OSLO, NORWAY — A link between folic acid consumption during pregnancy and reduced risk of birth defects has been known for years. Now, a study of Norway women found consuming folic acid during pregnancy was associated with a lower risk of autistic disorder in their children. The study appeared in the Feb. 13 issue of The Journal of the American Medical Association.

The study sample was 85,176 children from the Norwegian Mother and Child Cohort Study. Of the 61,042 children whose mothers took folic acid supplements, 64, or 0.1%, had autistic disorder. Of the 24,134 children whose mothers did not take folic acid supplements, 50, or 0.21%, had autistic disorder.

“Although these findings cannot establish causality, they do support prenatal folic acid supplementation,” the researchers concluded.

The reduced risk of autistic disorder was found among women who took folic acid supplements from 4 weeks before the start of pregnancy to 8 weeks after the start of pregnancy. No foods were fortified with folic acid at the time the women were recruited. Synthetic supplements represented the only source of folate apart from the women’s ordinary diets.

In the United States, the Food and Drug Administration mandated folic acid fortification of enriched grains in 1998. Since then, the number of babies born in the United States with neural tube birth defects has declined by about 36%.

Children in the Norway study were born from 2002 to 2008. By the end of follow-up on March 31, 2012, the age range was 3.3 years through 10.2 years with a mean of 6.4 years. Folic acid use increased by year of birth, going from 43% in 2002 to 84% in 2008.

The study also investigated fish oil supplements, but no association with reduced risk of autistic disorder was found.

Women who used folic acid within the exposure interval of 4 weeks before pregnancy to 8 weeks after the start of pregnancy were more likely to have college-level or university-level education, to have planned the pregnancy, to be non-smokers, to have pre-pregnancy body mass index below 25 and to be first-time mothers. The use of fish oil supplements followed patterns similar to those for folic acid use in the study sample.

“Use of folic acid supplements was associated with higher socioeconomic status and more health-conscious maternal behavior patterns in the study sample,” the researchers said. “We cannot exclude the possibility that some portion of the inverse association represents residual, unmeasured confounding. However, if residual confounding was substantial, we would have expected to find a lowering of risk associated with fish oil supplement use as well, because the use of fish oil was associated with the same parental characteristics in the study sample. No such lowering of risk was observed.”

Researchers from Norway involved in the study included those from the University of Oslo, the Norwegian Institute of Public Health in Oslo, the Lovisenberg Hospital in Oslo and the University of Bergen. Other researchers were from the UCL Institute of Child Health in London, Columbia University in New York, the New York State Psychiatric Institute, the National Institute of Neurological Disorders and Stroke in Bethesda, Md., and the University of Bristol in the United Kingdom.

Since 1998 the Norwegian Directorate of Health has recommended all women attempting to become pregnant should take folic acid supplements daily from one month before conception through the first trimester.