A woman having a child at 40 has a 50 per cent greater chance of having a child diagnosed with autism than a woman of 25-29, a study has found. The study, published in Autism Research on 8 February 2010, showed the risk of having a child with autism increased by 18 per cent with every five year increase in the mother's age.

Increased parental age has been previously linked to a raised risk of autism, but researchers had been unable to determine whether the mother's, father's or both parents' age contributed most risk. For example, Science Daily reported a previous study that found fathers over 40 were six times more likely to have a child with autism than a father under 30.

The study, carried out in the USA was led by Janie Shelton from the University of California, Davis Department of Public Health Sciences. The researchers compared children born in California between January 1990 and December 1999 who developed autism against those who didn't.

Cases of autism were identified using records from routine examinations of children under three. The final sample was 12,159 with autism and a control group of 4,935,776. Information about parents' age and education, and the birth record information, was used to construct models of how parental age affected the risk of autism.

While the study highlights an important link, Irva Hertz-Picciotto, Professor of Public Health Sciences and the study's lead author told Science Daily: 'We still need to out what it is about older parents that puts their children at greater risk for autism and other adverse outcomes, so that we can begin to design interventions'.

Further research may examine whether persistent environmental chemicals accumulate in the mother's body, increasing the autism risk, according to Science Daily. A UC Davis study in 2008 found some mothers of children with autism had antibodies to a fetal brain protein other women did not have.

Autism is a disorder that affects a child's development and is typically connected to impaired communication and social skills. The prevalence of children with autistic spectrum disorders has been increasing in the US, raising questions about possible causes.