“It’s important we not turn around and blame mothers,” Dr. Malaspina said. “The evidence is very, very strong that there is a paternal age effect.”

.Both Parents’ Ages Linked to Autism Risk

Health Guide: AutismIn a study published online on Monday in the journal Autism Research, the researchers analyzed almost five million births in California during the 1990s, and 12,159 cases of autism diagnosed in those children — a sample large enough to examine how the risk of autism was affected when one parent was a specific age and the other was the same age or considerably older or younger.

Previous research found that the risk of autism grew with the age of the father. The new study suggested that when the father was over 40 and the mother under 30, the increased risk was especially pronounced — 59 percent greater than for younger men.

By contrast, for women 30 and older, the risk of autism rose 13 percent when the father was over 40.

Every five-year increase in a mother’s age raised her risk of having a child with autism by 18 percent; a 40-year-old woman’s risk was 50 percent greater than that of a woman who became a mother in her late 20s, and 77 percent higher than that of a woman under 25.

But while the number of California women giving birth in their 40s rose sharply in the 1990s, the researchers said that could not account for the sevenfold rise in autism during the decade.

“The rise in autism is occurring among children of parents of all ages,” said Janie F. Shelton, a graduate student in epidemiology at the University of California, Davis, who was the paper’s lead author. “We can’t say that the shifting trend of maternal age is responsible for the increased rates of autism.”

The new findings appeared to question the conclusions of earlier research suggesting that the risk of autism spectrum disorders increased with advancing paternal age, but not with advancing maternal age.

One such study analyzed a large Israeli military database and found that children of fathers 40 or older were more than five times as likely to have an autism disorder as those whose fathers were under 30.

An author of that study, Dr. Dolores Malaspina, a psychiatrist at New York University Langone Medical Center, said Monday that mothers and fathers were usually so close in age that small statistical differences could appear to shift the effect of advanced age from one parent to another.

“It’s important we not turn around and blame mothers,” Dr. Malaspina said. “The evidence is very, very strong that there is a paternal age effect.”