Women who took extra folic acid in the weeks before and just after becoming pregnant were less likely to have a child with autism, in a new study from Norway.
Because lack of folic acid has been tied to brain and spinal cord birth defects, groups including the US Preventive Services Task Force (USPSTF) already call 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, in addition to the preventive effect that we already know it has against neural tube defects," said Dr Pal Suren from the Norwegian Institute of Public Health in Oslo, who led the research.
"It underlines the importance of starting early, preferably before the pregnancy," he told Reuters Health.
One in 88 children in the US has an autism spectrum disorder, according to the Centers for Disease Control and Prevention. That number has risen in recent years, but it's unclear whether that's due to more kids with symptoms or doctors who are more likely to recognise and diagnose autism.
The new study, which appears 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.
They followed just over 85,000 women and their children, born between 2002 and 2008.
When women were about halfway through their pregnancies, they reported on any supplements or vitamins they'd taken in the few weeks before becoming pregnant and the two months afterward - the time when folic acid is thought to have the strongest effect on development.
By the time their kids were between three and 10 years old, 270 had been diagnosed with an autism spectrum disorder, including 114 with autism itself.
Suren's team found one in 1000 babies born to women who reported taking folic acid early in pregnancy had autism, compared to about two in 1000 of those whose moms didn't take folic acid. On the other hand, there was no link between fish oil taken during pregnancy and autism risk.
That suggests it's something about folic acid, in particular, that influences a baby's autism risk.
"The biggest problem we have is, how do you know it's folic acid and not just a more health-conscious mother?" said Cathrine Hoyo, who has studied folic acid supplementation at the Duke University Medical Center in Durham, North Carolina.
"Doing that (analysis) really added another layer toward convincing us that there is something to it," Hoyo, who wasn't involved in the new research, told Reuters Health.
BEFORE PREGNANCY, TOO
Researchers said folic acid's effects on genes and DNA repair could explain its role in brain development disorders in growing babies, including autism.
The USPSTF recommends all women who are planning to or could become pregnant take between 400 and 800 micrograms of the vitamin daily. The US and Canada have also required flour to be fortified with folic acid since the late 1990s to cut down on birth defect risks.
Still, "The message for folic acid before pregnancy has been a little bit lost," said Rebecca Schmidt, who has studied prenatal vitamins and autism at the University of California, Davis.
"If you are not even just planning a pregnancy, but able to get pregnant, then you should be taking some sort of folic acid supplement," she told Reuters Health.