Older parents appear to be more at risk of having autistic children and the risk is seen with both mothers and fathers, new research shows.

'What we found was that actually it is both the parents' ages and when you control for one parent's age, you still see the effect of the other parent's age and vice versa," said

Dr Maureen Durkin of the University of Wisconsin School of Medicine and Public Health in Madison, the study's lead researcher.

The findings, reported in the American Journal of Epidemiology, may offer clues to understanding the causes of autism and why it is on the rise, but they should not be used to guide family planning decisions, Dr 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, there is a 97 per cent chance that the higher-risk child will be perfectly fine.

'The vast majority of children do not develop autism," she emphasised.

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 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 US Centers for Disease Control and Prevention's Autism and Developmental Disabilities Monitoring Network.

Adding up the risks

After the researchers accounted for factors that might influence the results, they found that children born to mothers aged 35 and older were 30 per cent 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 per cent.

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 mothers are more likely to have autistic kids, but Dr 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 mums and dads are at greater risk of having autistic children. 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 behavioural traits or psychological make-up of people who wait to have children that boosts autism risk in their offspring.

Reuters

This article was first published in The Straits Times on Dec 18, 2008.