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Why Prenatal Tests are Not Always Accurate

False Positives for a Painful Disorder

Stacie Chapman was 41 when she got pregnant, so when her doctor recommended that she get a screening, she and her husband decided it was the right move. Then, the test came back positive for Trisomy 18, also known as Edward’s Syndrome, a painful genetic condition that was “incompatible with life”. Not wanting their baby to suffer, they scheduled an appointment for an abortion the following day. Their physician called before they went through with it and suggested they wait because sometimes the screenings were not accurate. Eight weeks later, they had another ultrasound that told Stacie and her husband that their baby boy was totally healthy.

In another story from Portland, Oregon, Angie Nunes and her partner Zachary were told the screening was 99 percent accurate in detecting genetic abnormalities. They were devastated to learn that the test result was positive for Trisomy 18, and worried about how they would care for a child with such severe disabilities. However, at her 16-week ultrasound appointment, the doctor saw that she was likely carrying a healthy child.

Tests used to determine whether a child will be born with genetic abnormalities were once only offered to high-risk mothers. Today, they’re offered to pregnant women across the board. While some tests can be ordered for specific genetic disorders like Down’s or Edward’s Syndrome, others can screen for up to 250 different genetic abnormalities. But more and more research indicates that these tests are not nearly as accurate as you might think.

How does prenatal testing work?

The most recent method of screening for genetic abnormalities requires a blood sample from the expecting mother. The sample then goes to the lab, where it is screened for placental cell-free DNA (cfDNA), which are extremely small DNA bits released by the placenta and into the mother’s bloodstream.

What affects the accuracy of prenatal testing?

The most important factor that is often overlooked is that cfDNA screenings are not diagnostic. They are simply screening tests, which cannot guarantee with 100 percent accuracy whether the baby will be born with a genetic abnormality. The difference is that screening tests only determine the degree of likelihood that, when the child is born, he or she may have a genetic abnormality. Any screening that returns a positive result should be checked again with a diagnostic test like CVS amniocentesis, and that being said, even those are not 100 percent accurate. A screening test simply scans for likelihood, while a diagnostic test determines a condition with a degree of certainty.

Has the FDA published anything warning against the accuracy of prenatal screening?

In the fall of 2015, the FDA issued a paper that said these tests had “potential to yield both many false-positive and false-negative results”. There were two specific issues noted in this publication. First, the FDA stated that there is a lack of clinic validation that the tests are able to predict genetic abnormalities at an appropriate rate. Second, there were many false-positive results when these tests were taken by the general population.

What problems can arise from inaccurate prenatal testing results?

There are two vital issues that arise from inaccurate prenatal screening results:

  1. There is undue stress added to the expecting woman or parents regarding the child’s health.
  2. Abortion is often recommended for certain genetic disorders, even though the parents want the child.

Can these test results be trusted?

Since these cfDNA tests came on the market in 2011, hundreds of thousands of women have relied on their results. However, positive results can be wrong up to 50 percent of the time. This has resulted in potentially thousands of women with healthy pregnancies being encouraged to have abortions that they did not want or need. With these and so many other stories to look at, there’s no wonder a woman would feel a sense of caution when deciding on whether she should receive a genetic screening for her unborn child.

If you’re unexpectedly pregnant and looking for answers, Palm Beach Women’s Clinic can help with no-cost and confidential services.


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