Women who delay pregnancy are more likely to have a child with autism, a major study suggests.
The research found the risk to be 50 per cent higher for a woman of 40 than it is for a woman in her late 20s.
A number of studies have made a similar link but it had been unclear whether the age of the mother or the father was the more important.
The latest investigation by the University of California looked at all births in the state during the 1990s.
The risk of having a child with autism increased by 18 per cent - nearly one fifth - for every five-year increase in the mother's age. The extra risk was less evident with older fathers.
Janie Shelton, the study's lead author, said: 'This challenges a current theory in autism epidemiology that identifies the father's age as a key factor in increasing the risk of having a child with autism.
'It shows that while maternal age consistently increases the risk of autism, the father's age only contributes an increased risk when the father is older and the mother is under 30 years old.
'Among mothers over 30, increases in the father's age do not appear to further increase the risk of autism.'
Autism is an umbrella term for a range of developmental disorders that have a lifelong effect on the ability to interact socially. It is generally taken to affect one in 100 British children - a figure that some researchers think is too low.
The California study covered 4.9million births and 12,159 cases of autism, according to the report published online in the journal Autism Research.
For older mothers, the progression in the risk of having a child who later would be diagnosed with autism was apparent whatever the age of the father.
When the mother was significantly younger than the father the child's chance of developing autism rose. Children born to mothers under 25 and fathers over 40 were twice as likely to develop autism as those whose father was between 25 and 29.
The study was comprehensive in that it investigated how each parent's age - separately and together - was connected with the risk of autism.
Irva Hertz-Picciotto, a professor of public health sciences who contributed to the study, said it was not clear why having an older parent placed a child at risk for autism.
'We still need to figure out what it is about older parents that puts their children at greater risk for autism and other adverse outcomes, so that we can begin to design interventions,' she said.
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