To evaluate whether early folic acid supplementation during pregnancy prevents diagnosis of autism spectrum disorders in offspring.

Information on autism spectrum disorder diagnosis was obtained from the National Hospital Register and the Central Psychiatric Register. We estimated risk ratios for autism spectrum disorders for children whose mothers took folate or multivitamin supplements from 4 weeks prior from the last menstrual period through to 8 weeks after the last menstrual period (–4 to 8 weeks) by three 4-week periods.

We did not find an association between early folate or multivitamin intake for autism spectrum disorder (folic acid—adjusted risk ratio: 1.06, 95% confidence interval: 0.82–1.36; multivitamin—adjusted risk ratio: 1.00, 95% confidence interval: 0.82–1.22), autistic disorder (folic acid—adjusted risk ratio: 1.18, 95% confidence interval: 0.76–1.84; multivitamin—adjusted risk ratio: 1.22, 95% confidence interval: 0.87–1.69), Asperger’s syndrome (folic acid—adjusted risk ratio: 0.85, 95% confidence interval: 0.46–1.53; multivitamin—adjusted risk ratio: 0.95, 95% confidence interval: 0.62–1.46), or pervasive developmental disorder–not otherwise specified (folic acid—adjusted risk ratio: 1.07, 95% confidence interval: 0.75–1.54; multivitamin: adjusted risk ratio: 0.87, 95% confidence interval: 0.65–1.17) compared with women reporting no supplement use in the same period.

We did not find any evidence to corroborate previous reports of a reduced risk for autism spectrum disorders in offspring of women using folic acid supplements in early pregnancy.