According to a study of 85,176 children born in Norway just published in the Journal of the American Medical Assocation, mothers who reported taking a folic acid supplement in the weeks surrounding conception were 39% less likely to have a child later diagnosed with autism. Only 0.1% of children whose moms popped folic acid pills had autism compared to 0.2% of the those whose mothers didn't. The link between an autism diagnosis and folic acid held up even after controlling for factors like maternal age and economic status.
The link between folic acid and the milder forms of autism (Asperger's and Pervasive Developmental Disorder - Not Other Specified) did not remain significant and the authors argue it might be because they lacked enough subjects (i.e. statistical power).
The study jibes with previous research suggesting folic acid reduces the risk of autism including a study of preschoolers in California.
Norway has a much lower rate of autism than the U.S. This study found .32% of children had an ASD diagnosis or compared to 1.14% of U.S. children. So for every 1,000 kids born in the respective countries, 3.2 receive an autism diagnosis in Norway compared to 11.4 in the U.S.
Suprise, surprise. We have another surprised researcher: