Autism risk rises with mother's age

8th February 2010 - The older a mother is when she gives birth, the higher her child's risk of autism, new research shows.

A smaller effect is also seen for the age of the father, but only when the child’s father is over the age of 40 and the mother is under the age of 30.

The new findings come from a comparison of reported autism cases in the US state of California using birth records from 1990 to 1999. Over that time, there were nearly five million births and more than 12,000 autism cases.

The study results

• Women over 40 were 77% more likely than women under 25 to have a child with autism.
• Women over 40 were 51% more likely than women aged 25-29 to have a child with autism.
• Women aged 35-39 were 31% more likely than women aged 25-29 to have a child with autism.
• Women aged 30-34 were 12% more likely than women aged 25-29 to have a child with autism.
• Women under 25 were 14% less likely than women aged 25-29 to have a child with autism.
• Men over 40 were twice as likely as men aged 25-29 to have a child with autism, but only if the mother is under 25.

Understanding the trend

So, is the trend for women to delay childbirth behind the continuing rise of autism? That's not the case, say the researchers from the University of California. This accounts for less than 5% of the autism increase in the data studied.

So what's going on? That still isn't clear. Older parents' genes can undergo changes caused by ageing and by the environment.

"We need to understand biologically why this is happening," lead author Janie Shelton tells us, saying it would be premature to tell older mums not to have a child. “It could be the risk is associated with an exposure, and avoiding the exposure would be more important than not having kids at age 40."

Exactly what is a woman's risk of having a child with autism? The figures from the study come from 1990 to 1999. Autism cases increased throughout that decade - and throughout the next decade, too.

From 1990 to 1999, the risk of having a child with autism in California was:

• 1.6 per 1,000 women under 25
• 2.3 per 1,000 women aged 25-29
• 3.1 per 1,000 women aged 30-34
• 3.85 per 1,000 women aged 35-39
• 4.4 per 1,000 women aged 40 and older

The paper appears online ahead of publication in the journal Autism Research.

It's clear that a parent's age is only part of the autism puzzle, say autism experts.

This study does not say the advanced age of a mother or father causes autism, rather that it is one risk among many, alongside other genetic and environmental factors that lead to a child developing autism.

Autism in the UK

Around one in every 100 children in England has an autistic spectrum disorder (ASD).

Boys are three to four times more likely than girls to have ASDs.

Like in California, the NHS says the number of diagnosed ASD cases in the UK has increased over the past two decades. However, the condition may not actually be becoming more widespread. Some experts believe that doctors are now better at diagnosing ASD cases which has increased the number of cases identified.