Nuchal translucency refers to a small space at the back of the embryo’s neck. Because it is abnormally enlarged in fetuses with a chromosomal abnormality, its ultrasound measurement is used as part of the early detection of trisomy 21.

Nuchal translucency (CN) is a thin space located at the back of the fetal neck. It corresponds to a small detachment between the skin and the spine. On the first trimester ultrasound, it appears as a black space at the back of the baby’s head because it is a so-called anechoic fluid area (it does not return the ultrasound echo). At this stage of pregnancy, all babies have this area but it then disappears.

Babies with chromosomal abnormalities such as Down’s syndrome or certain lymphatic or heart abnormalities have been observed to have above average nuchal translucency. Hence the idea of ​​measuring it and comparing the result with a reference curve in order to make a first estimate of the risk of chromosomal anomaly.

The measurement of nuchal translucency is now used in the context of “combined screening of the first trimester” which combines 3 data:

• the assay of two serum markers (PAPP-A protein and free beta-hCG) carried out by taking blood between 8 and 14 weeks;
• measurement of nuchal translucency at the first ultrasound;
• maternal age.

These three data are entered into software in order to calculate a “combined risk” (not to make a diagnosis) of chromosomal abnormality.

This screening is governed by law (1). It is systematically offered to pregnant women but not imposed. If the combined screening could not be performed in the first trimester, a catch-up screening may be performed. This is either integrated sequential screening (serum markers in the second trimester and measurement of nuchal translucency at the first ultrasound), or screening by serum markers alone in the second trimester.

Nuchal translucency is measured during the first ultrasound performed between 11 WA and 13 WA + 6 days, on an embryo measuring between 45 and 84 mm in cranio-caudal length (LCC or head-buttock length). It must be done under specific conditions: on a sagittal section of the embryo (that of the LCC measurement), with an embryo at rest (2).

In France, the risk threshold is set at 1/250 (for 250 women with the same screening result, 1 child is affected). A karyotype (“map” of the baby’s chromosomes) will then be offered to the pregnant woman to rule out or confirm the diagnosis of trisomy. It can also be offered straight away in cases of nuchal translucency greater than or equal to 3,5 mm (3).

To achieve this karyotype, two examinations are possible:

• the trophoblast biopsy which consists in puncturing, via the abdominal route (more rarely vaginally) a sample of the trophoblast or future placenta. It can be performed early in the first trimester of pregnancy (between 11 and 14 weeks) in the event of a significant combined risk;
• an amniocentesis which consists of taking amniotic fluid, also via the abdominal route. This examination can be performed after 15 weeks.

The couple is free to carry out or not a trophoblast biopsy or an amniocentesis, which will then be taken care of as part of the prenatal diagnosis. These invasive examinations both carry a risk of miscarriage. For a long time, this risk was estimated at nearly 1%, but it would only be 0,1% according to the most recent data in the literature (4).

New screening tests have recently been developed. They consist in looking for an overrepresentation of chromosome 21 in free DNA circulating in maternal blood. They therefore make it possible to refine the risk estimate by a simple blood test and thus avoid certain invasive examinations. On the other hand, they do not replace the fetal karyotype, the only examination allowing the diagnosis of trisomy 21.

In 2017, the HAS issued new recommendations and recommends offering this DNA test after a combined screening of between 1/1000 and 1/51. For a risk greater than or equal to 1 in 50, the HAS maintains its recommendation to immediately propose a fetal karyotype, with the possibility for future mothers who so wish to perform a DNA test first (5).