Women who took folic acid supplements before and during pregnancy were about 40% less likely to have a baby later diagnosed with autism, according to a provocative new study generating high interest in the scientific community.

The dramatic increase in the prevalence of autism spectrum disorders, which affect one in 88 children, has generated intense interest in learning the causes of autism, as well as better ways to treat and prevent the condition.

In the new study, which included more than 85,000 Norwegian children, doctors asked pregnant women to fill out a questionnaire about supplement use, both before and during their pregnancies. Researchers then followed the children, born between 2002 to 2008, for an average of six years. The study was published online Tuesday in theJournal of the American Medical Association.

Doctors have encouraged women to take folic acid before and during pregnancy for years, because it can reduce the risk of birth defects.

In this study, the critical window for folic acid consumption was four weeks before conception through the eighth week of pregnancy. Overall, women who took supplements during this window were 27% less likely than others to have a baby with any autism spectrum disorder, which includes autism disorder — the most severe form — as well as Asperger's syndrome and pervasive developmental disorder-not otherwise specified (PDD-NOS).

Women who took folic acid during that window were about 40% less likely to have a child later diagnosed with autism disorder.

Taking folic acid in mid-pregnancy, measured at week 22, was not associated with a decreased risk. Researchers also found no link between fish oil supplements and autism risk.

In addition, researchers found no decrease in the individual risk of two milder subtypes of autism — Asperger's syndrome or pervasive developmental disorder — which tend to be diagnosed later. It's possible that children in the study, at an average age of 6, were too young for these disorders to have been diagnosed, says study co-author Pal Suren, of the Norwegian Institute of Public Health in Oslo.

The study's results confirm findings from earlier, more preliminary studies linking folic acid and autism risk, says Craig Newschaffer, director of Drexel University's Autism Institute in Philadelphia. Other studies also have found that children whose mothers took folic acid were less likely to have language delays.

While scientists will need to confirm the results with additional studies, Newschaffer says "it provides additional evidence that we may eventually be able to develop solid strategies to effectively prevent some forms of autism."

Folic acid, a B vitamin, is essential for synthesizing and repairing DNA, Suren says. It appears to play a key role in the first days and weeks of embryonic life, before women even know they're pregnant. Scientists can't explain, however, exactly how folic acid prevents birth defects.

Folate, the natural form of folic acid, is found in lentils, spinach, black beans, peanuts, orange juice, romaine lettuce and broccoli. Most people don't get enough folic acid from food, however. Two-thirds of women don't even know it's important, according to the March of Dimes.

For that reason, doctors recommend all women of childbearing age take 400 micrograms of folic acid a day. In the USA, grains such as flour, rice and cereal have been fortified with folic acid since 1998. Norway does not add folic acid to foods.

Many women wonder what they can do to reduce the risk of autism, especially if they already have one autistic child.

"We get questions from women all the time, asking, 'What can I do? What can I do?' " says Alycia Halladay, senior director for environmental and clinical sciences for Autism Speaks, an advocacy group.

But Halladay notes that taking folic acid doesn't guarantee that women won't have a child with autism. Some women in the study still had an autistic child after taking the supplements.

"I do worry that women who didn't take folic acid during that critical time period might feel somewhat responsible, that it's the 'mother's fault' again," Halladay says.

And although the symptoms of autism often become clear only after a child's first birthday, this study is one of many suggesting that the biological changes driving autism occur either before conception or during pregnancy, Suren says.

Doctors have isolated genetic causes for only about 15% of cases of autism, according to the National Institutes of Health, although studies show couples with one autistic child are at increased risk of having another.

Additional factors contributing to autism include premature birth, low birth weight, prenatal exposure to certain medications or air pollution and maternal infections during pregnancy. Children are also more likely to develop autism if they're born to older fathers or they're born less than a year after an older sibling, studies show.

Doctors say the condition should really be called "autisms." That's because research strongly suggests that autism is not a single condition, but a group of conditions with similar symptoms and different causes, says co-author Deborah Hirtz, of the National Institute of Neurological Disorders and Stroke, which helped fund the study.

Autism is an incredibly diverse diagnosis. Some severely disabled children are unable to speak and prone to injuring themselves, while many adults with Asperger's syndrome have successful careers in science.

Both doctors and parents have wondered why the prevalence of autism has grown so dramatically in recent decades.

The new study raises additional questions, says Cathrine Hoyo, a professor of epidemiology at Duke University School of Medicine in North Carolina. These include:Could changes in American diets — which now include fewer fruits and vegetables than in the past — affect folic acid levels, influencing autism rates? Could rising rates of overweight and obesity, which may affect how much folic acid women need, contribute to the problem?

"We're certainly not going to be able to find any one environmental factor that will be the cause of autism," Hirtz says. "I'm sure there will be multiple causes that interact with genetic susceptibilities."