For a large share of pregnant women, testing for Down syndrome and other chromosomal abnormalities is a standard part of prenatal care. This testing has gotten quite a bit of negative press in the last few days. In particular, news reports have questioned the value and accuracy of new technologies that claim to detect problems at a high rate with a simple blood test.
As a first shot, the New England Center for Investigative Reporting detailed several cases where this test resulted in either a false negative (the test said the fetus had the normal set of chromosomes when it did not) or a false positive (the test said there was a problem and there was not). NECIR gave several examples of women who terminated healthy pregnancies as a result of these inaccurate tests. Other outlets have piled on with concerns about the “alarming inaccuracy” of prenatal testing.
These new tests aren’t perfect, but the current discussion dramatically undervalues them, and they represent a huge leap in accuracy over what was previously available.
Without any intervention, chromosomal abnormalities (Down syndrome is the most common, others include Edwards syndrome and Patau syndrome) would occur in something like 1 in 700 live births, with the rate increasing significantly as the mother’s age increases.
If you want to know for sure whether a fetus has a chromosome imbalance, you have to look at its genome.