Children born to women who started taking folic acid supplements four to eight weeks before pregnancy appear to be at a lower risk of autism, a study showed this week.

Pal Suren of the Norwegian Institute of Public Health and colleagues looked into the use of folic acid supplements before and during early pregnancy, and any impact on the later risk of various disorders on the autism spectrum.

"Our main finding was that maternal use of folic acid supplements around the time of conception was associated with a lower risk of autistic disorder," the authors wrote in the Journal of the American Medical Association.

The journal recalled that many countries recommend flour be enriched with folic acid to lower the risk of birth defects, and that women are often advised to take folic acid supplements before and during pregnancy.

Despite the practice, European and North American studies have found that many pregnant women take less folate in their diet than is necessary to prevent neural tube defects.

Suren's research appears to confirm that the advice to take folic acid supplements is well-founded.

The 85,176 Norwegian children who took part in the study were born between 2002 and 2008.

Among the sample, 270 children, or 0.32 percent, were diagnosed with autism spectrum disorders, and researchers found that there was an inverse association between folic acid use and subsequent autism risks.

About 1 in 88 children, or 1.14 percent, in the United States have been identified with an autism spectrum disorder, according to the US Centers for Disease Control and Prevention.

Mothers who took folic acid supplements in early pregnancy had a 40 percent lower risk of having children with autistic disorder compared with mothers who did not take folic acid, the researchers found.

Folic acid is found in naturally high levels in foods such as dark leafy greens, asparagus and broccoli, as well as citrus fruits.