Nuchal translucency is measured during the first trimester of pregnancy. If it is abnormal, it can explain certain chromosomal abnormalities such as trisomy 21. What are the normal levels and values of nuchal translucency? The point with Dr. Françoise Boussion, gynecological-obstetrical sonographer.
During the first trimester of pregnancy, the practitioner following the pregnant woman measures the fetal nuchal translucency. It is during a pregnancy ultrasound carried out precisely between 11 and 12 weeks of pregnancy that this particular area on the body of the fetus can be checked. This exam is important in the follow-up of pregnancy, because a abnormal nuchal translucency can reveal chromosomal abnormalities in the fetus, especially Trisomy 21. What is nuchal translucency? What is the normal value? Insight from Dr. Françoise Boussion, gyneco-obstetrical sonographer at the University Hospital of Angers.
What is fetal nuchal translucency?
nuchal translucency is a small area at the neck of the fetus. Concretely, it is the thickness of the tissues between the skin and the cervical spine. Due to a small detachment between the spine and the skin, it is present in all fetuses during the first trimester of pregnancy, but then disappears. “As the fetus has not yet necessarily connected its lymphatic channels, the lymphatic fluid accumulates at the back of the neck: this is called nuchal translucency”explains Dr. Françoise Boussion, gyneco-obstetrical sonographer at the University Hospital of Angers.
Nuchal translucency is measured on ultrasound during the first trimester of pregnancy. “It is measured according to very strict criteria which have been established by the French college of fetal ultrasound, details the specialist. Each sonographer has been tested, noted and you must have an approval number to be able to do it. This requires a very strict cutting plan, it is necessary that the fetus is sufficiently magnified on the image and that the callipers (points to make the measurements, editor’s note) are well positioned for the neck measurement to be validated. With that, we get a score out of 9this is called the Herman score. If it is less than 7, the neck measurement is not considered valid.”
When to do ultrasound to measure nuchal translucency?
Usually it is measured on ultrasound between 11 and 13 and a half weeks of amenorrheai.e. between 11 and 12 weeks of pregnancy, when the embryo is between 45 and 84 mm. “Below 45 mm and above 84 mm, nuchal translucency has no value. The liquid accumulated in the cervical region of the embryo corresponds to the lymph and is physiological at this term. It becomes worrying when the thickness is too great, which may indicate a delay in the connection of the lymphatic channels to the heart”, says the expert.
What is the normal nuchal translucency value?
To be considered normal, nuchal translucency should be less than 3.5 mm. “If this is the case, we then offer screening for trisomy 21 by taking a blood test”says the sonographer.
What is the value of a thick nuchal translucency?
The nuchal translucency is considered thick if it is greater than 3.5 mm, “up to 10 mm or more”. “If the nuchal translucency is greater than 3.5 mm, we do not offer blood screening for trisomy 21, but immediately a biopsy of the placenta to look for a chromosomal abnormality”, develop the professional.
“If the nuchal translucency is greater than 3.5 mm, placental biopsy is suggested to look for a chromosomal abnormality.”
Two scenarios are then possible:
► The fetal karyotype is abnormal and the couple has the choice of whether or not to continue the pregnancy.
► There are no chromosomal abnormalities : in this case, an early morphological ultrasound is done around 17 weeks of amenorrhea, or 15 weeks of pregnancy, to look for a cardiac anomaly or other fetal anomalies. “We know that these are babies who have good prognoses but we monitor them carefully throughout pregnancy by doing an ultrasound every month with a sonographer specialized in this field.“, emphasizes Dr. Françoise Boussion.
What is the rate of nuchal translucency in case of trisomy 21?
Up to 3.5mm thick, nuchal translucency does not justify systematic karyotyping. “There is no real correlation between the thickness of the neck and the risk of trisomy 21, nuances the specialist. However, when the neck is very thick, up to 10 mm see more and that the liquid comes to be put around the head and the heart of the baby, one speaks then cystic hygroma and anasarca (two phenomena associated with trisomy 21, editor’s note). The risk of chromosomal abnormalities then rises to 50% and the risk of fetal death remains very high throughout the pregnancy..
When to worry about nuchal translucency?
A nuchal translucency greater than 3.5 mm may lead to a risk of genetic abnormalities, in particular trisomy 21 or trisomy 18, which is about 10%. “If we obtain a normal result at the level of the chromosomes, the risk of cardiac anomaly rises to 20%, continues Dr Françoise Boussion.
The measurement of the neck is therefore a very good marker of chromosomal abnormalities and cardiac abnormalities. If the baby has neither a chromosomal anomaly nor a cardiac anomaly, the children followed until their entry into CP nevertheless have an additional risk of 11% of having other cerebral, vascular, etc. abnormalities.. In any case, it is up to your practitioner to warn you if there is a risk for your baby. No need to interpret results alone in your area. Do not hesitate to ask all your questions to the health professional to reassure you.
Thanks to Dr Françoise Boussion, gyneco-obstetrical sonographer at the CHU d’Angers.