Women over 40 are nearly twice as likely to give birth to an autistic child than a mother under 30, researchers said Monday in a study that 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 unless the father is older and the mother is younger than 30, according to the 10-year study, which examined 4.9 million births in the 1990s.

"This study 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," said study lead author Janie Shelton.

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

The study, published in the February issue of the journal Autism Research, found that the incremental risk of having an autistic child increased by nearly a fifth -- 18 percent -- for every five-year increase in the mother's age.

The risk for a woman over 40 to have a child later diagnosed with autism was 50 percent 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 behavior.

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 incidence of autism in the United States is estimated at between one in 100 and one in 110 children in the United States. Onset of the disorder begins prior to age three.

The number of California women over 40 giving birth increased by more than 300 percent during the 1990s, but the number of autism cases increased by 600 percent. Yet the authors found that older mothers only accounted for about five percent 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 fetal 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," explained senior author Irva Hertz-Picciotto.

A 2008 UC Davis study also found that some mothers who gave birth to autistic children had antibodies to fetal 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.