Younger siblings of children with autism are less likely to be diagnosed with the disorder if their mothers take prenatal vitamins during the first month of pregnancy, a new study suggests.

The researchers at University of California, Davis believe that it’s the first study to suggest that prenatal vitamin use may reduce the risk of autism recurrence in high-risk families.

The study, published Wednesday in JAMA Psychiatry, looked at more than 240 children whose older siblings had autism.

Children of mothers who took prenatal vitamins in the first month of pregnancy were half as likely to be diagnosed with an autism spectrum disorder, the study found. Those who were found to be on the spectrum had “significantly lower” autism symptom severity and higher cognitive scores.

“In highly genetically susceptible families where they’re already affected by autism, our study suggests there might be a way to … mitigate that risk,” said Rebecca Schmidt, an assistant professor at UC Davis Department of Public Health Sciences and co-author of the study.

She said researchers still need to find out exactly how prenatal vitamins and supplements are working to reduce the risk of autism.

“We know that some of the nutrients in prenatal vitamins, like folic acid, are really critical during brain development and it’s also critical for many of these pathways that might influence autism risk,” she said.

The UC Davis researchers recruited participants from the Markers of Autism Risk in Babies: Learning Early Signs (MARBLES) cohort study in California. The study looked at younger siblings of kids with autism born between 2006 and 2015.

The study found that a vast majority of mothers reported taking a prenatal vitamin at some point during their pregnancy, but only 34 per cent took the vitamins before becoming pregnant.

Mothers who took prenatal vitamins in the first month of pregnancy had more education, were more likely to own their homes and have private health insurance. They were also more likely to have a planned pregnancy compared to mothers who did not take prenatal vitamins before they got pregnant, researchers said.

Schmidt said the findings reinforce the importance of prenatal care, both before and during pregnancy.

“It’s recommended already that moms take prenatal vitamins, or folic acid supplements, starting before pregnancy to catch this early developmental window,” she said.

Dr. Lonnie Zwaigenbaum, a pediatrics professor and autism researcher at the University of Alberta, said the latest study is consistent with previous research that linked prenatal vitamins to a reduced risk of autism, both in the general population and among families already affected by the disorder.

He called the study results “remarkable.”

“For the growing number of families who have a child with autism and are looking ahead to future pregnancies, they’re really demonstrating that the potential reduction in absolute risk is really quite significant,” he told CTV News.

Previous research has shown that younger siblings of children with an autism spectrum disorder are about 13 times more likely to develop ASD than the general population.

According to the Public Health Agency of Canada, an estimated one in every 66 Canadian children and youth aged five to 17 has autism spectrum disorder.