Older mothers and fathers are more likely to have a child with autism, according to a new US study.
In the study, researchers found that first-born children of older parents were three times more likely to develop autism than later children of younger parents.
"The results provide the most compelling evidence to date that autism risk increases with both maternal and paternal age, and decreases with birth order," the Telegraph quoted researchers, as saying.
"The increase in autism risk with both maternal and paternal age has potential implications for public health planning and investigations of autism," they added.
In the study, the researchers examined records of 240,000 children born in 1994, and identified 1,251 as having autism.
After adjusting for a range of factors, both maternal and paternal age were found to be independently linked to autism.
The study revealed that for mothers aged 35 or over, the risk of having an autistic child was 30 per cent higher than for mothers aged 25 to 29.
Mothers under 20 had a 30 per cent lower risk than those aged 35 to 29. Fathers aged 40 or over had a 40 per cent higher risk of having an autistic child than fathers aged 25 to 29.
However, there is no clear explanation for why parental age increases the risk of having an autistic child.
The study is published in the American Journal of Epidemiology.