The federal government is revising its official autism prevalence estimate upward to reflect a 30 percent increase over numbers reported just two years ago.

New data indicate that 1 in 68 children have autism, the U.S. Centers for Disease Control and Prevention said Thursday. That works out to roughly 1.2 million Americans under age 21. Previously, the agency said that the rate stood at 1 in 88, a figure released in 2012.

The latest estimate comes from data collected on 8-year-olds in 2010 in 11 communities across the country. Findings published in the CDC’s Morbidity and Mortality Weekly Report indicate that autism rates ranged from 1 in 175 children in Alabama to 1 in 45 in New Jersey.

Nearly half of those with autism have average or above average IQs, the CDC said. That marks a significant change from just a decade ago when only a third of kids on the spectrum fell within that range.

It’s unclear what exactly is driving the change, said Coleen Boyle, director of the National Center on Birth Defects and Developmental Disabilities at the CDC. It could be that doctors are getting better at diagnosing autism in those with typical IQ scores or that there are more kids with autism or a combination of both factors, she said.

Boys continue to be diagnosed at a rate that’s nearly five times that of girls with 1 in 42 males considered to be on the spectrum, the CDC said. Autism is also flagged more often in white children than in those who are black or Hispanic.

Most children with autism are not diagnosed until after age 4 even though experts say the condition can be spotted as early as age 2. The timing is significant since research shows that intervention is most successful when started early.

“The most important thing for parents to do is to act early when there is a concern about a child’s development,” said Marshalyn Yeargin-Allsopp, chief of the CDC’s Developmental Disabilities Branch. “If you have a concern about how your child plays, learns, speaks, acts or moves, take action. Don’t wait.”

The increased autism prevalence rate does not come as a complete surprise. Findings from a separate CDC survey released last year suggested that the condition could affect 1 in 50 kids. However, that data was based on a telephone survey of parents. The current study relies on records from providers of education and health services to children with developmental disabilities and is considered more accurate.

Autism rates have grown rapidly in recent years. This is the fourth time that the CDC has revised its official prevalence estimate since 2002 when autism was believed to affect 1 in 150 American children.

Paul Wang, a pediatrician who is head of medical research at Autism Speaks, said he expects the numbers to continue to rise as awareness grows and resources become more available across the country.

“The numbers are real,” Wang said. “These are real people who need everything from behavioral and medical services to employment and housing supports.”