After more than a decade of steady increases in the rate of children diagnosed with autism in the United States, the rate has plateaued in the past three years, researchers said Tuesday.

The findings were based on a nationwide study in which more than 30,000 parents reported whether or not their children had been diagnosed with autism spectrum disorder (ASD).

"The estimated ASD prevalence was 2.41 percent among US children and adolescents in 2014-2016, with no statistically significant increase over the three years," said the research letter by experts at the University of Iowa, published in the Journal of the American Medical Association (JAMA).

The first observation of a plateau was made by a separate group in 2012, when the rate flattened out to 1.46 percent, according to the Autism and Developmental Disabilities Monitoring (ADDM) Network.

Federal health authorities say that means about one in 68 children in the United States have the neurodevelopmental disability, whose causes remain poorly understood.

The ADDM had documented a "continuous increase from 0.67 percent in 2000 to 1.47 percent in 2010."

The 2.4 percent rate described in the JAMA report translate to one in 47 children, but researchers cautioned that the discrepancy may be explained by "differences in study design and participant characteristics."

The JAMA report, based on the annual National Health Interview Survey, did not delve into "underlying causes for the findings and cannot make conclusions about their medical significance."

The US Centers for Disease Control and Prevention also noted a plateau in the autism rate in 2016, but said it was "too soon to know whether ASD prevalence in the United States might be starting to stabilize."