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A large study conducted in Norway has come to a striking conclusion. Women who take folic acid supplements early in pregnancy are much less likely to have a child with autism. The study appears in the latest issue of the Journal of the American Medical Association. As NPR's Jon Hamilton reports, the results have implications for mothers everywhere.

JON HAMILTON, BYLINE: This study looked at more than 85,000 Norwegian mothers and their kids. Ian Lipkin of Columbia University says he and others involved in the study wanted to know whether folic acid, a synthetic version of the B vitamin folate, could influence a child's risk of developing autism. So they looked to see which moms in this huge group were taking the vitamin around the time they got pregnant. Lipkin says he was startled by what they found.

IAN LIPKIN: It looks like there's about a 40 percent reduction in risk of autism if one takes the folate vitamin. That's a huge effect.

HAMILTON: Lipkin says this link between folic acid and autism is especially striking because the supplement is so cheap and easy to get.

LIPKIN: The notion that a very simple, nontoxic food supplement could reduce your risk is striking, profound, important and requires emphasis.

HAMILTON: There are caveats, though. Risk reduction was limited to severe autism, not milder forms such as Asperger's. Also, Lipkin says, women needed to start taking the supplements before conception.

LIPKIN: The most potent effect was seen in those children whose mothers took that folate before pregnancy and continued for eight weeks into pregnancy.

HAMILTON: Doctors already recommend folic acid to reduce the risk of having a child with spina bifida. But Lipkin says many women in the U.S. still are not taking it when they become pregnant. He says one reason may be that spina bifida is very rare.

LIPKIN: But when you start talking about autism, a disorder that has an incidence of 1 percent or higher, that really does bring it to home. That is a substantial risk. That's not something that's rare.

HAMILTON: Lipkin says women often do not get enough folate through diet alone, even if they eat a lot of healthful foods.

LIPKIN: I have nothing at all against green vegetables and beans and all the other things that are important in a balanced diet and that have lots of folate, but you only absorb about half of the dietary folate that you take in.

HAMILTON: The study of Norwegian mothers is the biggest to link folic acid to a reduced risk of autism, but smaller studies have shown the same thing. Susan Hyman is an autism researcher at the University of Rochester.

SUSAN HYMAN: This study is reassuring that folic acid supplementation not only is safe but actually decreases the rate of autism.

HAMILTON: Hyman says the study also shows that brain disorders are often linked to events that take place soon after conception.

HYMAN: Cells are dividing at a great rate, and they're defining what's going to become brain. And it's really those very first weeks to months of brain development that can provide long-lasting differences in neurologic function.

HAMILTON: Hyman says previous studies have found that factors including obesity and diabetes can also influence a mother's risk of having a child with autism. So she says the message to women who may become pregnant is pretty clear.

HYMAN: Eat a balanced diet, exercise, decrease the stress in their lives and take the supplements containing folic acid that their health care provider recommends so that they can have the healthiest baby possible.

HAMILTON: The Norwegian study is just the latest to suggest that the beginning of pregnancy is a critical period. Researchers say diseases from cancer to schizophrenia appear to be influenced by events that happen in the earliest weeks of life. Jon Hamilton, NPR News.

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