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 autism epidemic, California researchers reported Monday.
The findings are expected to draw widespread attention because of the intense public interest in autism, but their true impact is expected to be simply in suggesting further avenues of research.
Surprisingly, the father's age 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 by 300 percent in the 1990s, while the diagnosis of autism increased by 600 percent.
At first glance, it might seem that the rise in older pregnancies could be responsible for the rise in autism, which is now thought to affect as many as 1 child in 100.
But the authors of the paper, from the University of California, Davis, calculate that older mothers account for less than 5 percent of the increase in autism diagnoses.
"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."