Compare first-trimester nuchal translucency with a CRL-based reference or review a second-trimester nuchal fold measurement with clear timing context.

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The cited log-quadratic model covers 40-84 mm; its paper reports poorer fit below 45 mm.
This mode is a measurement comparison, not a diagnosis.

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 ≈ (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.