A new study says a woman's chance of having a child with autism increases as she ages. It also says the risk may be less with older dads than previously thought.

University of California at Davis researcher Janie Shelton is the lead author. She says that although the father's age can contribute, the risk is overwhelmed by the mother's age.

Mothers older than 40 were about 50 percent more likely to have autistic children than those in their 20s. The risk for fathers older than 40 was 36 percent higher than for men in their 20s.

Experts say older parents shouldn't be overly anxious, however, because the risk of a woman over 40 having an autistic child is still less than 4 in 1,000.