The nuchal translucency, in NT, is a screening test that evaluates the risk of chromosomal abnormalities during the first stages of fetal life.
Statistically speaking, the interpretation of nuchal translucency results allows the identification of 75-80% of the fetuses affected by Down syndrome, with a percentage of false positives of 5-8%.
In addition to indicating an increased risk of trisomy 21 and other more rare chromosomal diseases (such as trisomy 18), nuchal translucency makes it possible to quantify the probability that the fetus carries some skeletal and cardiac malformations.
When do you run?
Nuchal translucency is performed between the 11th and 14th week of pregnancy, subjecting the uterus of the expectant mother to a series of sound waves not perceptible to the human ear (ultrasound). These waves, absolutely harmless for both the mother and the fetus, are variously reflected by the tissues in relation to their density, then picked up by the same probe that generated them, converted into an electrical signal and processed by a computer to provide tissue images examined.
In the posterior region of the fetal neck there is an area that does not reflect ultrasound, therefore anechogenic and translucent; in this area we find a small physiological accumulation of fluid between the skin and the underlying paravertebral tissues. The collection of this fluid begins to appear around the 10th week of pregnancy and increases in thickness in the following weeks, and then decreases until it vanishes after the 14th week.
Nuchal translucency: clinical significance
Scholars have noted that the excessive thickness of this translucent area (called plica nucale) is due to an increased risk of Down Syndrome or other chromosomal pathologies.
In general, the greater the thickness of the nuchal translucency, the greater the risk that the fetus is affected by chromosomal abnormalities (in particular from Down syndrome and less frequently from trisomy 13 or 18). Nevertheless, the increase of nuchal translucency is not always related to pathologies, but it can be a momentary event; moreover, when it has pathological significance it may also be present in the absence of chromosomal defects, for example due to heart disease, metabolic alterations with hypoprotidemia, congenital or acquired anemias (infections), skeletal dysplasias, etc.
When talking about nuchal translucency, it is important to remember that we are always talking about a screening test, and not a real diagnostic exam. The result of the examination, therefore, does not tell us if the fetus is healthy or ill, but only the probability that it is. In other words, negativity reduces risk but does not clear it, while positivity does NOT necessarily imply that the fetus is affected by the disease; rather, it requires further diagnostic tests, which will be carried out by chromosomal examination on fetal cells after villocentesis or amniocentesis; these investigations, in general, are not carried out directly as a first examination, because they are burdened by a very small but not negligible risk of abortion (0.5-1%).
Being a screening method, the exam result does not speak of positivity or negativity, but - thanks to the help of a software developed by the Fetal Medicine Foundation (London) and the evaluation of other parameters (maternal age, weight, smoking etc.) - expresses the risk in percentage statistical terms (for example 1 possible pathological case out of 1000 or a possible pathological case out of 100). If a positive risk profile emerges from the study, then the report should not be confused with the ability of the test to identify fetuses affected by Down syndrome. Rather, it means that among all those cases for which the examination reports "attention, there is a certain risk that requires further investigations", a certain percentage of fetuses actually affected by down syndrome (in this case 75-80%) is also included . At the subsequent amniocentical or villocentical investigation, therefore, the vast majority of fetuses will be confirmed to be devoid of any chromosomal anomaly; this is because pregnant women are generally sent to these surveys with a risk profile equal to or greater than 1: 250 (one or more chances in 250 that the fetus is ill).
To improve the reliability of nuchal translucency as an early screening method for Down syndrome, the exam is now integrated with the so-called duotest, which consists in the simultaneous blood dosage of the chorionic gonadotropin (beta-β-hCG subunit) and of the PAPP- A (pregnancy-associated plasma protein = plasma protein A associated with pregnancy).
An increase in β-hCG and a decrease in PAPP-A in maternal venous blood are considered indicative of an increased risk of Down Syndrome.
The concentrations of these two substances (PAPP-a and B-hCG) - multiplied by the intrinsic risk depending on the maternal age, and integrated with the ultrasound data of nuchal translucency - allow up to 90% of the cases of Down Syndrome to be recognized, with a false positive percentage of 5%. Although these are respectable percentages, when the estimated risk is substantial, it is always suggested to undergo further invasive diagnostic tests. Fetal cytogenetic investigation, performed by villocentesis or amniocentesis, may therefore exclude (as in most cases) or confirm definitively, Down Syndrome, Trisomy 18 and other chromosomal abnormalities.
Examination of nuchal translucency may also be associated with ultrasound examination of the nasal bone. Examining the profile of the fetus during an ultrasound scan between the 11th and 14th week has important implications for screening, since in this period about 60-70% of fetuses with Down syndrome do not show a visible nasal bone.
An elevated nuchal translucency, even when the combined test (ultrasound + bi-test) is negative, can be associated with an increased risk of fetal structural anomaly (in particular, as already mentioned, on the heart). In this case it is advisable a 2nd level ultrasound to be performed at 16 and / or 20-22 weeks of gestation, with particular reference to the study of the heart.
How is the nuchal translucency exam performed? How long does it last? Is it painful?
Sometimes the exam can last more than 30-45 minutes, due to the need to perform multiple measurements with the fetus in well established positions. For the same reason, the patient may be asked to cough or to change position, in order to determine the displacement of the fetus for a correct scan. The examination is sometimes hindered by particular conditions, such as maternal obesity or bulky fibroids in the anterior wall of the uterus.
It is essential that the detection of nuchal translucency be carried out by expert and qualified operators, so that the subsequent risk analysis can be standardized, precise and reliable.
Ultrasound measurement of nuchal translucency can be performed transabdominal or transvaginal; in the latter case, the exam allows obtaining a better image resolution and a correct scan, but it can also cause a certain annoyance to the pregnant woman.
