"Parental age doesn't appear to be synergistic. That is having an older mom and an older dad doesn't increase risk more than having one or the other," said Marissa King, a professor at the Yale School of Management, who was not involved in the study.

"The data clearly demonstrate that older parents are more likely to have kids with autism, but it doesn't establish why that is the case," King told Reuters Health in an email.

The findings throw a monkey wrench into the idea that perhaps older sperm or eggs have more mutations that could increase the odds of having a child develop autism.

Erik Thorlund Parner at the University of Aarhus School of Public Health in Denmark said that if genetic problems arising from older sperm or eggs explain the results, then having both an older sperm and an older egg together should total up to an even higher risk of autism for the child.

Yet Parner, who led the study, and his colleagues did not find a higher risk of autism among kids with two older parents compared with just one.

The researchers collected information on more than 9,500 children in Denmark who had been diagnosed with an autism spectrum disorder.

These disorders range from mild Asperger's Syndrome to severe mental retardation and social disability.

In the new study, kids born to fathers in their late thirties had up to a 28 percent higher risk of being diagnosed with an autism spectrum disorder than kids born to dads under age 35. The increase in risk was not tied to the mother's age.

For kids born to dads over age 40, the risk for developing an autism spectrum disorder was 37 to 55 percent greater than for kids of dads under age 35, and the mother's age didn't seem to matter.

According to the Centers for Disease Control and Prevention, about 10 in 1,100 children have an autism spectrum disorder. A 55-percent increase in risk, for instance, would push that number up to about 15 out of every 1,100 children.

Similar to the findings on dads, children whose moms had them in their late 30s were 21 to 37 percent more likely to develop autism than kids with moms under age 35, and the dad's age did not change the numbers.

And just like with dads over age 40, kids born to moms over age 40 were 28 to 65 percent more likely to have autism than kids with moms under age 35, regardless of the age of their dad.

"The result was surprising in that there was no additive effect of maternal and paternal age," Parner said in an email to Reuters Health.

"We do not really have a qualified guess on the explanation to the non-additive effect," he said. "I don't believe that one can yet say anything conclusive about whether (having an) older parent is biologically related to autism."

Some earlier studies have also found that older parents are more likely to have a child with autism, but they have not had consistent results.

For example, one study found that having an older mother increases a child's risk of autism, but having an older father only increases the risk if the mom is under the age of 30 (see Reuters Health report of February 8, 2010).

Parner said he and his colleagues are planning on an even larger analysis of autism and parental age using data from Denmark, Finland, Israel, Norway, Sweden and Western Australia.

SOURCE: bit.ly/wFRsAK Annals of Epidemiology, online January 24, 2012.

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