Autism link to firstborn of older parents

Published on Saturday 25 October 2008 23:50

FIRSTBORN children of older parents have three times as much chance of being diagnosed with autism than later born children of younger parents, new research has revealed.

A study of 240,000 teenagers carried out by researchers in the US has found a strong link between the likelihood of developing the condition and a higher age of parents.

Mothers aged 35 and over were cited as having a 30% higher risk of having an autistic child than mothers in their twenties, while for fathers in their forties and over the risk was estimated to be even greater at around 40% higher when compared with fathers aged under 30.

The study also suggested that later born children were less likely to be afflicted with autistic disorders than firstborn offspring.

The research team behind the study has claimed their work provides "the most compelling evidence to date that autism risk increases with both maternal and paternal age, and decreases with birth order".

The study, published in the American Journal of Epidemiology, has been hailed as a possible explanation for the apparent increase in incidence of the condition in the western world, where social trends mean increasing numbers of couples put off having children until later in life, and tend to opt for smaller families – meaning a higher proportion of babies are firstborn or only children.

There is no clue to suggest why parental age should increase the risk of a child being autistic, but the authors suggest the sperm of older fathers could be more likely to pass on genetic mutations, while older mothers might be more susceptible to chromosome alterations. Another theory suggests that older parents may be more likely to spot developmental difficulties in their children, and therefore more likely to seek help and a formal diagnosis.

Firstborn children are also more prone to suffer from other childhood disorders, including type I diabetes. One theory to explain the phenomenon is the "hygiene hypothesis", which suggests that first-time parents are more protective and expose their children to fewer infections in early childhood, making them likely to develop autoimmune diseases.