A large study of one of the fetal DNA tests that screen fetuses for Down syndrome have shown that they are accurate in low-risk women, which may mean they will become more widely used. But experts warn that women who test positive for carrying a fetus with Down syndrome should still have more invasive tests done to confirm the diagnosis, especially if they are considering an abortion.
Down syndrome is caused by a third copy in one of the pairs of human chromosomes, specifically chromosome 21. Fetal DNA tests measure the mother's blood for fragments of the fetus's DNA, and also can detect two other forms of chromosomal abnormalities. They have already been shown to be accurate in women over age 35, who are at greater risk for having a child with Down syndrome. Fetal DNA tests are safer than other tests for genetic abnormalities, which can cause miscarriages in about 1 in 600 pregnancies when used.
The study was published in the New England Journal of Medicine. It involved 16,000 women and compared the Harmony test from the pharmaceutical firm Roche to standard prenatal screening for Down syndrome, which consists of looking for certain biomarkers in the mother's blood along with a fetal ultrasound. Standard screening detected 30 cases of Down syndrome, but missed 8 cases, while the Roche test detected all 38. The false positive rate, cases where Down syndrome was wrongly detected, was 0.06% with the Roche test compared to 5.4% with standard screening.
However, the fetal DNA test could not give a result in 500 women and there were babies born with chromosomal defects that the test is not designed to detect.
A letter in the same issue of the journal strongly urged women who test positive for Down syndrome or other abnormalities using fetal DNA tests to be screened with other technology before deciding to terminate a pregnancy. The letter noted that there were 56 false positives in a series of women who had tested positive using fetal DNA tests.
Critics say that fetal DNA tests are being heavily promoted in ways that can be misleading, with ads that promise perfectly accurate results. The technology is so new that even obstetricians are still just learning about it.