A new study appears to shed light on why older mothers have an increased risk of having a child with autism.

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 others and what they are doing.

It is already known that many people with autism have gene mutations that are responsible for the condition. However recent research suggests that these mutations may explain only half the risk for developing autism, with the other half related to environmental factors, such as a pregnant woman's stress levels and conditions in her womb.

US scientists decided to look into the role of age more. Fathers aged 40 and older are more likely to have children with autism than younger fathers. This is thought to be related to gene mutations that accumulate over time in their sperm-making cells.

However, mothers over the age of 35 have a similar risk and until now, researchers have known very little about why this is. The scientists decided to assess both genetic and environmental influences that might account for this increased risk.

They monitored the progress of 47 children with ASD and 48 without. All mothers of the children were over the age of 35. They analysed the buccal epithelial cells that line the inside of the cheek to assess genetic and environmental differences between the children.

They found two groups of genes that were distinctive in children with ASD. These two gene groups tended to interact with genes that were already known to be mutated in those with ASD.

"Genes interact with each other to create molecular pathways that carry out important functions. Our findings suggest that, at least in some individuals with an ASD, the same pathways in the brain seem to be hit by both mutations and epigenetic changes. So the severity of someone's ASD may depend on whether or not a gene mutation is accompanied by epigenetic alterations to related genes," they suggested.

However, are environmental influences responsible for the changes that lead to these genes becoming unregulated, the scientists wondered. By eliminating other potential causes, such as chromosomal abnormalities, they concluded that environmental factors have an important role to play.

"In the case of older mothers at risk for having children with ASD, one possible environmental influence might be the aging process itself, which could disturb epigenetic patterns in their eggs. Although much more work is needed, our study reveals a plausible way that environmental influences - which we know are important in ASD - might be exerting their effects," the team added.

Details of these findings are published in the journal, PLOS Genetics.