Nuchal Translucency Risk Ratio Calculator

First-trimester screening starts from a background (a priori) risk, usually based on maternal age and gestational age. Each screening marker, such as nuchal translucency, contributes a likelihood ratio. The adjusted risk equals the background odds multiplied by the likelihood ratio. This calculator does that odds multiplication for you.

A likelihood ratio is a number that scales the odds. A ratio above 1 raises the risk; a ratio below 1 lowers it. Your screening report or clinician provides the likelihood ratio derived from the measured markers. This calculator does not derive that ratio from a raw nuchal translucency measurement, because that conversion uses proprietary reference distributions.

Turning a raw nuchal translucency measurement into a likelihood ratio requires gestational-age-specific reference medians and distributions held by accredited screening programs. We will not fabricate those, so this tool instead takes the likelihood ratio that your accredited screening report already provides, and combines it correctly with your background risk.

Risk is shown both as a probability and in the familiar 1 in N form. The math converts the background 1 in N to odds, multiplies by the likelihood ratio, then converts back. For example, a 1 in 1,000 background risk with a likelihood ratio of 5 gives roughly a 1 in 200 adjusted risk.

No. It performs the standard odds arithmetic of sequential screening only. It does not interpret results or replace accredited screening and counseling. Always rely on your obstetric provider and screening service for interpretation and decisions.