Women who reported not taking prenatal vitamins immediately before and during a pregnancy were nearly twice as likely to have a child with autism, compared to women who did, according to UC Davis researchers.

And if the woman also had a mutation in a high-risk gene, that risk jumped to seven times.

Participating families were asked whether they took prenatal vitamins, multivitamins or other supplements at any time during the three months prior to and during their pregnancies and breastfeeding, and at what dosage.

The researchers found that for mothers who took supplements prenatally or during the first month of pregnancy, their risk of having an autistic child was cut in half, compared to those who didn't.

However, for moms who began using vitamins in the second month of pregnancy, there was no effect. This suggests that by the time most women learn that they are pregnant, starting on supplements has little or no help in terms of autism risk.

Prenatal supplements also look to be especially effective for genetically susceptible moms-to-be and their babies, and two high-risk genes have been linked: The methylenetetrahydrofolate reductase (MTHFR) gene and catechol-O-methyltransferase (COMT) gene. For women with the MTHFR variant who reported not taking prenatal vitamins, the risk of having an autistic child was 4.5 times normal. For those who did not take vitamins and had the COMT variant, the risk was 7 times normal.

The authors postulate that folic acid in prenatal supplements likely protect against deficits in early fetal brain development. Folate is known to be vital for neurodevelopment and studies have found that supplemental folic acid has the potential to prevent up to 70 percent of neural tube defects, the authors add.

Further research will be needed to rule out any biased reporting, but "the good news is that if this finding is replicated, it will provide an inexpensive, relatively simple evidence-based action that women can take to reduce risks for their child, which is to take prenatal vitamins as early as possible in a pregnancy and even when planning for pregnancy," says epidemiologist Irva Hertz-Picciotto, the senior author of the study.