The finding came from a survey of parents which revealed that the number of American children with autism spectrum disorder has increased significantly since 2007. As of 2012, one in 50 kids aged 6 to 17 has a form of autism, vs. one in 88 children just five years prior.
"This estimate was a bit surprising. There may be more children with autism spectrum disorder than previously thought," said Stephen Blumberg, a senior scientist at the CDC's National Center for Health Statistics.
A typical school bus carries approximately 50 kids, therefore, there is normally one child with autism on every school bus filled to capacity in the U.S., Blumberg pointed out.
The principal cause for the rise in the prevalence of autism seems to be improved diagnoses, particularly in older kids.
The survey also showed that boys had a four-fold increased risk of being diagnosed with autism compared to girls. Previous research found that males were at increased risk of the disorder because they were more affected by mutations in the autism susceptibility gene.
"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," Blumberg added.
There were no other factors, even survey bias, which could account for the rise. The majority of the kids with the disorder were diagnosed since the previous survey in 2008.
"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."
This could be why the majority of those newly diagnosed school kids are inclined to have milder forms of autism. "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.
The conclusions were drawn from data that was collected and analyzed from the National Survey of Children's Health, which is a national phone survey involving almost 96,000 American households. During the survey, the parents were asked whether they have a child with autism.
Written by Sarah Glynn