In the 1990s, the number of California women giving birth over the age of 40 jumped 300-percent and the diagnosis of autism grew 600-percent.
It might appear one is responsible for the other, but the numbers don't add up the way you'd think.
"When you really look at the data that's being reported, it's really a small increase," said Dr. Larry Yin.
Dr. Yin is the medical director for the Boone Fetter Clinic at Children's Hospital L.A. It's a unique autism practice where doctors from different specialties team up to fully assess a child's situation. He says the new UC Davis study of 5 million births adds to the mounting evidence that genetics play a role.
"There may be that there are multiple causes, but today we don't really have that answer," said Dr. Yin.
So the older a woman is the higher their risk for having a child diagnosed with autism. According to the study, a woman who is 40 years old has a 50 percent greater risk than a woman who is 25.
"But when you take a look at the big picture it's really only accounts for 4 percent of children who had autism who had mothers over 40," said Dr. Yin.
But the most surprising part of the study is that a mother under 30 who has a child with a man over 40 has an even greater risk of having a child with autism than parents both over 40.
"That actually had the highest risk for having a child with autism. Why that is nobody really knows," said Dr. Yin.
So should couples consider age when they're trying to have a baby? Dr. Yin says not any more than you would for other conditions.
He says much more research has to be done before doctors can make any recommendations. The report is in the Journal Autism Research.