Read the test of Nuchal Translucency

This article is also available in Italian.

What can we expect when we go to do the test of Nuchal Translucency?

Whereas there is no association between the Nuchal Translucency and serum concentration of free beta-hCG and PAPP-A, the values of biochemical and ultrasound can be combined to get a screening test more sensitive (see Valutazione della Translucenza Nucale).

With graphs below, you can immediately identify the percentage of risk calculated from the ultrasound data of the first trimester (11-14 weeks).

The values which we find are:

• The crown-rump length (CRL) which must be between 45 and 84 mm and in gestational age between 11 and 13 weeks +6 days. The graph of the CRL is related gestational age (in weeks).
  
• The Nuchal Translucency (also called NT), expressed in millimeters, is related to the CRL:
  
• The Maternal Age, is most important to know the “Risk of Trisomia 21 in the first trimester”.
  

Then enter your data, if they are located on the first red line down, the risk is 5% and, higher it value, the higher the risk, if it move in the red line higher, the risk is 95%.

For the last graph, the risk of Trisomy 21, you must enter your age and draw a straight vertical line up to the green line, now, drawing a horizontal line will get a percentage of the value of risk.

This table is from the site www.ilmiobaby.com, and indicates which is the risk that a woman, on the basis of age, has to have a child with Down’s syndrome. The variations of the thickness of NT modify this probability.

The risk based on maternal age, was calculated starting on the risk of basis, derived from the analysis of more than 100,000 patients in the course of a project of the Fetal Medicine Foundation. Furthermore, the risk of Trisomy 21 to term of pregnancy is lower than at this time because 30% of pregnancies with fetuses suffering hesitate in a miscarriage.

Is common in many congenital anomalies, fetuses and newborns have dysmorphic facial. The skin of Down is plentiful and flaccid (see NT), the face and the occiput flat, the nose small and these characteristics are common to almost all people suffering from the syndrome. Regarding the nasal bone (NB), the paper published in 2001 by the Fetal Medicine Foundation (Cicero S, Curcio P, Papageorghiou A Sonek J, Nicolaides KH. Absence of nasal bone in Fetuses with trisomy 21 at 11-14 weeks of gestation: an observational study. Lancet 2001, 358:1665-67) has shown the association of Down syndrome with the nasal bone absence; indeed in 70% of fetuses affected by Down syndrome, at the ultrasound control from the 11th to the 14th week, the nasal bone was not visible.

Then, in the absence of nasal bone, the risk of chromosomal defects increases by 30 times, and this risk increases 50 times when the assessment is made in the second trimester (the measure nasal bone less than 2.5 mm made a 20 W of amenorrhea).

This procedure is not yet confirmed by multicenter studies, and for now, is little practiced, but above all require special skills and experience of echographers and high-level ultrasound.

The Ultra-Screen may identify 90% of children with Down syndrome and Trisomy 18. The test can also identify other types of abnormal chromosomes, the genetic defects and congenital anomalies.

The examination may not, in any event, ensure that the child is absolutely free from congenital anomalies, in fact 5% of cases the examination may be a false positive, that is, children are born healthy despite the examination is suspected the presence of the disease.

In the vast majority of cases, however, there is no congenital defect and is therefore highly unlikely that the child suffers from Down syndrome or Trisomy 18.

As for the blood sample tested, are measured concentrations of two substances produced by the placenta and fetus. One is the free beta-HCG, namely the free-beta subunit human chorionic gonadotropin and the other is the PAPP-A that is an abbreviation of the term Pregnancy Associated Plasma Protein A.

The lower the PAPP-A, and the greater free beta-HCG, the greater the possibility of chromosomal diseases. In pregnancies with fetuses affected by trisomy 21 at 11-14 weeks, the serum concentration of free beta-HCG (about 2 MoM) is higher than in chromosomally normal fetuses, whereas PAPP-A is lower (about 0.5 MoM).

The values of these doses are compared with those of so-called medians reference (in practice this is the development of results from hundreds or thousands doses to determine which are the normal values in certain population: for this reason the dosage should be made in centers that have this amount of data) and those with a parameter of ultrasound (called Nuchal Translucency). The results of hormonal assays, together with the data of translucency nucale and other parameters, are processed using appropriate software.

The result is a risk index expressed numerically as a fraction (1 / 500) that should be compared to the threshold value (also called “cut off“; the cut-off risk of 1:300 was equal to that which has the best characteristics in terms of sensitivity of the test): values above the “cut off” represent a positive (as increased risk), while values below the “cut off” represent a negative.