LOS ANGELES – Women who give birth after age 40 are nearly twice as likely to have a child with autism as those under 25, but it is unlikely that delayed parenthood plays a big role in the current autism epidemic, California researchers reported Monday.

The findings were expected to draw attention, but their true impact was expected to be simply in suggesting further avenues of research.

Surprisingly, the age of the father plays little role in the likelihood of the disorder unless the mother is younger than 30 and the father is over 40, according to the analysis of all births in California in the 1990s.

The number of women over age 40 in California giving birth increased 300 percent in the 1990s, while the diagnosis of autism increased 600 percent.

At first glance, it might seem that the rise in older pregnancies could be responsible for the jump in autism, which is now thought to affect as many as one child in every 100. But the authors of the University of California-Davis paper calculate that older mothers account for less than 5 percent of the increase in autism diagnoses. The rise was seen across every age group.

"There is a long history of blaming parents" for the development of autism, said senior author Dr. Irva Hertz-Picciotto, a professor of public health sciences and a researcher at the UC Davis MIND Institute. "We're not saying this is the fault of mothers or fathers. We're just saying this is a correlation that will direct research in the future."

Researchers have known that the age of the parents plays a role in a child's risk of autism, but how big a role and how that role varies with the sex of the parent is uncertain, with contradictory results reported in different studies.

The California researchers analyzed all single births in California during the 1990s for which information was available about the ages of both parents, a total of about 4.9 million births and 12,529 cases of autism.

Because of the large sample size, they were able to show how the risk of autism was affected by each parent's age. They reported in the February issue of the journal Autism Research that women over 40 were 77 percent more likely to deliver an autistic child than those younger than 25 and 51 percent more likely than those ages 25 to 29, independent of the age of the father.

For men over 40, there was a 59 percent increased risk of autism if the mother was younger than 30, but virtually no increased risk if the mother was over 30.

The researchers also calculated that the recent trend toward delayed childbearing contributed about a 4.6 percent increase in autism diagnoses over the decade.

"Five percent is probably indicating that there is something besides maternal age going on because we are seeing a rise in every age group of parents," UC Davis graduate student Janie E. Shelton, one of the researchers, said. "We can't say if it is age or something that is a proxy for age," such as lifetime exposure to pollutants.

Thomas H. Maugh II,

Los Angeles Times