Enter a crown-rump length (CRL) to see NT reference values. If you also have the NT measured at that same scan, use the second panel to see its approximate position in the reference distribution.
CRL-based reference results
These are reference-distribution estimates, not a diagnosis or a prediction of an individual screening result.
Measured NT reference comparison
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NT reference curves by CRL
NT alone cannot calculate the chance of a chromosome condition. Screening interpretation combines NT with other information and should follow your clinician’s and local program’s guidance.
Choose a focused NT tool
Use the main calculator above for both reference values and a measured NT comparison.
- Nuchal translucency reference calculator — one form with an optional NT measurement.
- Estimate your nuchal translucency — CRL reference values only.
- Assess your nuchal translucency — compare a measured NT with the same-scan CRL.
Methodology
The first panel estimates reference NT values from CRL. The second compares a measured NT with reference values for the CRL 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
- Wright et al., NT reference distribution
- Perinatal Institute, CRL dating policy
- NHS FASP screening handbook
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.