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.
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