The number of U.S. children with developmental disabilities has been climbing over the past decade, reaching nearly one in six in 2006 to 2008, a new government report shows.

"The take-home message for parents would be to promote early identification and screening of children," Sheree Boulet, of the Centers for Disease Control and Prevention in Atlanta, told Reuters Health. "These children require more services."

The study, based on ongoing national surveys of children under 18, looked at a range of disabilities, including attention deficit hyperactivity disorder (ADHD), autism, blindness, cerebral palsy, hearing loss, seizures, stuttering or stammering and other developmental delays.

From 1997 to 2008, the proportion of children with at least one of the conditions rose from less than 13 percent to more than 15 percent -- representing an extra 1.8 million kids.

"We don't know for sure why the increase happened," said Boulet, adding that several factors may be at play.

For instance, there is a bigger emphasis on early treatment today, and parents are more likely to be aware of the conditions, so kids who might not have been diagnosed in the past are being recognized now.

Part of the increase might also be due to a change in risk factors, such as parents getting older and having more preterm babies, Boulet said.

Most of the rise was driven by the rate of ADHD, which went from 5.7 percent to 7.6 percent over the 12-year study.

Autism rates showed the fastest growth, from 0.2 percent to 0.7 percent, while hearing loss dropped by nearly a third.

"We need to plan for this proportion of children needing services," Maureen Durkin, an epidemiologist at the University of Wisconsin-Madison, told Reuters Health.

"In many places it is becoming harder for families to get insurance," added Durkin, who was not involved in the new work.

While the findings are consistent with earlier reports, she noted, "the big limitation of this work is that it is based on parent reports."

Not all parents may be aware that their child has a developmental disability, and so the numbers might actually be an underestimate.

On the other hand, the fact that there are more treatments available for ADHD today probably means that parents are more likely to know about the condition, Durkin said.

According to the CDC report, published in the journal Pediatrics, boys were twice as likely as girls to have a developmental disability.

In addition, children from low-income families had higher rates of disabilities across the board, except for autism.

Durkin said the reason is unclear, but explained that poorer nutrition, more pre-term births and less intellectual stimulation could be involved.

Boulet explained that curbing risk factors like maternal obesity and smoking will help trim the chances of having a child with disabilities.