In the past few years, "non-invasive" prenatal screening tests have flooded the market, which claim to be able to predict chromosomal abnormalities like the one that causes Down Syndrome with near-perfect accuracy. But a new report has found that these blood tests may be vastly overstating how accurate they are, leading some pregnant people to choose to have abortions after thinking their fetus has been diagnosed with a devastating birth defect.

A report from Beth Daley at the New England Center for Investigative Reporting explores the story of a Rhode Island woman named Stacie Chapman, who very nearly terminated a much-wanted pregnancy at three months after a prenatal blood test called MaterniT21 predicted that her baby probably had Trisomy 18, also known as Edwards Syndrome, a serious chromosomal disorder that can lead to severe birth defects. (The median lifespan for a baby with Edwards is 15 days, and many die long before that.)

Chapman called her husband sobbing when she heard the news, then scheduled an abortion for the following day. Her doctor urged her to wait, and a follow-up test showed that her baby didn't in fact have Edwards; her son Lincoln Samuel just turned 1 and is perfectly healthy.

That's because while MaternitT21 and many other prenatal screening tests describe themselves as having a 99 percent detection rate, there's a very, very big difference between detecting a potential problem and diagnosing it definitively. From Daley's report:

But the new screens are not perfect, and can indicate fetuses may have serious genetic abnormalities when they do not, the industry's own research shows.

For example, among older pregnant women such as Chapman, for whom chromosomal abnormalities are more common, a positive test result for Edwards syndrome is accurate around 64 percent of the time, according to a recent study by Quest Diagnostics, a large provider of medical tests and other services. That means Baby Lincoln had about a 36 percent chance of not having the condition.

Among younger women who are at lower risk for fetal abnormalities, the error rate is even higher. Only about 40 percent of the positive tests showing high risk for Edwards syndrome turned out to be accurate, according to a recent well-regarded Illumina study of its Verifi screen.

To truly diagnose a birth defect, a more invasive procedure like amniocentesis is usually required, a test some pregnant people are reluctant to undergo because it carries a small risk of miscarriage.

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To complicate matters, the tests don't have to be approved by the Food and Drug Administration. FDA-approved drugs and tests are very limited in the types of claims they can make; prenatal screening tests aren't, and the information about them, doctors who use them told Daley, largely comes from salespeople marketing them. Besides MaternitT21, the first on the market, made by a company called Sequonom, Natera Inc. offers one called Panorama. In 2012, the first full year it was on the market, Maternit21 sold at least 50,000 tests.

A Portland couple, Zachary Diamond and Angie Nunes, had the same experience, according to NBC, learning from a test that their baby likely had Edwards Syndrome as well. They, too, nearly aborted, before an ultrasound at 16 weeks revealed that he was healthy. Even sadder is the case of Belinda Boydston, an Arizona woman whose prenatal screening showed treatable heart defects but no chromosomal abnormalities. But her son was born with Edwards and died within four days.

"We went from the greatest high to the greatest low of our lives," Boydston told NBC. "We wouldn't have terminated and we got four days with him but I wish we had known."

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The test-makers told both Daley and NBC that they stand by the accuracy of their tests

MaterniT21 does have a disclaimer, which looks like this:

No test is perfect.

While results of the MaterniT21 PLUS test are highly accurate, false positive and false negative results may occur in rare cases. A negative result or the absence of an Additional Finding does not ensure an unaffected pregnancy. The results of this testing, including the benefits and limitations, should be discussed with your health care provider.

A promotional image for MaterniT21. Image via Facebook.