The trisomies of chromosomes 21, 18 and 13 and also other genetic conditions can be detected precisely already during pregnancy by an amniocentesis. This invasive procedure comprises taking a small amount of amniotic fluid, which surrounds the fetus, by a hollow needle. The DNA from the fetal cells floating in the amniotic fluid is prepared and analyzed. Given its high accuracy, amniocentesis is still the gold standard of prenatal analysis. It can be performed between weeks 15 and 18 of gestation. The procedure is quite unpleasant for the pregnant women, but the main issue with this method is the rate of 0.5% to 2% of pregnancies that undergo a spontaneous abortion as side effect of the procedure. To avoid this risk, other less invasive methods were sought.
By analyzing the appearance of the fetal neck fold (nuchal translucency) between 11 and 14 weeks of gestation, it is possible to ascertain the risk that the fetus has a trisomy 21. The accuracy of the calculation can be improved by measuring the concentration of two hormones, “PAPP-A” and “ß-HCG” in mothers’ blood. If the result is “low risk”, then a trisomy 21 can almost be excluded. However, in the adverse case of “high risk for a trisomy 21” many results are false positive. This is why every “high risk” result has to be verified by an amniocentesis. The method of measuring the nuchal translucency and the hormonal concentration, however, has reduced the number of invasive procedures. This is why it is referred to as a screening test. It is not as accurate as the diagnostic amniocentesis but is not invasive and has helped to reduce the number of amniocenteses.
A much more accurate screening method for trisomies 21, 18 and 13 was introduced only a few years ago: non-invasive prenatal screening testing (NIPTs). With a simple blood draw from the pregnant women the risk for the three trisomies can be ascertained with 99% accuracy. The worldwide triumph of the NIPTs can be attributed particularly to the less than 1% false positive rate, which led to a strong decline in the number of invasive procedures. However, even for the NIPTs a confirmation of a positive result is strongly recommended.
Because of their accuracy for testing trisomies 21, 18 and 13 NPTs are recommended in several medical guidelines and paid for by public health insurances in various countries.