Taking folic acid before conception and early in pregnancy is associated with a significantly lower risk of the most severe form of autism, a new study suggests.

Researchers tracked more than 85,000 mothers in Norway, finding that children of those who took folic acid supplements in the four weeks before becoming pregnant and up to eight weeks into their pregnancy were 40 percent less likely to develop autistic disorder.

Whether or not a pregnant mother took the supplements, however, did not appear to alter a child’s chances of having pervasive developmental disorder, not otherwise specified. And, researchers identified too few children with Asperger’s syndrome to determine whether or not the supplement impacted a child’s odds of developing the high-functioning form of autism.

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The study published Wednesday in the Journal of the American Medical Association followed women who had babies born between 2002 and 2008. Researchers documented the mothers’ dietary habits during pregnancy and study participants were periodically surveyed for the three to 10-year period after their children were born to identify who would later be diagnosed with autism. Ultimately, 270 kids who participated in the study were found to be on the autism spectrum.

When a woman took folic acid appeared to make a significant difference, the study found. Mothers who took the supplement before and just after becoming pregnant were the only ones to see a reduced risk of autism for their children.

Meanwhile, however, the use of fish oil supplements and other vitamins and minerals during pregnancy did not appear to alter autism risk, the study found.

Taking folic acid supplements during pregnancy is known to ward off spina bifida and other neural tube defects. However, more research is needed to affirm its impact on autism, researchers said.

Nonetheless, the findings offer reassurances, say researchers from the U.S. Centers for Disease Control and Prevention, since the study debunks previous concerns that folic acid may increase risk for autism.

“This should ensure that folic acid intake can continue to serve as a tool for the prevention of neural tube birth defects,” the CDC researchers wrote in an editorial published alongside the study in JAMA. “The potential for a nutritional supplement to reduce the risk of autistic disorder is provocative and should be confirmed in other populations.”