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As of 2012, one in 50 kids between the ages of 6 and 17 has some form of autism, compared with one in 88 only five years earlier, according to the U.S. Centers for Disease Control and Prevention.
"This estimate was a bit surprising," said report author Stephen Blumberg, a senior scientist at the CDC's National Center for Health Statistics. "There may be more children with autism spectrum disorder than previously thought."
The average school bus holds about 50 children, so there is typically one child with autism spectrum disorder on every full school bus in America, Blumberg noted.
Michael Rosanoff, associate director for Public Health Research and Scientific Review at Autism Speaks, said that "this study added to the evidence suggesting that we are underestimating the prevalence of autism in the United States."
This report, however, underestimated the real prevalence of autism, Rosanoff said. "It's probably much higher," he said.
The main reason for the increase in the prevalence of autism appears to be better diagnoses, especially in older children, Blumberg said.
In addition, boys were more than four times more likely to be diagnosed with autism than girls, which has been the historical trend, Blumberg said.
"For the most part, the increase in the prevalence is largely due to an increase in the prevalence in reported autism spectrum disorder for boys," he said.
None of the other factors, such as survey bias, could explain the increase, he added. Most of the children who were diagnosed with autism spectrum disorder were diagnosed since the last survey in 2008, Blumberg noted.
"By ruling out other explanations and noting the increase in recent diagnoses, that suggests to us that improved ascertainment -- recognizing children who were previously unrecognized as having autism spectrum disorder -- is the reason," he said.
This may be the reason most of those newly diagnosed children tend to have milder forms of autism, Blumberg said.
"It would certainly make sense that those with unrecognized autism spectrum disorder may have symptoms that are milder than children who have been diagnosed earlier," he said.
Rosanoff agreed that more children with milder autism are being diagnosed.
"What we are seeing is that children who have not been diagnosed in the past are now being diagnosed," he said. "That is likely due to doctors and other health care providers being better at recognizing the more milder symptoms of autism and being able to diagnose those."
These children are most likely having trouble with social skills, which limits their ability to interact with others in the classroom and in social situations, Rosanoff said.
Diagnosing these children is important, Rosanoff said, because even though they may be doing well in the classroom they could benefit from help with their autism.
"With appropriate diagnosis and access to services, a child with autism can improve in the way they function and how they are able to be successful in life," he said.
To reach their conclusions, researchers gathered data from the National Survey of Children's Health, which is a national telephone survey of nearly 96,000 American households. As part of the survey, parents are asked whether they have a child diagnosed with autism.
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