It’s a large increase since the last report but experts stress it doesn’t necessarily mean more children are developing autism.

Instead, the numbers suggests that more children are being diagnosed as they get older – probably because of many factors, including increased awareness and more services both to diagnose children and help them, the researchers said.

“Our findings suggest that the increase in prevalence may be due to increased recognition of autism spectrum disorders in recent years, especially when the symptoms were mild,” Stephen Blumberg of the National Center for Health Statistics, which published the report, said in a telephone interview.

“Parents are more aware. Professionals are more aware. There may be more access to diagnostic services.”

Autism describes a range of conditions and disorders – some of which a few years ago were not even recognized as conditions. It can range from the very mild social awkwardness seen in some cases of Asperger’s syndrome, to severe and debilitating symptoms that prevent children from interacting in a normal way, prevent learning and often require medication. Some children with autism were classified as mentally retarded in years past, while others struggled quietly with no idea they could benefit from therapy.

Parents should not worry that something new has been happening to kids, says Dr. Marshalyn Yeargin-Allsopp of the Centers for Disease Control and Prevention, which is monitoring autism in several ways. “We don’t want them to be frightened by the numbers. We want them to recognize that there are things they can do that make a difference in their child’s life,” she said.

The present study asked 100,000 parents across the country a range of health questions, including whether their child had been diagnosed on the autism spectrum and whether he or she currently had the diagnosis. The autism spectrum includes autism, the most severe form, as well as Asperger’s syndrome and pervasive developmental disorder-not otherwise specified (PDD-NOS).

The study looked at children ages 6-17 and was based on parent reports, while last year’s study looked at 8-year-olds whose diagnosis was noted in school district or other official records.

The fact that the new study found such high rates implies that “there will likely be more demand for (autism-related) services than we had previously thought,” said study author Stephen J. Blumberg, a senior scientist at the National Center for Health Statistics.

The new study, like most others, found that boys are four times more likely to have autism than girls.

The parents’ answers to the two survey questions also suggests that 15% to 20% of children who were once diagnosed with autism no longer have the condition. Blumberg said the study cannot say whether they lost the diagnosis because they outgrew the condition, or because they were misdiagnosed in the first place.

The higher numbers recorded in the new study suggest that officials are getting better at counting kids with autism – not that more have the condition, several experts said.

“I don’t see any evidence that there’s a true increase in the prevalence of autism,” said Roy Richard Grinker, a professor of anthropology at George Washington University, Washington, D.C.