As people with Down syndrome live longer, a new analysis suggests that this population is growing, but that may not hold true for long.

Researchers looked at data on the number of people born with or living with Down syndrome in nine states where such information is most readily available.

They found that the Down syndrome population steadily increased between 1950 and 2010, according to a study published this month in the American Journal of Medical Genetics, Part A.

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However, at the same time, expectant parents are increasingly willing to terminate pregnancies when Down syndrome is diagnosed prenatally, researchers said, and that could influence this population’s growth over time.

“The numbers of people with Down syndrome living in a state is influenced both by the numbers of babies born each year and by the expanding lifespan of people with Down syndrome,” said Brian Skotko, co-director of the Massachusetts General Hospital Down Syndrome Program and a senior author of the study. “People with Down syndrome are living longer than ever before, but the increased use of prenatal testing for this and other genetic diseases has had a counterbalancing effect, leading to a plateauing of total numbers in many states.”

Researchers observed varying geographic trends in data in the most recent year studied. Of the nine states included, Florida had the highest rate of births with 1 in 729 babies having the chromosomal disorder while Kentucky had the lowest at 1 in 2,256.

Terminations were estimated to be most common in New Jersey and least frequent in Indiana and Michigan, the study found.

Other states included in the analysis were Massachusetts, New York, Illinois and Arizona.

The study is based on data collected through 2010. The following year, however, a new noninvasive prenatal screening test came on the market and Skotko said future research will assess the impact of the new test.