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:
- (i) chromosomal anomalies including Downs syndrome, Edwards syndrome, Turners syndrome, Patau syndrome;
- (ii) fetal structural defects, in particular congenital cardiac defect;
- (iii) viral infection such as Parvovirus B19
- (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