The number of babies born with Down syndrome is up 30 percent and an increasing number of American kids and teens have the chromosomal disorder, new government research indicates.

In the broadest study of its kind, researchers from the Centers for Disease Control and Prevention looked at birth defect records from 10 regions across the country and found that between 1979 and 2003 the number of children born with Down syndrome grew 31.1 percent. That brings the number of Americans ages 19 and younger with the chromosomal disorder to 83,400 or 1 in 971.

The growth in prevalence was most notable among younger children and among children born to older mothers, researchers report in the December issue of Pediatrics.

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The reason for the spike is unclear, but researchers point to several possible factors. Notably, mothers are giving birth at older ages and older mothers are five times more likely to have a child with Down syndrome. Also, medical advances mean kids born with the disorder today are more likely to survive and there is the possibility of a natural increase in incidence of the disorder.

Down syndrome appears to be more common among boys than girls, the study indicates. The condition is also seen more frequently in Hispanic children at birth, though the number of these children appears to level off with that of white children as they age. Black children appear less likely to have Down syndrome.

This study marks the broadest analysis of Down syndrome prevalence in the United States to date. Previously just one study looked at the occurrence of Down syndrome, but it was based on data from a single metro area.

Knowing how many people live with Down syndrome is expected to become increasingly important. “Down syndrome prevalence at birth is increasing and the median age at death for individuals with Down syndrome has increased in recent years (from 25 years in 1983 to 49 in 1997) in the United Sates, the number of children and adolescents who have Down syndrome and live in the United States is likely to increase over the next several years,” the researchers write. The new data is expected to help communities provide appropriate services.

Down syndrome occurs when an individual is born with an extra chromosome. It is the leading cause of intellectual disability.