But a new study published today in the Journal of the American Medical Association found it may also prevent autism.
The study comes at a time when pregnant women are increasingly worried about how their actions will affect their growing fetuses - from eating certain foods to getting vaccines to dying their hair.
"Many women have significant concerns, anxiety and stress during pregnancy about the things that they eat, consume or come into contact with in an environmental sense," said Dr. Jennifer Ashton, a senior medical contributor to ABC News.
The JAMA study, which used data from the Norwegian Mother and Child Cohort Study, found that mothers who took folic acid four weeks before and eight weeks after pregnancy had a 40 percent reduced risk of giving birth to a child with autism. While the researchers found an association between folic acid deficiency and autism, that does not mean that folic acid taken during pregnancy would result in fewer autism cases.
"We know that folic acid deficiency leads to defects in the development of the nervous system," said Dr. Ari Brown, a pediatrician in Texas, who was not involved in the JAMA study, referring to spina bifida. "So it would not be surprising that a deficiency might also affect brain development in other ways."
The Norwegian Mother and Child Cohort Study followed more than 85,000 babies born between 2002 and 2008, and their parents. About 270 babies whose parents participated in the study were born with a developmental disorder on the autism spectrum.
Mothers reported whether they were taking folic acid before and during early pregnancy before they found out whether their children had autism, which eliminated some potential bias, said molecular epidemiologist Rebecca Schmidt, a professor at the University of California, Davis School of Medicine. In 2011, Schmidt was one of the first scientists to publish a study that found that autism arises because of both genetic and external factors, including women's prenatal vitamin intake before conception.
"Given the replication of findings showing reduced risk of autism associated with folic acid supplements taken near conception, more research is needed to investigate whether this association is causal," she said. "Interestingly, both studies reported … a nearly 40 percent reduction in risk for autism."
The number of children with autism spectrum disorders in the United States rose to one in 88 in 2012, up from one in 110 in 2006, according to the Centers for Disease Control and Prevention.
It is not clear whether mothers who did not take folic acid had other risk factors for bearing children with autism, said Dr. Schlomo Shinnar, a professor of neurology at the Albert Einstein College of Medicine in New York. He said that the Norwegian study population was also more homogeneous than the population in the 2011 University of California, Davis study, and that Norway might have different diagnostic tests for autism spectrum disorders than we do in the United States.
"The findings are of great interest," Shinnar said.
Alfred Romeo, a counselor at the Organization of Teratology Specialists affiliate in Utah, said he works on a hotline set up for pregnant women who are worried about how their medications, vaccines and food choices will affect their unborn children. Romeo's center gets about 9,000 calls a year from 10 states. "Autism is a hot topic," he said. "We're watching all the research studies to see if anything increases or decreases the rate of autism." The organization has not yet reviewed the JAMA study.
Ashton said she tries to help the patients in her OB/GYN practice to weigh the benefits and risks of things that might affect their pregnancies, such as medications, chemical exposure or foods. She said she knows from her own pregnancy how confusing and frightening it can be, and she aims to alleviate some of that by reassuring mothers that fetuses are resilient.
"Society can sometimes do a really good job of laying blame and guilt, and when there is no medical proof that it is the mother's fault," she said. "I usually tell women pregnancy is no different than parenting. There are never 100 percent guarantees of anything."