Page last updated at 23:00 GMT, Tuesday, 30 June 2009 00:00 UK

Problem pregnancy 'autism risk'

Boy with autism
There has been an increase in the number of autism diagnoses

Complications during pregnancy and giving birth later in life may increase the risk of having a child with autism, a review of dozens of studies suggests.

Researchers found the bulk of studies into maternal age and autism suggest the risk increases with age, and that fathers' age may play a role too.

The mothers of autistic children were also more likely to have suffered diabetes or bleeding during pregnancy.

The US review of 40 studies appears in the British Journal of Psychiatry.

The recorded number of children with autism has risen exponentially in the past 30 years but experts say this is largely due to improved detection and diagnosis, as well as a broadening of the criteria.

The cause of the condition is unclear, and the review team from the Harvard School of Public Health said there was "insufficient evidence" to point to any one prenatal factor as being significant.

Sperm mutations

They did however note that nine out of 13 studies suggested an increased risk for older mothers, a demographic group which has grown in the last three decades.

This ranged from a risk 27% higher for those aged between 30-34 compared to those aged 25-29, and over 100% higher for those over 40 compared to those under 30.

For fathers, every five years increased the chances of a child with autism by nearly 4%.

It is like trying to complete a huge jigsaw puzzle - we still just don't know how all the pieces fit together
Richard Mills
Research Autism

The biological reasons for why this may be are unclear, but the researchers speculated that potential chromosomal abnormalities in the eggs of older women and mutations in the sperm of older men may be a factor.

Gestational diabetes - which affects four in 100 pregnancies - was associated with a two-fold increase in the risk of autism, while bleeding in pregnancy was alleged to carry an 81% increased risk.

However, the team noted that there was little information given about when in pregnancy bleeding occurred. Common and often inconsequential in early pregnancy, later on it can signify serious problems.

Such bleeding may deprive the baby of oxygen - a condition known as fetal hypoxia - and this is turn impacts upon the developing brain, potentially raising the risk of autism.

The team also found associations with medication use, with a particularly strong link with drugs for psychiatric problems.

However, they acknowledged it was impossible to tell whether this was a result of the medication itself or the genetic traits which may be shared between autism and conditions requiring such treatment.

Working together

Researchers said the key challenge was to work out how genetics and the environment interacted with each other to produce autism.

"The rising prevalence, coupled with the severe emotional and financial impact on the families, underscores the need for large, prospective, population-based studies with the goal of elucidating the modifiable risk factors, particularly those during the prenatal period," wrote lead author Hannah Gardner.

"Future investigations of prenatal exposures should also collect DNA to study potential gene-environment interactions."

Richard Mills of Research Autism said such reviews of existing studies were "very useful indeed".

"Age is a very interesting line of inquiry, but it is very hard to tease out one clear factor. It is like trying to complete a huge jigsaw puzzle - we still just don't know how all the pieces fit together."



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