Then, she identified children born during that period that were eventually diagnosed with autism.
"We found that maternal age does increase the risk of autism and it's about 18 percent per five-year increment that she ages," Shelton said.
And by the time a woman reaches 40, the risk of having a child with autism is 50 percent greater than that of a woman in her mid or late 20s.
So, the likely question is, are we seeing more autistic kids because women are having children later in life?
The lead researcher says not necessarily.
"And we found that it was a very small percentage. Less than 5 percent of an approximate 600 percent increase in the case load," Shelton said.
600 percent represents the increase in the number of cases of autism in California during the 90s -- an increase that researchers haven't been able to explain.
The study released on Monday also found the risk of autism increased among older fathers but only when the mother was 30 years of age or younger.
Still, researchers don't know what it is about older parents that puts their children at greater risk of autism. But at UC Davis they're working on it.
Researchers are studying how genetics and the environment may have a role to play in autism.
Yip Yu has a 4-year-old son with mild autism.
"We started a little bit late because we wanted to make sure that we were right for each other before we started a family," he said.
For this study, more than 5.6 million birth records were analyzed.