The prenatal tests include all examinations and screenings during pregnancy. The ultrasound diagnostics is a major part of prenatal care for pregnant women. The ultrasound may be combined with additional tests for better detection of certain abnormalities or complications. The most popular test is the fetal Down syndrome test.
The tests we perform to detect trisomy 21 (Down syndrome) can be basically divided into two types – invasive (taking a direct sample from the placenta, amniotic fluid or fetal blood) and non-invasive (indirectly from the mother's blood).
Non-invasive prenatal test (NIPT
The test is called non-invasive because it is safe for the mother and the fetus. It is a highly sensitive screening for the most common genetic diseases of the fetus, by detecting fragments of the baby's DNA in the mother's blood. It can be performed after 10 weeks of gestation, until the end of the pregnancy, and is done by drawing a few milliliters of blood from the woman's vein.
This test is most often used to detect Down syndrome (trisomy 21), Edwards’ syndrome (trisomy 18) and Patau’s syndrome (trisomy 13). The prenatal test is extremely precise for these syndromes, but is still not 100% accurate. The only completely reliable methods are the invasive diagnostic procedures – chorionic villus sampling and amniocentesis (see below).
Having made this important clarification, let's return to the syndromes. Since trisomy 18 and 13 are associated with multiple anomalies of the brain, heart, limbs, kidneys, abdominal wall, etc., they are easy to detect with simple ultrasound as early as 12-13 weeks of gestation (wog) during the screening in first trimester. These anomalies are severe enough and have a poor prognosis, and the parents can take a decision to terminate the pregnancy for medical reasons immediately after the examination. In my opinion, these sensitive but expensive tests are not needed to detect these anomalies. With such conditions, we must think about diagnostic procedures that can confirm the result (chorion biopsy/amniocentesis and abortive material) and genetic counseling of the couple, in view of subsequent pregnancies. In rare cases, there may be an incorrect distribution of the genetic material (balanced translocation of chromosomes) in one of the parents and this may be the cause of a series of miscarriages and failures to conceive, in which case we recommend preconception genetic testing.
Things are different for the Down syndrome. This syndrome may show multiple deviations in the ultrasound and biochemistry tests that can help us detect its existence as early as the first trimester screening, which is done by a specialist in fetal medicine during 11-13 wog. But it can also have barely noticeable and discrete manifestations or appear at a late stage of pregnancy, so that its detection is much more difficult, even for an experienced specialist. In my practice, I only follow established medical guidelines, according to which, if the first trimester screening shows a high risk of Down syndrome, one of the options is to do a non-invasive prenatal test, which is up to 99.8% accurate for this syndrome.
Invasive diagnostic procedures – chorionic villus sampling and amniocentesis
These are procedures that include taking of small amount of amniotic fluid (amniocentesis), or a few villi from the placenta (chorionic villus sampling). The material is then sent to a genetics laboratory to test for certain conditions. Amniocentesis is performed between 16-24 wog, or later, while the chorion biopsy - in 11-15 wog.
It is important to know the main indications (reasons) for these procedures:
Amniocentesis and chorion biopsy (chorionic villus sampling) can be used for detection of a large number of genetic diseases of the fetus, such as thalassemia, sickle cell anemia, cystic fibrosis, as well as chromosomal defects like Down's syndrome, etc. Genetic tests are important not only for the current pregnancy, but also for future ones.
The fear of the invasive procedures is most often due to the fear of the needle itself, and many pregnant women avoid doing them for this very reason. Both manipulations are done with local anesthesia on the abdomen, so there is no pain, only slight discomfort, and their duration is about 30 seconds.
The procedures are generally not very technically complex, but of course, they have their risks. In the hands of an experienced specialist, the risk of abortion is 1 in 1000 such interventions. There are also some other risks such as giving an infection to the mother, not getting a result from the manipulation, tightening of the uterus and other less common complaints. It is important to emphasize that this is an extremely good method of diagnosis that is used in conjunction with the developing genetics and the many advanced genetic tests that we already have.
Author: Dr. Boris Stoilov