Pregnancy/Maternity

The nuchal translucency (NT) ultrasound

What is it?

This is a collection of fluid under the skin at the back of a baby's neck at 11-14 weeks that can be measured using an ultrasound. It determines the risk of genetic abnormalities, including Down syndrome.

How?

The nuchal translucency measures the translucid space between the muscles and the skin at the back of a baby's neck, a space that is only visible between the 9th and 14th week of pregnancy. The increase in this space is associated with a higher risk of chromosomal abnormalities and a dozen other malformations.

An abnormal measure is a measure above the 95th percentile for gestational age, which means 2.2 mm at 11 weeks and 2.8 mm at 14 weeks. Unfortunately, this sign disappears before the routine ultrasound at 18-20 weeks. Only a trained specialist is allowed to measure this space, since it’s is a medical procedure.

Efficiency

About 75 per cent of babies with Down syndrome were correctly identified using the.nuchal translucency test. As this test is not 100% reliable, it is essential to undergo a Chorionic villus sampling (CVS) or amniocentesis, which gives 100 % reliable results. However, 1/200 to 1/300 women develop an infection or some other complication as a result of their amniocentesis that then results in miscarriage.

Advantages
  • No risk for the mother or the fetus
  • Confirms the gestational age
  • Detects multiple pregnancies
  • Increased sensitivity to chromosomal and cardiac abnormalities
  • Early detection of other malformations
Disadvantages
  • Additional test
  • Additional costs
  • More Chorionic villus samplings (CVS) and amniocentesis
  • Requires equipment with a good resolution and experts for the measurement and interpretation
Down syndrome

The incidence of Down syndrome varies according to the mother’s age. Despite the fact that the incidence increases with the mother’s age, 70 to 80 % of Down syndrome births occur among mothers under 35 years old who don’t have access to an amniocentesis.

Incidence of Down syndrome

(according to the mother’s age)

15 years old

1 out of 1500

20 years old

1 out of 1450

25 years old

1 out of 1260

30 years old

1 out of 820

32 years old

1 out of 600

34 years old

1 out of 420

35 years old

1 out of 340

40 years old

1 out of 100

45 years old

1 out of 25

48 years old

1 out of 11

Warning

There are characteristics that are only detectable in the second trimester, such as some heart defects, for example. For this reason, it is essential to have two ultrasounds: the first one at 12 weeks and the second one between the 18th and 20th week.

Sources : Forum 2002, Dr JEAN-MARTIN BAILLARGEON, Centre Hospitalier de l'Université de Montréal, Hôpital Saint-Luc, Radiologie, Section Échographie fœtale


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