Compare a nuchal translucency (NT) measurement with the expected median for crown-rump length (CRL), or check a second-trimester nuchal fold against 6 mm.
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 first mode compares measured NT with the log-quadratic median regression reported by Bestwick et al. (2014), using CRL and NT in millimeters. The study covers CRL 40–84 mm; fit is poorer below 45 mm, and the curve turns down after about 78 mm. This is an educational reference comparison, not a percentile or personal risk score.
Variables:
- NT median is the regression-estimated study median nuchal translucency (mm)
- CRL is the crown-rump length (mm)
Calculate the median as 10 raised to the displayed regression value, then divide measured NT by that median. Clinical interpretation requires validated reference data and the full clinical record.
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 second-trimester nuchal fold thickness. SMFM Consult Series #57 defines a thickened fold as at least 6 mm at 15–20 weeks. This calculator compares 6 mm only in its 16–20 week band. The later, after-20-through-24-week option reports the measurement without applying a cutoff.
