ATLANTA -- A government report released Thursday shows no change in how common autism is among U.S. children.

About 1 in 68 school-aged children have autism or related disorders. That's the same as it was when health officials checked two years earlier.

The lack of change is noteworthy because autism estimates had been steadily increasing. In 2007, the government estimated only 1 in 150 children had autism.

But it's too soon to tell whether the number is stabilizing, said Daisy Christensen, lead author of the new report from the Centers for Disease Control and Prevention.

For decades, autism meant kids with severe language, intellectual and social impairments and unusual, repetitious behaviours. But the definition has gradually expanded and now includes milder, related conditions. The causes of autism are still not known.

There are no blood or biological tests for autism, so diagnosis is not an exact science. It's identified by making judgments about a child's behaviour.

Experts say that in recent decades, teachers and parents have been increasingly likely to say a child with learning and behaviour problems is autistic, so at least some of the apparent increase through the years was due to different labeling.

It is possible that there's a levelling off in how frequently children with certain behaviours are labeled autistic, said Catherine Rice, an Emory University autism expert who works with the CDC.

Autism estimates levelled off between 2000 and 2002, and then increased, so it's too soon to make much of the latest figure, she said.

The new data is from 2012. The report is based on a tracking system in 11 states that focuses on 8-year-olds because most autism is diagnosed by that age. The researchers check health and school records to see which children meet criteria for autism, even if they haven't been formally diagnosed. It is one of three autism estimates by the CDC but is considered the most rigorous.