Nuchal Translucency (NT) Calculator — MoM & Risk Category

Formula

How this is calculated

Nuchal translucency is the fluid-filled space at the back of a fetus's neck, measured by ultrasound between 11 weeks 0 days and 13 weeks 6 days gestation (CRL 45–84 mm). As the fetus grows during this window, NT naturally increases; results are therefore compared not to a fixed number but to the expected median NT for the fetus's CRL. The Multiple of Median (MoM) expresses this comparison: MoM = measured NT ÷ median NT at the same CRL. A value near 1.0 MoM is average.

The expected median NT in this calculator uses a linear approximation anchored at the well-established reference points from Snijders et al. (1998 Ultrasound Obstet Gynecol): median NT ≈ 1.2 mm at CRL 45 mm and 1.7 mm at CRL 84 mm. Gestational age is estimated from CRL using the Robinson & Fleming (1975) formula: GA (days) = 42.22 + 1.8836 × CRL. Both are educational approximations; precise clinical work uses population-specific regression curves.

An NT below 2.5 mm is generally considered within the normal range at any CRL in the screening window. An NT of 3.5 mm or above is associated with increased risk of chromosomal abnormalities (particularly trisomy 21, 18 and 13) and structural defects; clinical guidelines recommend referral for genetic counselling and further investigation. Integrated risk assessment also weighs maternal age, free beta-hCG, PAPP-A, nasal bone and other markers — this calculator provides only the NT component.