After the babies were born, investigators followed up with the children through the end of March 2012. They wanted to find out how many developed any form of autism. Two hundred seventy of the children had some form of autism. The study found no reduction in the risk of some forms of autism. But it did find a lower risk for the most severe form. There were not enough children with a milder form known as Asperger's syndrome to make any findings.

The cause of autism is not known, but several genes may play a part.

Folic acid has been linked to the development of a protective covering called the neural tube in a fetus. Women who do not get enough folic acid are at risk of giving birth to infants with part of their spinal cord or brain exposed. This disabling condition is called spina bifida.

Ezra Susser of Columbia University in New York City was a co-author of the new study of folic acid.

“These supplements are already recommended for other reasons. So this increases the reasons why we should recommend them.”

Dark leafy vegetables, asparagus, broccoli and citrus fruit are rich sources of folate. Folate is the naturally occurring form of folic acid. Folate is also found in peas, lentils, beans, eggs, yeast and liver.

Studies have shown that many pregnant women get less folate in their diet than they need to prevent neural tube defects. So folic acid supplements are commonly recommended for pregnant women. Some countries add folic acid to flour. Norway does not do that.