Older mothers are more likely to give birth to a child with autism, largely irrespective of the father’s age, one of the biggest population-based studies of parental age and autism has found.

The risk of having a child with autism increases by 18 percent for every five years a woman ages before giving birth, according to research published online Monday in the journal Autism Research. That means a 40-year-old woman is 50 percent more likely to have a child with autism than a mother who is age 25 to 29.

A father’s age only seems to matter when the mother is under age 30, but the father is older. For example, children of mothers under age 25 and fathers over 40 are twice as likely to have autism as compared to children with young mothers and fathers.

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“This study challenges a current theory in autism epidemiology that identifies the father’s age as a key factor in increasing the risk of having a child with autism,” said Janie Shelton, the study’s lead author and a doctoral student at the University of California, Davis. “It shows that while maternal age consistently increases the risk of autism, the father’s age only contributes an increased risk when the father is older and the mother is under 30-years-old. Among mothers over 30, increases in the father’s age do not appear to further increase the risk of autism.”

The findings are based on California birth records from 1990 to 1999 which were cross-referenced with autism diagnosis information from state regional centers. Ultimately researchers focused on 4.9 million births, resulting in 12,159 cases of autism.

It remains unclear why children of older mothers are more likely to have autism. Nonetheless, parental age is likely just one factor, researchers say. While autism diagnoses increased 600 percent among Californians in the 1990s, parental age likely only contributed to about 5 percent of the increase.