Why So Many Babies Are Still Being Born With Down Syndrome

As prenatal tests improve, more and more women are finding out if their fetus has an extra chromosome, but they're still carrying to term.

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Prenatal diagnosis -- the ability to diagnose abnormalities before a baby is born -- is undergoing a revolution due to the recent arrival of tests that can accurately detect fetal genetic abnormalities, including Down syndrome, by testing the mother's blood. For the past 30 years, obstetricians like me have used the mother's age, ultrasound markers, and levels of certain blood chemicals to guess whether a fetus might have Down syndrome, or other genetic abnormalities. But it took an invasive test -- an amniocentesis or a chorionic villus sampling -- to be certain, and these tests occasionally caused miscarriage. It was an inexact guessing game that was extremely difficult to explain to patients.

There are a host of reasons why patients and doctors want to know in advance whether a child will be affected by a large number of genetic diseases, but by far the most common concern patients have is whether their baby will have Down syndrome -- a condition in which the child has an extra copy of chromosome 21 and will have cognitive impairment and be at risk for other abnormalities, from heart defects to leukemia to early dementia.

The number of babies born every year affected with Down syndrome has increased slightly in the United States to about 6,000 annually.

But there are really only two reasons why parents undergo testing to determine whether their baby will have Down syndrome: to prepare to raise a child with special needs, or to terminate the pregnancy. (Those of you who hold strong 'pro-choice' or 'pro-life' views are getting your hackles up, I know. So I'm going to go ahead and apologize in advance for ignoring your agendas entirely in this post.)

The number of babies born in the United States each year affected with Down syndrome is the result of several factors, including the number of fetuses conceived that carry the third copy of chromosome 21 (older mothers are more likely to conceive Down syndrome-affected fetuses, and the childbearing population in the United States is aging), the percentage of pregnant mothers who choose to test for Down syndrome, and the percentage of women who learn they are carrying a fetus affected with Down syndrome who choose to terminate. Fetuses affected with Down syndrome are more likely to miscarry than normal fetuses, but this hasn't changed over time.

Interestingly, the number of babies born every year affected with Down syndrome has increased slightly in the United States to about 6,000 annually according to the Centers for Disease Control (CDC), even as the trends I just mentioned have swung dramatically.

A recent article in Prenatal Diagnosis provides the best glimpse into the choices women made about abortion for Down syndrome over the past couple of decades, and the authors' conclusions are that fewer women who learn their fetus has Down syndrome are opting to terminate their pregnancy, and the percentage has probably declined over time to someplace between 60 and 90 percent. The conventional wisdom, based on a paper in the same journal from 1999, was that over 90 percent of Down syndrome-affected pregnancies were terminated, although the current paper casts doubt that the percentage was ever that high.

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Adam Wolfberg is an obstetrician specializing in high-risk pregnancies with Boston Maternal-Fetal Medicine and is a clinical instructor at Harvard Medical School. He is the author of Fragile Beginnings.

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