Calculator Input
NT Reference Graph
The graph compares your observed measurement with the estimated median, 95th centile, and 99th centile reference curves across CRL values.
Example Data Table
| Gestational Age | CRL (mm) | Observed NT (mm) | Expected Median (mm) | MoM | Approx. Interpretation |
|---|---|---|---|---|---|
| 11w 4d | 47.0 | 1.40 | 1.41 | 0.99 | Near median reference |
| 12w 2d | 58.0 | 1.90 | 1.55 | 1.23 | Within expected range |
| 12w 6d | 69.0 | 2.30 | 1.70 | 1.35 | Upper but still near range |
| 13w 3d | 77.0 | 3.60 | 1.80 | 2.00 | High review threshold |
Formula Used
Expected Median NT = 0.80 + (0.013 × CRL)95th Centile NT = 1.15 + (0.018 × CRL)99th Centile NT = 1.55 + (0.023 × CRL)MoM = Observed NT ÷ Expected Median NTDelta NT = Observed NT − Expected Median NTEstimated Percentileis interpolated between the median, 95th centile, and 99th centile lines.
These equations create a practical educational reference curve for quick estimation and reporting. They are not a validated diagnostic engine, and they should not replace specialist ultrasound interpretation or a formal prenatal screening pathway.
How to Use This Calculator
- Enter the gestational weeks and extra days from the scan visit.
- Enter the crown-rump length in millimeters.
- Enter the measured nuchal translucency value in millimeters.
- Add maternal age and fetal heart rate if you want extra context shown.
- Choose singleton or twin pregnancy for context labeling.
- Press Calculate NT to show the result above the form.
- Review the observed NT, expected median, MoM, centiles, percentile, and interpretation.
- Use the export buttons to download a CSV or PDF report.
FAQs
1) What is nuchal translucency?
Nuchal translucency is the fluid-filled space seen at the back of the fetal neck during an early pregnancy ultrasound. The measurement is compared with fetal size and gestational timing to judge whether it looks expected, upper-range, or unusually enlarged.
2) When is NT usually measured?
It is usually measured during the first-trimester screening window. This calculator highlights the common window when gestation and crown-rump length fit the usual scan range used for NT-based screening interpretation.
3) Does a high NT confirm a diagnosis?
No. A higher NT does not confirm a chromosome condition, heart problem, or structural issue. It signals that specialist review may be needed, usually alongside the full ultrasound assessment and, when appropriate, additional screening or diagnostic testing.
4) Why does CRL matter so much?
NT changes as the fetus grows. A raw NT value without crown-rump length can be misleading, because the expected measurement is different at smaller and larger fetal sizes. That is why this calculator uses CRL to build the reference lines.
5) What does MoM mean here?
MoM means multiple of the median. It shows how your entered NT compares with the estimated median value for the same CRL. A MoM near 1.00 is close to the reference median, while higher values sit above it.
6) Why is 3.5 mm highlighted?
Many clinical pathways treat 3.5 mm or more as a clear review threshold in early pregnancy. This page marks that level as a high-alert result, but the final meaning still depends on the full scan and clinical context.
7) Can this replace combined screening?
No. Combined screening uses more than NT alone. It also includes blood markers and a validated risk-calculation pathway. This calculator is designed for educational estimation, reporting, and quick reference only.
8) Should twins use the same interpretation?
Use extra caution. Twin pregnancies often need more specialized assessment. This tool can still show the entered NT against a reference curve, but it should not be treated as a definitive interpretation for multiple gestations.