It should be noted that nowadays there are other types of tests that can give indications or exclude them in terms of further invasive tests, with considerably higher reliability than the previously described screening tests. These are based on the analysis of free fetal DNA from the mother’s blood. The reliability of these testes is high for the detection of Down, Edwards, Patau and Turner syndromes and anomalies related to sex chromosomes. The problem with these tests is the high cost.
The final diagnosis is made by some of the invasive tests such as a chorionic villus sampling performed at the end of the first trimester, then amniocentesis (taking amniotic fluid) or cordocentesis (taking blood from the umbilical cord). Since each of these tests implies the risk of miscarriage, it is done only when there are indications for it.