Older parents may be more likely to have a child who develops autism, a new study suggests.

This risk is particularly associated with mothers over the age of 30.

Autism, also known as autism spectrum disorder (ASD), is a biological disorder of the brain that impairs communication and social skills. Signs can include an absence of eye contact, apparent aloofness, avoidance of physical contact even with family, difficulty dealing with interruptions to routine and a lack of interest in other children and what they are doing.

US and Swedish scientists set out to investigate the impact of parental age on the condition. They analysed data relating to over 417,000 children born in Sweden between 1984 and 2003.

The study found that a man's risk of having a child who developed autism increased with age across his lifespan. However, the relationship was more complex in women.

Before the age of 30, the risk of a woman having a child with autism was very low. However, after the age of 30, this risk rose rapidly.

The scientists called for further research in this area.

"The open question at hand really is, what biological mechanisms underlie these age effects?" asked the study's lead author, Dr Brian Lee, of Drexel University in Philadelphia.

However, he reminded people that the overall risk of having a child who develops autism remains low.

"The absolute risk of having a child with ASD is still approximately one in 100 in the overall sample, and less than two in 100 even for mothers up to age 45,", he said.

Details of these findings are published in the International Journal of Epidemiology.