Normal Nuchal Translucency Size on Ultrasound 11–14 Weeks
Nuchal translucency (NT) refers to the subcutaneous fluid collection at the posterior neck of the fetus, visualized sonographically in the first trimester. Accurate NT measurement between 11 and 14 weeks of gestation is a cornerstone of first-trimester aneuploidy screening. An increased NT is associated with chromosomal defects, structural cardiac anomalies, and adverse perinatal outcomes.
Normal Reference Values
| Measurement |
|---|
| <3 mm |
Clinical Significance
A nuchal translucency measurement of less than 3 mm is considered within the normal range between 11 and 14 weeks of gestation. When NT is at or above this threshold, the risk of chromosomal abnormalities rises significantly and warrants further evaluation, including combined first-trimester screening, cell-free fetal DNA testing, or invasive diagnostic procedures such as chorionic villus sampling.
Even when the karyotype is normal, a persistently elevated NT (≥3.5–4 mm or greater) is associated with structural anomalies — particularly congenital heart defects — as well as genetic syndromes, skeletal dysplasias, and adverse neurodevelopmental outcomes. It is important to note that NT is a continuous variable; risk increases with increasing NT measurement rather than representing a simple normal/abnormal binary.
Common clinical scenarios associated with increased nuchal translucency include:
- Trisomy 21 (Down syndrome)
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
- Turner syndrome (45,X)
- Congenital cardiac defects with normal karyotype
Reference: Pandya PP, Kondylios A, Hilbert L et al. Chromosomal defects and outcome in 1015 fetuses with increased nuchal translucency. Ultrasound Obstet Gynecol. 1995;5(1):15–9.
Imaging Notes
NT measurement is performed with transabdominal or transvaginal ultrasound between 11 weeks 0 days and 13 weeks 6 days (crown-rump length 45–84 mm). The fetus should be in a neutral mid-sagittal position, magnified so that the head and upper thorax occupy the majority of the screen. Calipers are placed on the inner borders of the nuchal fold, perpendicular to the fetal long axis, at its widest point. Care must be taken to distinguish the fetal skin from the underlying amnion, as confusion between the two is a recognized source of measurement error.
Standardized training and quality-control programs (e.g., Fetal Medicine Foundation certification) are strongly recommended, as inter-operator variability significantly affects screening performance. At least three measurements should be obtained and the largest recorded value used for risk calculation.