What did the California birth study find about maternal age and autism spectrum disorder?

A study of more than 4.9 million births in California between 1990 and 1999 examined the relationship between parental age and autism spectrum disorder. The research found that approximately 5 percent of the nearly 12,160 children who were later diagnosed with autism were born to older mothers.1

Specifically, the study identified that mothers aged 40 or older faced the highest risk of giving birth to a child who would later develop autism. While the study also noted that older paternal age was a risk factor, this was only observed in cases where the mother was age 30 or younger. The underlying biological or environmental reasons for these associations remain unclear.1