After twenty years of steady increases in the number of children being diagnosed on the autism spectrum, the U.S. Centers for Disease Control reported no such rise over the last three years, according to a nationwide survey.

More than 30,000 parents were asked if a health professional had diagnosed their child or adolescent with an autism spectrum disorder (ASD), which includes Asperger’s syndrome. In the annual study, the rate of those reporting such a diagnosis has remained relatively unchanged since 2014, according to a research letter in the Journal of the American Medical Association.

Currently, the causes of the ASD neuro-developmental symptoms, which alter communication abilities and social skills, are still unknown.

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“Previous surveys have reported a steady increase in ASD prevalence in U.S. children over the past two decades,” the letter says. But in recent years, the numbers have hit a “plateau” — between 2014 and 2016, the estimate of children and adolescents with autism was about 2.4 percent, “with no statistically significant increase over the three years.”

The survey information was gathered through the Autism and Developmental Disabilities Monitoring Network, which is funded by the U.S. Centers for Disease Control and Prevention.

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The data only reflects the number of reported cases, so the researchers cannot make conclusions about its medical significance.

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