A 10-year study looking at 4.9 million births in the 1990s showed that older mothers are more likely than younger ones to have a child with autism, and older fathers can contribute to the risk of the disorder when their partners are under 30.

The study, published online in the journal Autism Research is one of the largest population-based studies to quantify how each parent’s age — separately and together, affects the risk of having a child with autism.

The study found that for each five-year increase in a mother’s age, the risk of having an autistic child increased incrementally by 18%. In mothers 25 years and younger, there were 16 autism cases per 10,000 births. For 25 to 29-year-old mothers, there were 23 autism cases, for 30 to 34-year-old mothers, 31 autism cases, for 35 to 39-year-old mothers, 38, and 40 years old and up there were 44.

The father’s age had almost no impact on the child’s risk of autism unless the father is older and the mother is younger than 30.

Other research has showed that advanced maternal age is a risk factor for a variety of other birth-related conditions such as: miscarriage and a higher chance of developing pre-eclampsia and Down’s syndrome is more common if women give birth later.

Recent data shows one out of every 100 children in the United States is autistic.

Autism can causes concerns with social skills and communication, and is associated with repetitive and restrictive behaviour. Abnormal brain development in the womb is thought to trigger the disorder, which mostly affects boys.