Why So Many Babies Are Still Being Born With Down Syndrome

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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.

So what's going on? If the abortion rate is declining, why isn't the number of babies born with Down syndrome rapidly increasing? Several factors are at play:

  • The number of babies born to Hispanic women is increasing, and Hispanic women are least likely to terminate a fetus affected with Down syndrome.
  • It used to be that women had to make a conscious decision to have the test for Down syndrome, and the women who chose to be tested were probably more likely to terminate if the series of tests was positive. Due to guidance from the American College of Obstetricians and Gynecologists, more and more women are getting tested for Down syndrome, perhaps including more women who don't terminate their pregnancy when the test comes back positive.
  • However, since women who don't get the Down syndrome tests can't terminate for this reason (because they never find out their fetus is affected), and more women are being tested, the overall number of terminations for Down syndrome may have increased.

Perhaps the most important factor is a sea change in society's approach to individuals with Down syndrome. Explains lead author of the recent paper, Jaime L. Natoli, a senior consultant in the department of clinical analysis at the Southern California Permanente Medical Group, in response to emailed questions: "Families have significantly more educational, social, and financial support than they had in the past. For example, from a social standpoint, women of childbearing age are from perhaps the first generation who grew up in an era where individuals with Down syndrome were in their schools or daycare centers -- perhaps not the mainstream integration that we see today, but still a level of exposure that was very different than in generations prior. They grew up watching kids with Down syndrome on Sesame Street."

What will the impact of these new tests be on the number of babies born with Down syndrome, since it is now easier to make a diagnosis without risking miscarriage? Says Natoli, "I cannot predict if the termination rate will go up, down, or stay the same. A lot of people think it will go up, but I wouldn't be surprised at all if it went down."

My guess is that new tests will have little impact. They are much easier to understand than the old ones, and eventually insurance companies will pay for them for most women. I suspect that in 10 years most women carrying a fetus affected with Down syndrome will receive a diagnosis early in their pregnancy. This will mean that more women will have to make the gut-wrenching decision about whether to continue the pregnancy or abort. And I predict that the number of babies born affected with Down syndrome will stay about the same.

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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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