Women who give birth after age 40 face a higher risk of having an autistic child, regardless of the father’s age, according to a comprehensive study of all births in the state of California in the 1990s. Researchers from the University of California, Davis, found that a woman who gave birth after age 40 was 50% more likely to have an autistic child compared with women who gave birth between ages 25 and 29. These findings add to previous research indicating a correlation between advanced maternal age and a child’s autism risk, but clarify how both a mother and father’s age contributes to this risk.
Researchers found that a woman’s age when she gives birth is directly related to the child’s autism risk—for every five additional years of maternal age, the likelihood of giving birth to an autistic child increased by 18%. While previous studies have shown that a father’s advanced age can also increase autism risk, this study found that, for women over age 30, the father’s age makes no difference. It is only when men over age 40 parent children with women age 30 or younger than the father’s age increases autism risk, researchers found. When women under age 25 had children with men over 40, for example, those children faced double the risk of being diagnosed as autistic compared with children born to a mother under age 25 and a father between ages 25 and 29.
During the study period, from January 1, 1990 to December 31, 1999, the number of women in California who gave birth after age 40 increased by 300%. Yet, during that same decade, autism cases in the state surged—by some 600%. While this latest study adds to evidence for a link between maternal age and autism risk, the authors point out that the higher rate of older women giving birth can only account for a small fraction—5%—of the massive increase in autism cases.
The study, published in the February issue of the journal Autism Research included 4.9 million children born in California during the 1990s.