That's according to a study of hundreds of thousands of Norwegian children published in mid-February in the Journal of the American Medical Association.
It's the second large study in the past few years tying the B vitamin, which is commonly associated with preventing brain and spinal defects such as spina bifida, to autism.
Women who took folic acid supplements before and during pregnancy in the Norwegian study were about 40 percent less likely to have a baby later diagnosed with autism than were those who took none.
Researchers tracked more than 85,000 Norwegian children born between 2002 and 2008, said Dr. Pal Suren, lead author of the study from the division of epidemiology at the Norwegian Institute of Public Health in Oslo. Doctors asked pregnant women to fill out a questionnaire about supplement use at their first ultrasound appointment, and followed their children for an average of six years.
Autistic disorder was present in 0.10 percent of children whose mothers took folic acid, compared with 0.21 percent in children whose mothers did not take folic acid.
The prevalence of autism spectrum disorders has been increasing dramatically in the United States over the past decade and now affects one in 88 children, according to the Centers for Disease Control and Prevention.
While there is no single known cause of autism, strong genetic factors and a number of environmental factors such as parental age and low birth weight contribute to the disorder, said Thomas Frazier, interim director of the Cleveland Clinic's Center for Autism.
Folic acid, or vitamin B9, plays a key role in the very early development of the brain and spinal cord of an embryo, often before a woman knows she's pregnant.
While folate, the natural form of folic acid, is found in foods such as black beans, spinach, broccoli and fortified bread and cereal, most women do not get enough without a supplement. The Institute of Medicine recommends 400 micrograms a day, the amount found in most multivitamins, for women to prepare for pregnancy.
The women in the Norwegian study took 400 micrograms of folic acid a day for four weeks before and eight weeks after conception. This is a critical period for development, said Frazier.
"That's the time period when you might see neural tube defects," such as spina bifida, said Frazier. "The spinal cord starts out basically as a tube, and if that tube doesn't end up developing properly, it won't end up being whole and connected."
This is the second recent study to find a link between autism and folic acid. In 2011, the CHARGE, or CHildhood Autism Risks from Genetics and Environment, study at the University of California-Davis found that folate intake was associated with reduced autism risk.
One advantage of the Norwegian study, though, is that mothers there took folic acid supplements alone, not in a multivitamin, which makes the results more easily attributable to the folic acid itself.
The researchers also asked the women in the study about their use of other supplements, such as omega-3 fatty acids and fish oil. They found that while similar groups of women took these supplements -- highly educated women, older women and women having their first baby -- omega-3 and fish oil use had no influence on autism risk.
The results of the Norway study don't mean that folic acid deficiency is the driving force behind the rise in prevalence of autism cases, Frazier said.
"Most people with autism are probably a result of some pretty powerful genetic factors and then some environmental factors layered on top of that," he said. "This is a two-fold increased risk, and that's large, but one factor in and of itself, even folic acid, isn't going to be enough to push someone toward a diagnosis of autism."
Suren and Frazier said the Norwegian study should remind women how important folic acid is to a healthy pregnancy. Yet the March of Dimes reports that about two-thirds of women are not aware of the vitamin's key role in embryonic development.
"The fact that folic acid can prevent brain and spinal disorders suggests that it's critical at these early stages of development, and it's logical to think that because it has benefits with regard to neural tube defects, that it may also have other beneficial effects," Suren said.
"You don't want to just catch people after they find out they're pregnant," Frazier said.