Nuchal translucency
Excess fluid within the nuchal space, typically observed during the 12-week scan, may be indicative of a chromosomal, syndromic, structural or genetic condition.
Overview
A nuchal translucency (NT), also called nuchal fold or nuchal thickness, is a measurable area at the back of the fetal neck. It is examined using ultrasound as part of combined screening for Down, Edwards and Patau syndromes from 11 to 13 weeks and six days of pregnancy.
To measure the NT, the fetal crown-rump length should be between 45 millimetres and 84 millimetres.
A small amount of fluid (up to 3.5 millimetres) within the nuchal space (between the back of the neck and the skin) is normal. Excess fluid can also be a feature in a normal fetus; however, it may be indicative of a chromosomal, syndromic, structural or genetic condition.
A higher NT measurement may indicate an increased chance of congenital heart conditions, Turner syndrome, trisomy and viral infection. The probability varies depending on the extent of the nuchal thickness.
Risks associated with an increased NT
Table 1: Higher NT measurements and associated risks
| NT measurement | Chance of chromosomal anomaly | Chance of pregnancy loss | Chance of major fetal anomaly | Fetus will be alive and well |
| 3.5mm–4.4mm | 21.1% | 2.7% | 10% | 70% |
| 4.5mm–5.4mm | 33.3% | 3.4% | 18.5% | 50% |
| 5.5mm–6.4mm | 50.5% | 10.1% | 24.2% | 30% |
| Over 6.5mm | 64.5% | 19% | 46.2% | 15% |
NT screening can detect around 80% of fetuses with Down syndrome and other major aneuploidies for a false positive rate of 5%.
Resources
For clinicians
- Fetal Medicine Foundation: Nuchal translucency scan
- NHS England: National Genomic Test Directory
For patients
- Antenatal Results and Choices
- International Society of Ultrasound in Obstetrics and Gynaecology: Nuchal translucency
- Leeds Teaching Hospitals NHS Trust: Increased nuchal translucency measurement: Information for patients