The Centers for Disease Control and Prevention's National Center for Health Statistics and the Health Resources and Services Administration said the change in prevalence estimates was greatest for boys and for adolescents age 14-17.
Officials said the increase was most likely not due to an environmental factor but because parents, healthcare staff and teachers were more aware of autism behaviors. Children were also diagnosed more often during ages 14-17.
Children who were first diagnosed in or after 2008 were more likely to have milder ASD than those diagnosed in or before 2007, the report said.
Much of the increase in the prevalence estimates from 2007 to 2011-12 for school-age children was the result of diagnoses of children with previously unrecognized ASD, the study said.
In 2007, a study of medical records found 1-in-86 children had ASD, while the parental survey in 2011-12 found 1-in-50 had ASD.
The researchers used data from the National Survey of Children's Health, a nationally representative phone survey of households with children conducted every four years.
Information about parent-reported ASD diagnosis was obtained for 63,967 children age 6-17 in 2007 and for 65,556 in 2011-12.