Enter the Crown-Rump Length (CRL) into the calculator to estimate an approximate median nuchal translucency (NT) using a simplified (non-clinical) model. (NT is sometimes confused with the second-trimester nuchal fold measurement.)

Nuchal Fold Calculator

Assess nuchal translucency or nuchal fold thickness against gestational reference values.

First Trimester (NT)
Second Trimester (NF)
Enter NT measurement in mm.
CRL should be between 45 and 84 mm (≈11–14 weeks).
Enter nuchal fold measurement in mm.
Select gestational age (15–22 weeks).
Result
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▸ How this is calculated
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For educational reference only. Nuchal measurements are part of a screening assessment that includes maternal age, biochemistry, and other markers. Discuss results with your obstetric provider.

Medical disclaimer: This calculator provides a simplified, non-clinical estimate and is not for diagnosis or medical decision-making. NT must be measured and interpreted by a qualified clinician using validated reference charts (percentiles/MoM), proper ultrasound technique, and the full clinical context.

Nuchal Translucency (NT) Formula

The following formula is a simplified linear model used by this calculator to approximate a median nuchal translucency (NT) in millimeters from the crown-rump length (CRL) in millimeters. It is intended only across the usual NT screening range of CRL ~45–84 mm (about 11–13+6 weeks).

NT \approx (CRL * 0.018) + 0.4

Variables:

  • NT is the approximate median nuchal translucency (mm)
  • CRL is the crown-rump length (mm)

To approximate NT, multiply the CRL by 0.018, then add 0.4. In clinical practice, NT interpretation is typically based on validated reference charts/percentiles (or MoM) for a given CRL/gestational age rather than a single linear equation.

What is Nuchal Translucency (NT)?

Nuchal translucency (NT) is the fluid-filled space at the back of a fetus’s neck measured on ultrasound during the first-trimester screening window (about 11–13+6 weeks, when CRL is roughly 45–84 mm). Increased NT can be associated with a higher risk of certain chromosomal conditions (such as trisomy 21/Down syndrome) and some structural or genetic conditions, but NT alone is not diagnostic and follow-up testing may be needed.

Note: NT is different from the second-trimester nuchal fold thickness measurement, which refers to soft-tissue thickness (not a fluid space) and is assessed later in pregnancy.