Pregnancy

What is nuchal translucency

Nuchal translucency is an ultrasound and therefore non-invasive measurement of the fluid found at the level of the neck of the fetus (nape). This ultrasound which allows to obtain some information on some chromosomal abnormalities of the fetus if it is performed by an expert doctor and during correct weeks of pregnancy.

According to the most recent guidelines of the Fetal Medicine Foundation (FMF) it is advisable to separate the sampling from the ultrasound, and therefore perform the hormone dosage between 9+0 and 10+6 weeks of pregnancy (the Bi-test) , while should perform the ultrasound between 11+4 and 13+6  weeks of pregnancy .

Nuchal translucency, if performed by a good sonographer,  makes it possible to predict with a certain probability whether the fetus is affected by chromosomal abnormalities , in particular trisomy 21 (“Down syndrome”), trisomy 13 and 18. However, the latter two they are much rarer and usually result in a miscarriage.

It is important to underline that nuchal translucency has a probabilistic value and does not give certain results as are more invasive techniques such as CVS or amniocentesis ( the latter two, however, expose to a risk of miscarriage , albeit low).

How is nuchal translucency performed?

Nuchal translucency measures the maximum thickness of the space between the skin and the soft tissue behind the neck. This is where the lymphatic-like fluid is formed.

If the liquid occupies a thickness greater than 2.5-3 millimeters, there is a risk that the child may have chromosomal abnormalities, such as Down syndrome or other trisomies (13 or 18) or cardiac or neural tube malformations or other other organs.

At this point the doctor recommends carrying out CVS or amniocentes to be sure of the pathology. Amniocentesis is usually performed between the 15th and 18th week of pregnancy.

Translucency alone gives a correct result only in 75% of cases to which we must also add 5% of false positives.

Therefore, other parameters are associated with this ultrasound technique to increase its reliability . And what are they?

At the same time as the ultrasound, 2 hormones are dosed ( Bi-test ). During pregnancy mother and child produce 2 hormones: PAPP-A and free-Beta HCG whose levels are altered in case of trisomies.

When a blood sample is taken to measure the 2 hormones and translucency is performed at the same time, we speak of “Ultrascreen” or Combined Test .

The Ultrascreen  associates a total of 4 parameters: the mother’s age, the child’s age which is obtained from ultrasound measurements generally the head-coccyx length, the nuchal translucency and the Bi-Test .

Recently, the measurement of the nasal bones of the fetus has also been added to these elements as it has been seen that in Down’s Syndrome the formation of the nasal bones is late or absent in the fetus. Ultrascreen and measurement of the nasal bones reach a predictive capacity of 98% (always 5% false positives).

Dr Kathryn Barlow

Kathryn Barlow is an OB/GYN doctor, which is the medical specialty that deals with the care of women's reproductive health, including pregnancy and childbirth.

Obstetricians provide care to women during pregnancy, labor, and delivery, while gynecologists focus on the health of the female reproductive system, including the ovaries, uterus, vagina, and breasts. OB/GYN doctors are trained to provide medical and surgical care for a wide range of conditions related to women's reproductive health.

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