First-born children of older parents were three times more likely to develop autism than later children of younger parents, researchers found.

First-born children of older parents are more likely to be autistic
A major US study has provided the strongest evidence yet that older mothers and fathers are more likely to have a child with autism.

By Roger Dobson
Last Updated: 7:48PM BST 25 Oct 2008

First-born children of older parents were three times more likely to develop autism than later children of younger parents, researchers found.

"The results provide the most compelling evidence to date that autism risk increases with both maternal and paternal age, and decreases with birth order," according to the research team, whose study was funded by the US Centers for Disease Control and Prevention.

The findings could help to provide an explanation for the recent increase in the number of children diagnosed with autism, which has coincided with a trend towards parents having children at an older age, and to smaller family sizes.

The researchers say in their paper, reported in the American Journal of Epidemiology: "The increase in autism risk with both maternal and paternal age has potential implications for public health planning and investigations of autism."

In the study, the researchers examined records of 240,000 children born in 1994, and identified 1,251 as having autism. After taking into account a range of factors, both maternal and paternal age were found to be independently associated with autism.

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.

Many western countries have seen a trend for women to have their first child later in life, while the decline in average family size means that a higher proportion of today's children are first-born.

There is no clear explanation for why parental age increases the risk of having an autistic child. The researchers speculate that older fathers could be more likely to pass on gene mutations, while older mothers may be more prone to age-related chromasome changes or pregnancy complications. Older parents are also more likely to have undergone fertility treatment.

Another theory is that older parents may be more aware of developmental abnormalities, and therefore more likely to seek treatment and have their child diagnosed with disorders such as autism.

Firstborn children are also more prone to suffer from other childhood disorders, including type I diabetes. One possible explanation is the so-called "hygiene hypothesis", which suggests that firstborns are exposed to fewer infections from other children early in childhood and as a result are more likely to develop autoimmune diseases.