Autism spectrum disorder (ASD) includes a spectrum of behavioral and cognitive disturbances that begin in childhood and is associated with multiple genetic and environmental factors. The recognition and diagnosis of ASD have increased, with US prevalence rates having increased from 6.7 per 1000 in 2000 to 16.8 per 1000 in 2014 at age 8 years. Although most data suggest that increased ascertainment has contributed to the increased prevalence, concern that modifiable factors may contribute to ASD remains.