Nuchal translucency ultrasound

1. What is nuchal translucency (NT)?

  • For every fetus, a collection of fluid is noted under the skin behind the neck in the first trimester. This can be picked up on ultrasound examination; 
  • Nuchal translucency (NT) refers to the appearance of this fluid-filled space on ultrasound;
  • Accurate measurement of the NT thickness can be obtained by ultrasound at 11-13 6/7 weeks of gestation;
  • The normal range of the NT thickness increases with the fetal length (crown-rump length)
2. What does it mean if the NT measurement is increased?
  • Increase in NT thickness refers to measurements outside the upper limit of the normal range;
  • Increase in NT thickness can be a normal variant and transient. However, it is an important marker for following conditions:
  1. (i) chromosomal anomalies including Downs syndrome, Edwards syndrome, Turners syndrome, Patau syndrome;
  2. (ii) fetal structural defects, in particular congenital cardiac defect;
  3. (iii) viral infection such as Parvovirus B19
  4. (iv) some genetic abnormalities;
  • The more deviated the NT is from the normal range, the higher the chance of having fetal abnormalities;
  • An early fetal morphology scan is needed if the NT is found to be increased;
  • Options of invasive prenatal diagnosis should be discussed

3. How is NT measured on ultrasound?

  • Usually a transabdominal ultrasound; 
  • Gestation age should be between 11-13 6/7 weeks;
  • Crown-rump length of fetus should be between 45-84 mm;
  • NT thickness is measured at sagittal plane of the fetal head (see photos)

4. Is NT ultrasound still needed if NIPT is normal? 

  • NT is an important screening test not just for fetal chromosomal abnormalities, but also for fetal structural defects in particular congenital heart diseases. NT is also a good screening test for some genetic conditions;
  • It has been shown that among fetuses with an increase in NT > 99th centile in the first trimester, use of NIPT alone will miss up to 19 % of genetic abnormalities;
  • Hence, a NT scan is still recommended even if NIPT is chosen
This article is contributed by Dr. T.N. Danny Leung
Updated on 25.09.2020