Women who took extra folic acid in the weeks before and just after becoming pregnant were less likely to have a child with autism, a study in Norway showed.

Since a lack of folic acid has been tied to brain and spinal cord birth defects, medical groups such as the U.S. Preventive Services Task Force have long called for women who may become pregnant to take daily supplements containing the B vitamin.

The new study "provides an additional reason to take folic acid," said Dr. Pal Suren from the Norwegian Institute of Public Health in Oslo, who led the research. Its preventive effect against neural tube defects was well known, he said. "It underlines the importance of starting early, preferably before the pregnancy," he said.

One in 88 children in the United States has an autism spectrum disorder, according to the Centers for Disease Control and Prevention. The number has risen in recent years.

The Norwegian study, appearing in the Journal of the American Medical Association, doesn't prove low folic acid in pregnant women causes their babies to develop autism, or that high doses can prevent it. And Suren and his colleagues didn't see an effect of folic acid on other autism spectrum disorders, such as Asperger's.

Suren's team found one in 1,000 babies born to women who reported taking folic acid early in pregnancy had autism, compared with about two in 1,000 of those whose moms didn't take it. There was no link between fish oil taken during pregnancy and autism risk, suggesting it's something about folic acid, in particular, that influences a baby's autism risk.

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