Doctors have discovered a more accurate way to prenatally test for chromosome disorders like Down syndrome, and it's less stressful for the mother to boot. Normally, if an ultrasound and blood test show a woman's risk of an affected pregnancy is high, doctors can perform additional tests to identify possible Down syndrome, Edwards syndrome, or Patau syndrome using a needle that collects fluid or tissue from a woman's womb. These procedures—either an amniocentesis or chorionic villus sampling—aren't fun, nor are the weeks of worry that can precede them. But researcher Nicholas Wald says new "reflex DNA screening" led to a 100-fold reduction in false positives at five UK maternity wards and prevented "nearly all invasive diagnostic tests in unaffected pregnancies," per a release.
In this method, described in Genetics in Medicine, doctors take a blood sample from a woman around 11 weeks of pregnancy and split it into two. If the first sample shows a woman's risk for an affected pregnancy is 1 in 800 or higher, doctors use the second sample to search for DNA from the placenta that would indicate a disorder, reports the Guardian. Over a 16-month study period involving 23,000 women, the test detected 95% of affected pregnancies, compared to 81% with the previous method. The rate of false positives, meanwhile, fell to 0.02% from 2.4%. In the end, only one of 26 women who underwent the diagnostic test had an unaffected pregnancy, adds Wald, who notes "the extra costs of the DNA tests are offset against savings from fewer amniocenteses and associated counseling." (Read more Down Syndrome stories.)