Nuchal Translucency Reference Calculator

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Use the CRL from your ultrasound first. If you also have the NT measurement from that same scan, add it for an approximate reference percentile and chart.

Use the measured CRL from the ultrasound, to one decimal place. This reference applies from 45.0 to 84.0 mm.

Optional. Enter the measurement exactly as reported, to one decimal place.

I have NT and gestational age, but not CRL

Look for CRL or crown-rump length on the ultrasound report, or ask the ultrasound service. This calculator does not turn weeks and days into a precise CRL because NT reference values should use the CRL from the same scan.

Peer-Reviewed Science
Updated July 2026
See Methodology
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Methodology

Enter CRL first. Add an NT measurement only if it came from the same scan.

The calculator estimates gestational age from CRL with the Robinson-Fleming dating equation. NT reference values and approximate percentiles use the unaffected-pregnancy distribution described by Wright and colleagues. These are reference estimates, not a diagnosis or a personal chromosome-condition chance.

NT screening is normally interpreted with the full ultrasound and, where relevant, other screening information. The CRL and NT should be from the same scan. If your report gives NT and gestational age but not CRL, ask your ultrasound service for the CRL rather than converting weeks and days into a precise percentile.

Sources

If your result needs follow-up

An NT result is one part of screening and cannot calculate an individual chance by itself. Follow the plan from your ultrasound service or clinician. If your report says the NT is 3.5 mm or more, contact the service that arranged the scan if you have not already been given follow-up advice.