Nuchal Translucency Calculator

The nuchal translucency (NT) test is an ultrasound screening performed between 11 and 14 weeks of pregnancy. It measures the fluid-filled space at the back of the baby's neck. A thicker NT measurement can indicate a higher risk of chromosomal conditions such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), or Patau syndrome (trisomy 13). The NT result is typically combined with blood tests (PAPP-A and hCG) for a more accurate risk assessment.

The expected NT value is calculated based on the baby's Crown-Rump Length (CRL) using the formula: Expected NT = 0.437 + (0.01969 × CRL in mm). This formula, derived from large population studies, provides a baseline against which your measured NT can be compared. Your measured NT is then expressed as a multiple of the median (MoM) or a percentile rank.

At 12 weeks (CRL approximately 60 mm), the expected NT is around 1.6 mm. An NT measurement below the 95th percentile is generally considered within the normal range. Most guidelines use an NT of less than 3.0 mm as the upper limit of normal, though risk assessment considers the NT in conjunction with CRL and maternal blood markers for a fuller picture.

Yes, an NT of 0.8 mm at 12 weeks is well within the normal range. The expected NT at this gestational age is approximately 1.6 mm, so 0.8 mm would fall below the median — which is not a cause for concern. Very low NT values are not associated with increased genetic risk.

An NT measurement at or above 3.0 mm, or above the 99th percentile for the CRL, is generally considered elevated and warrants further investigation. Higher NT values are associated with increased risks of chromosomal abnormalities, cardiac defects, and other structural issues. However, many babies with a high NT measurement are born healthy — the result is a risk indicator, not a diagnosis.

An abnormally thick NT is associated with chromosomal conditions including Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13), and Turner syndrome (45,X). It can also be linked to congenital heart defects, skeletal dysplasias, and other structural abnormalities. The risk rises progressively with NT thickness above the 95th percentile.

No, a normal NT scan cannot guarantee the health of the fetus. It significantly reduces the probability of certain chromosomal conditions but is a screening test, not a diagnostic one. Some chromosomal and structural conditions may not affect NT thickness. For a more definitive answer, diagnostic tests such as chorionic villus sampling (CVS) or amniocentesis can be offered to high-risk pregnancies.

NT naturally increases with gestational age, which is why it must always be interpreted in relation to the CRL (a proxy for gestational age). The NT scan window is 11 to 14 weeks (CRL 45–84 mm). Outside this window, the measurement is less reliable. This calculator uses the CRL to estimate gestational age and calculate the expected NT value for comparison.