Older mothers are more likely than younger ones to have a child with autism, and older fathers significantly contribute to the risk of the disorder when their partners are under 30, researchers are reporting.

In 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.