Print edition: 
LOS ANGELES — Women who give birth after age 40 are nearly twice as likely to have a child with autism as those under 25, California researchers reported Monday.
The father's age plays little role in the disorder's likelihood 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 one child in every hundred. But the paper's authors, 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 Irva Hertz-Picciotto of the UC Davis MIND Institute, the report's senior author. "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 long thought that the parents' age plays a role in the risk of developing autism, but how big a role and how that role varies with the sex of the parent has remained confusing, with contradictory results reported in different studies.
To investigate, Hertz-Picciotto, Janie Shelton and Daniel Tancredi analyzed all the singleton 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 likelihood of autism was affected by each parent's age. They reported in 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 aged 25 to 29, independent of the father's age.
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," Shelton said. "We don't know what the biology is. ... We can't say if it is age or something that is a proxy for age," such as lifetime exposure to environmental pollutants, which accumulate in the body over the years.
Also, noted Hertz-Picciotto, older women are more likely to have problems with fertility and require intervention. They may be followed more closely during pregnancy, which would mean more ultrasounds. They are more likely to suffer gestational diabetes and to develop autoimmune disorders, which have been shown to play a role in autism. All are fertile areas for further research.
"We still have a real long way to go" in determining the causes of autism, she concluded.
