THURSDAY, May 4 (HealthDay News) — The first-ever national estimate of the prevalence of autism in the United States finds the behavioral disorder affects up to one in every 175 school-age children — a total of more than 300,000 youngsters.

That finding comes from researchers at the U.S. Centers for Disease Control and Prevention in Atlanta, who presented their conclusions at a press conference on Thursday.

The agency’s estimates are based on interviews conducted in 2003 and 2004 with the parents of nearly 98,000 children aged 4 to 17. In two national surveys — the National Health Interview Survey (NHIS) and the National Survey of Children’s Health (NCHS) — parents were asked: “Has a doctor or health-care provider ever told you that [your child] has autism?”

“Estimates of diagnosed autism from these surveys were 5.7 per 1,000 school-age children from the NHIS, and 5.5 per 1,000 school-age children from the NSCH,” said Laura Schieve, an epidemiologist with the CDC’s National Center on Birth Defects and Developmental Disabilities (NCBDDD).

Those ratios correspond to about one in every 175 children and one in every 181 children, respectively, the researchers said.

“Together, these two national surveys of parents suggest that over 300,000 school-age children had a diagnosis of autism in 2003-2004,” Schieve said.

The findings appear in this week’s issue of the CDC puiblication Morbidity and Mortality Weekly Report.

As borne out in previous studies, “both surveys indicated that boys were nearly four times more likely to have been diagnosed with autism than girls,” Schieve said. Hispanic parents were slightly less likely than non-Hispanic whites to report a child with autism, although the researchers said this may be due to cultural or other factors, including access to medical care.

The new national estimates echo previous surveys, most of which were based on doctor and/or teacher reports and confined to specific states or regions, Schieve said. Those studies had led experts to a slightly higher estimate of the prevalence of autistic-spectrum disorders among U.S. children — about one in every 166 children.

One autism expert said the CDC findings echo previous research.

Dr. Melissa Nishawala is clinical director of the New York University Child Study Center’s Autism Spectrum Disorders Service. She said a similar, large survey in England that also relied heavily on parental reports found autism rates there to be about 6.3 cases per every 1,000 children.

The CDC experts stressed that because these were the first such surveys conducted, the results say nothing about either the underlying causes of autism, or ongoing prevalence trends.

“These surveys are designed to provide us with a snapshot, a picture of the world at a given time — in this case, the results cover the years 2003-2004,” said NCBDDD Director Dr. Jose Cordero.

The prevalence of diagnosed autistic spectrum disorders — which include autism and two less severe conditions, Asperger disorder and pervasive developmental disorder — was found to be less among 4- to 5-year-olds (4.8 cases per 1,000 children) than among children aged 6 to 8 (7.5 per 1,000).

Schieve and Cordero attributed that statistical difference to delays in diagnosis.

“Although student loan calculator often autism can be identified as early as 18 months, many children won’t be diagnosed until they start school,” Schieve said. “So, some of these 4- to-5-year-old children who might later go on to get an autism diagnosis may not have been diagnosed at the time of this survey.” Previous studies have shown that parents often miss an early diagnosis of autistic disorders, only picking up on it later when youngsters enter school.

For that reason, the real prevalence of autism may slightly exceed the numbers supplied by these su