A new study looking at the prevalence of Down syndrome over the last 60 years finds that the condition may be far less common than previously thought.

As of 2010, just over 206,000 Americans had the chromosomal disorder, according to findings published online this month in the journal Genetics in Medicine.

That figure — which equates to about 6.7 cases per 10,000 people — falls short of the 400,000 estimate often reported by advocacy groups.

“Almost every book, web page and research article about Down syndrome begins with what should be a simple fact — the number of people living with the condition in the U.S. — but what we found is that most of those statistics are gross overestimations of the actual data,” said Brian Skotko, co-director of the Massachusetts General Hospital Down Syndrome Program and a senior author of the report.

The study is believed to be the first to look at fluctuations in the Down syndrome population over the course of 60 years and relies on an analysis of several databases reflecting birth trends, overall population shifts and changes in longevity for those with the chromosomal disorder, among other factors.

Less than 50,000 Americans had Down syndrome in 1950, researchers found.

By 2010, that figure had grown fourfold. The vast majority — 138,000 — were white. Meanwhile, roughly 33,000 were Hispanic and just over 27,000 were black.

“The number of Hispanics with Down syndrome has been growing incrementally, reflecting the growth of that population, while numbers among non-Hispanic whites have been slowing,” Skotko said. “Now we can begin to ask whether our resources, health care and support systems are evolving to meet the needs of the changing population.”

A number of factors are likely impacting population trends among those with Down syndrome, researchers said. In recent years, prenatal testing and elective termination have become more available. At the same time, however, people with the disorder are living longer thanks to better treatments for heart defects that are common among this group and other co-occurring conditions like thyroid disease, obstructive sleep apnea and leukemia.

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