Nuchal Translucency Calculator

What is Nuchal Translucency (NT)?

Nuchal Translucency (NT) is the sonographic measurement of the subcutaneous fluid collection behind the fetal neck in the first trimester of pregnancy. Between 11 weeks 0 days and 13 weeks 6 days of gestation (corresponding to a fetal crown-rump length of 45 mm to 84 mm), all fetuses possess a small fluid layer.

An increased thickness of this fluid layer is a primary ultrasound marker associated with chromosomal aneuploidies—most notably Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome)—as well as major congenital heart defects, diaphragmatic hernias, and genetic syndromes.

How NT Thickness Relates to Crown-Rump Length (CRL)

Because normal NT thickness increases incrementally with fetal growth, measurements must be interpreted against the crown-rump length rather than as a static cutoff. According to the Fetal Medicine Foundation (FMF) nomograms, the expected median NT thickness is modeled using regression analysis:

$$\text{Expected Median NT (mm)} = 0.437 + (0.01969 \times \text{CRL in mm})$$

Gestational age can also be estimated from CRL via the Robinson formula:

$$\text{Gestational Age (days)} = 8.052 \times \sqrt{\text{CRL}} + 23.73$$

Reference Ranges and Percentiles

At CRL 45 mm (approx 11 weeks), the median NT is approximately 1.3 mm, with a 95th percentile around 1.8 mm. By CRL 84 mm (approx 13w6d), the median NT rises to approximately 2.1 mm, with a 95th percentile around 2.7 mm.

Clinical Thresholds and Follow-up Testing

• Normal Range (< 95th percentile): Routine prenatal care. Often combined with serum biochemistry (free β-hCG and PAPP-A) or non-invasive prenatal testing (NIPT/cell-free DNA).
• Intermediate Elevation (95th percentile to 3.4 mm): Elevated risk. Genetic counseling and targeted mid-trimester fetal anatomical survey recommended.
• Significant Elevation (≥ 3.5 mm / > 99th percentile): Substantial risk of aneuploidy or structural heart anomaly. Diagnostic testing via chorionic villus sampling (CVS) or amniocentesis, plus fetal echocardiography at 18–20 weeks, is strongly advised.