Women over 40 are nearly twice as likely to give birth to an autistic child than a mother under 30, researchers say.

A new study has found more evidence of links between autism and maternal age.

The father's age had nearly no impact on the child's risk of autism, according to the 10-year study that examined 4.9 million births in the 1990s.

The study's lead author, Janie Shelton, said the research challenged a current theory in autism epidemiology that identified the father's age as a key factor in increasing the risk of having a child with autism.

She and her fellow University of California researchers examined data from all births in their State for a decade.

The study found that the incremental risk of having an autistic child increased by nearly one-fifth - 18 per cent - for every five-year increase in the mother's age.

The risk of a woman over 40 to have a child later diagnosed with autism was 50 per cent greater than for a mother aged between 25 and 29.

For mothers over 30, an older father did not appear to further increase the risk of autism, a pervasive developmental disorder of deficient social and communication skills, as well as repetitive and restricted behaviour.

When the father was older and the mother was under 30, the child also had a high risk of developing autism, according to the study.

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. But that risk dissipated among mothers over 30.

The number of California women over 40 giving birth increased by more than 300 per cent during the 1990s, but the number of autism cases increased by 600 per cent.

Yet the authors found that older mothers only accounted for about 5 per cent of the increase.

Prior studies have also linked the parents' age to a child's risk for autism, but with contradictory results on whether the mother, the father or both played the biggest role.

They have also observed that advanced maternal age can contribute to a broad spectrum of other birth-related conditions, such as infertility, low birth weight, early foetal loss and chromosome problems.

"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," senior author Irva Hertz-Picciotto said.

A 2008 UC Davis study also found that some mothers who gave birth to autistic children had antibodies to foetal brain protein, unlike mothers of typical children.

The study authors suggested that some persistent environmental chemicals that accumulate in the body may be to blame in part.