Women taking a folic acid supplement before and during pregnancy can halve the risk of having a child being diagnosed with autism later. But the supplement has no effect if it is started more than eight weeks into the pregnancy.

These findings are the result of a new study carried out at the Norwegian Institute of Public Health. In the study, women who took folic acid supplements from four weeks before conception to eight weeks into pregnancy had a 40% lower risk of giving birth to children with childhood autism (classic autism).

“It appears that the crucial time interval is from four weeks before conception to eight weeks into pregnancy,” said Pal Suren, MD and doctoral fellow at the Norwegian Institute of Public Health. The study is based on the Norwegian Mother and Child Cohort Study and the Norway Autism Birth Cohort Study.