The analysis of fetal free DNA is already available in some specialized laboratories of molecular diagnosis, and is recommended in women who had pregnancies at risk, where the probability of the fetus suffering a chromosomal abnormality is higher. However, experts point out that this technique could be extended to all pregnancies as a basic non-invasive prenatal check-up for Patau syndrome, trisomy of chromosome 13; Edwards syndrome, trisomy of chromosome 18 and Down syndrome, trisomy of chromosome 21.
To date, non-invasive prenatal diagnosis is based on maternal blood tests and ultrasound scans to determine nuchal translucency. Some genomic alterations cause an accumulation of fluid in the area of the neck, so on the twelfth week this parameter is measured. This technique has a considerable error since, after all, that image is obtained by radiofrequencies and is of low quality. Normally when these tests indicate an anomaly or in cases of high-risk pregnancies, an INVASIVE confirmation test is used. It can be an amniocentesis, where a sample of amniotic fluid is taken, that which bathes the baby, to analyze suspended fetal DNA samples, or an analysis of chorionic villi, where a piece of the placenta is studied. The advantage of this last technique is that this sample can be taken through the cervix without having to perforate the placenta.
One of the main advantages of free fetal DNA sequencing that drastically decrease false positives. Nearly 90% of women who would mistakenly think their baby has an abnormality would get a result closer to the reality of their pregnancy.
It is very important to inform women correctly about what is being done and what the results mean. It has been proven that many pregnant women distrust a technique that is simply based on a blood sample, although it is more effective than conventional techniques. In addition, experts warn that this test has one last peculiarity to take into account. Sometimes “false positives” do not correspond to the fetal chromosomal endowment, but to the maternal one. That is, it may happen that the fetus is in perfect condition but the mother has an unknown asymptomatic chromosomal alteration, or in the worst case, may be developing a tumor. This reaffirms how important it is to correctly inform the mother.
Finally, the manuals include a series of situations where it is more common for false negatives to appear. First, multiple pregnancies are included where one of the fetuses dies prematurely and is reabsorbed. In these cases the remains of the fetal free DNA of this brother can circulate through the mother’s blood, clouding the results. On the other hand is the case of women who have received a donation of bone marrow. Fragments of cfDNA from donated cells may appear and be confused with those of the fetus. Third, there is the possibility that fetal mosaicism appears, an anomaly where only the placenta has an aberrant chromosomal endowment, whereas the fetus does not. This is due to the fact that in the first stages of gestation, in that initial cell ball, the precursor cell of the trophoblast suffers an erroneous division. All of its descendants will inherit that anomaly, while the cells that make up the embryo are independent of this event. In this case only amniocentesis will give us a true result of the state of the fetus.
Be that as it may, prenatal diagnosis, like any other field of medicine, is determined to move forward and embrace new, more reliable and affordable molecular analysis techniques every day.