A new 10-year study which analyzed nearly 5 million births in California has found a link between autism and maternal age — with mothers over the age of 40 having 51% higher odds of having children with autism compared to women between the ages 25 and 29.

“The risk of having a child with full syndrome autism increases with maternal age,” said researchers from the University of California, Davis. Although previous studies have found links between a child’s health and a mother’s age at conception, this is the first study to have such a large research pool and focused on autism.

What about maternal age increases autism risk?

“As people age, we know there are changes to our DNA that occur,” Geraldine Dawson, chief science officer of Autism Speaks, said to CNN. “There have been studies that show we have increased damage to our DNA as parents age. They’re more likely to have children of low birth rate and more birth complications. It’s not surprising that those factors would slightly increase the risk for autism as well as other neurological disorders.”