Prenatal Testing: When a result is not a diagnosis

When fetal DNA testing for chromosomal disorders was first introduced in 2011, many experts feared that the improved results would be oversold, leading women to take irreversible steps based on a test that, while an improvement over earlier testing, remains far from conclusive.

Both the new generation of tests and the old were performed on a blood draw from the mother; newer tests seek to analyze the fetal DNA that floats freely in the mother’s blood. At 10 weeks pregnant, when these tests are normally done, about 10 percent of the DNA in a mother’s blood will be fetal DNA from the placenta. A study published in The New England Journal of Medicine in February found that a fetal DNA test was 10 times better in predicting cases of Down syndrome than the standard blood test and ultrasound screening, and five times better in predicting the other disorder examined, Trisomy 18. But both new and old tests require an additional procedure, usually amniocentesis, to confirm the accuracy of the result.

Read more from the New York Times.