Researchers in Norway looked at data from 85,000 pregnancies. They found women who took the supplement four weeks before pregnancy, and through the eighth week of pregnancy, were 39 percent less likely to have children with autism.
The study is the largest to date on the benefits of folic acid for autism prevention.
It marks one of the first tangible things a woman can do to reduce her risk of giving birth to a child with the disorder.
Some experts say more research needs to be done to confirm the link.