The findings were expected to draw widespread attention because of the intense public interest in autism, but their true impact was expected to be simply in suggesting further avenues of research.
The number of women over age 40 in California giving birth increased by 300% in the 1990s, while the diagnosis of autism increased by 600%. 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 100. But the authors, from UC Davis, calculate that older mothers account for less than 5% 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 who has been studying potential causes for the autism increase. "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 known that the age of the parents plays a role in a child's 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, graduate student Janie E. Shelton and epidemiologist Daniel J. Tancredi of UC Davis 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 12,159 cases of autism.
Because of the large sample size, they were able to show how the risk was affected by each parent's age. They reported in the February issue of the journal Autism Research that women over 40 were 77% more likely to deliver an autistic child than those younger than 25 and 51% more likely than those age 25 to 29, independent of the age of the father.
For men over 40, there was a 59% 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% 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.
Also, noted Hertz-Picciotto, older women may be followed more closely during pregnancy, which would mean more ultrasounds - which a few researchers have suggested might play a role in autism. Older women are more likely to suffer gestational diabetes and to develop autoimmune disorders, both of which have been linked to an increased risk of autism.
"We still have a real long way to go" in determining the causes of autism, she concluded.