age and pregnancy terminations at any gestational age following the prenatal diagnosis of NTDs were included. Participants A total of 25249 and 83 996 births before and after the programme were included respectively. Primary and secondary outcome measures The prevalence of NTDs by sex and subtype, Male:female rate ratios and their 95% CI were calculated. Results Overall, NTDs were less prevalent among men than among women (rate ratio (RR) 0.92; 95% CI 0.90 to 0.94), so was anencephaly (RR 0.77; 95%CI 0.73 to 0.81) and encephalocele (RR 0.75; 95%CI 0.61 to 0.92), while spina bifida showed a male predominance (RR 1.10; 95%CI 1.05 to 1.15). The overall prevalence of NTDs decreased by 78/10 000 in men and 108.7/10 000 in women from 2003 to 2004 to 2011 to 2016. There was a significant sex difference in the magnitude of reduction, being greater in women than men, particularly for anencephaly. Conclusions The prevalence of NTDs decreased in both sexes after the implementation of a massive FA supplementation programme. While female predominance was observed in open NTDs and total NTDs, they also had a greater rate of decrease in NTDs after the supplementation programme.