Does high-dose gestational folic acid increase the risk for autism? The birth order hypothesis

There has been a dramatic increase in the incidence of autism spectrum disorder (ASD) in recent decades but the causes have not been elucidated. To date, numerous studies have shown that the FDA-recommended doses of folic acid (400 mcg/d) render a protective effect against ASD. Yet, a recent prospective study has claimed that while self-reported folic acid supplementation was associated with decreased risk of ASD, very high levels of maternal plasma folate levels (<60.3 nmol/L) were associated with 2.5 time increased risk of ASD. This study has led to high levels of public anxiety because many women use high dose folic acid to prevent neural tube defects. We hypothesize that because ASD children have been documented to be much more likely to be first or second born, and women consume significantly more folic acid during their first and second pregnancies, the claim that high dose folic acid causes ASD is based on a previously unrecognized birth order bias. This article presents evidence for the wrong claim that high dose folic acid causes ASD. The question whether high exposure level of folic acid is associated with increased risk of ASD is not merely a theoretical issue, because many women at increased risk for NTD in their offspring need substantially higher daily doses of folic acid (1 mg, or 5 mg), than the FDA-recommended 400 mcg daily.