By Anne Harding

NEW YORK (Reuters Health) - Advanced parental age does indeed appear to boost autism risk in children, and the risk is seen with both mothers and fathers, new research shows.

"What we found was that actually it's both parents age, and when you control for one parent's age you still see the effect of the other parent's age, and vice versa," Dr. Maureen Durkin of the University of Wisconsin School of Medicine and Public Health in Madison, the lead researcher of the study reported in the American Journal of Epidemiology, told Reuters Health.

The findings may offer clues to understanding the causes of autism and why it's on the rise, but they shouldn't be used to guide family planning decisions, Durkin said. Even though the oldest child born to two older parents is three times as likely to be autistic than a middle or youngest child with younger parents, she explained, there's still a 97 percent chance that the higher-risk child will be perfectly fine. "The vast majority of children don't develop autism," she emphasized.

Several studies have suggested links between a father's age or the age of both parents and a child's likelihood of having autism. The current study included twice as many autism cases as any other research on this issue to date, which made it possible to tease out the effects of both maternal and paternal age.

The researchers looked at 253,347 children born in 1994 at 10 sites included in the Centers for Disease Control and Prevention's Autism and Developmental Disabilities Monitoring Network. There were 1,251 children who met standard criteria for an autism spectrum disorder at age 8 for whom information on both parents' age was available.

After the researchers accounted for factors that might influence the results, they found that children born to mothers aged 35 and older were 30 percent more likely than those whose mothers were 25 to 29 years old to have been diagnosed with autism. Having a father who was 40 or older boosted risk by 40 percent.

The effects of parental age were additive; firstborn kids with two older parents were at more than triple the risk of autism compared to third or later children born to mothers 20 to 34 years old and fathers under 40.

Past studies have suggested that more educated moms are more likely to have autistic kids, but Durkin and her team found this was because these women were older than less educated women, not because they had more years of schooling.

There are several possible explanations for why older moms and dads are at greater risk of having autistic children, the researchers say. Older parents have had a longer time to sustain genetic damage to their sperm or egg cells, as well as to store up environmental contaminants in their bodies.

They are also more likely to have used assisted reproduction technologies, which have been tied to poor pregnancy outcomes. And there could just be something about the behavioral traits or psychological makeup of people who wait to have children that boosts autism risk in their offspring.

The findings could also help explain why autism appears to be on the rise in the US, the researchers add, since the percentage of children who are born to mothers 35 and older and fathers 40 and older has risen steadily since 1980.

SOURCE: American Journal of Epidemiology, December 1, 2008.

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