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Nuchal Translucency Calculator

HealthMedical Reference

Measured at the same scan, 45 to 84 mm.

The maximum thickness reported on the scan.

NT for this CRL —
Gestational age from CRL
—
Median NT at this CRL (mm)
—
95th centile at this CRL (mm)
—
NT multiple of the median
—
NT minus 95th centile (mm)
—
NT as a multiple of the median
The chart appears with your result

Nuchal translucency is judged against crown-rump length, because the normal NT grows as the fetus grows. At a CRL of 45 mm the 95th centile is about 2.1 mm; at 84 mm it is about 2.7 mm. An NT of 3.0 mm at CRL 84 mm is above the 95th centile, and any value of 3.5 mm or more counts as increased.

About this tool

Parents reading a first-trimester scan report, and students learning combined screening, often see an NT figure in millimetres with no reference beside it. This calculator places that figure against the crown-rump length taken at the same scan. It gives the gestational age implied by the CRL, the median and 95th-centile NT for that length, the NT expressed as a multiple of the median (MoM), and how far the reading sits above or below the cut-off. A scale shows the MoM against usual and raised bands. It does not produce a Down syndrome risk: the combined test also uses maternal age, free beta-hCG and PAPP-A, and accredited labs apply their own software and medians.

How to use it

  1. Enter the crown-rump length

    Type the CRL in millimetres from the scan report. Only 45 to 84 mm is accepted, the range in which NT is measured.

  2. Enter the NT measurement

    Type the nuchal translucency in millimetres, as reported.

  3. Read the result and scale

    The category, gestational age, centiles and MoM update as you type; the scale places the MoM in a band.

Examples

CRL 45 mm, NT 1.5 mm

Crown-rump length (CRL)
45
Nuchal translucency (NT)
1.5

Result NT for this CRL: Within the usual range
Gestational age from CRL: 11 weeks 0 days
Median NT at this CRL (mm): 1.2
95th centile at this CRL (mm): 2.1
NT multiple of the median: 1.25
NT minus 95th centile (mm): -0.6

  1. Gestational age = 8.052 × √45 + 23.73 = 77.7 days → 11 weeks 0 days
  2. Position in CRL range = (45 − 45) ÷ 39 = 0
  3. Median NT = 1.2 + 0.7 × 0 = 1.2 mm
  4. 95th centile = 2.1 + 0.6 × 0 = 2.1 mm
  5. MoM = 1.5 ÷ 1.2 = 1.25

At the start of the 11+0 window the 95th centile is 2.1 mm, so 1.5 mm sits comfortably below it.

CRL 84 mm, NT 3.0 mm

Crown-rump length (CRL)
84
Nuchal translucency (NT)
3

Result NT for this CRL: Above the 95th centile
Gestational age from CRL: 13 weeks 6 days
Median NT at this CRL (mm): 1.9
95th centile at this CRL (mm): 2.7
NT multiple of the median: 1.58
NT minus 95th centile (mm): 0.3

  1. Gestational age = 8.052 × √84 + 23.73 = 97.5 days → 13 weeks 6 days
  2. Position in CRL range = (84 − 45) ÷ 39 = 1
  3. Median NT = 1.2 + 0.7 × 1 = 1.9 mm
  4. 95th centile = 2.1 + 0.6 × 1 = 2.7 mm
  5. MoM = 3 ÷ 1.9 = 1.58

Near 13+6 weeks the cut-off has risen to 2.7 mm; 3.0 mm is above it but under the 3.5 mm mark.

CRL 64.5 mm, NT 3.8 mm

Crown-rump length (CRL)
64.5
Nuchal translucency (NT)
3.8

Result NT for this CRL: Increased (3.5 mm or more)
Gestational age from CRL: 12 weeks 4 days
Median NT at this CRL (mm): 1.55
95th centile at this CRL (mm): 2.4
NT multiple of the median: 2.45
NT minus 95th centile (mm): 1.4

  1. Gestational age = 8.052 × √64.5 + 23.73 = 88.4 days → 12 weeks 4 days
  2. Position in CRL range = (64.5 − 45) ÷ 39 = 0.5
  3. Median NT = 1.2 + 0.7 × 0.5 = 1.55 mm
  4. 95th centile = 2.1 + 0.6 × 0.5 = 2.4 mm
  5. MoM = 3.8 ÷ 1.55 = 2.45

Any NT of 3.5 mm or more is treated as increased whatever the CRL.

How it is calculated

GA (days) = 8.052 × √CRL + 23.73; median = 1.2 + 0.7 × f; P95 = 2.1 + 0.6 × f; f = (CRL − 45) ÷ 39; MoM = NT ÷ median

CRL
Crown-rump length in mm
f
Position within the 45–84 mm CRL range, 0 to 1
P95
95th centile of NT in mm
MoM
NT as a multiple of the median for that CRL

Gestational age comes from the Robinson and Fleming CRL formula. The median and 95th centile follow the end points reported by Snijders and colleagues in the Fetal Medicine Foundation study of about 96,000 pregnancies (1.2 and 2.1 mm at CRL 45 mm, 1.9 and 2.7 mm at 84 mm), interpolated in a straight line between them. An NT of 3.5 mm or more is flagged as increased regardless of CRL, as that is roughly the 99th centile across the window.

Sources

When not to use it

  • Do not use it as a chromosomal risk result; the combined test needs blood markers and age.
  • It does not apply outside a CRL of 45 to 84 mm.
  • Twin pregnancies need separate per-fetus assessment by the screening unit.

Common mistakes

  • Entering the CRL in centimetres instead of millimetres.
  • Judging every NT against a single 3 mm cut-off instead of the CRL-specific centile.
  • Comparing an NT from one scan with a CRL from another day.

Frequently asked questions

Why does the normal NT depend on crown-rump length?

The fluid space behind the fetal neck widens as the fetus grows through weeks 11 to 13. A 2.4 mm reading is above the 95th centile at CRL 45 mm but within range at CRL 84 mm, so the CRL from the same scan is needed to interpret it.

What does an NT multiple of the median mean?

It divides the measured NT by the median NT for that CRL. A MoM of 1.0 is exactly typical; 2.0 means twice the median. Screening laboratories use MoM or the difference from the median so that values from different gestations can be combined with blood markers.

Is an NT above the 95th centile a diagnosis?

No. About 5 in 100 fetuses measure above the 95th centile by definition, and most of them are chromosomally normal. A raised NT increases the combined screening risk, and the screening team then discusses options such as NIPT, CVS or amniocentesis.

Why is 3.5 mm treated separately?

From about 3.5 mm the NT lies above the 99th centile at every CRL in the window, and it is linked with chromosomal and heart conditions independent of the risk figure. Many units offer diagnostic testing and a fetal echocardiogram at this level.

When is NT measured?

Between 11 weeks 0 days and 13 weeks 6 days, which corresponds to a CRL of about 45 to 84 mm. The gestational age shown here is calculated from the CRL with the Robinson formula, the standard dating method in the first trimester.

Will my lab's figures match these exactly?

Not always. Laboratories use their own medians and log-based models, and the straight-line interpolation here is a close approximation of the published curve. Treat the output as an explanation of your report, not a replacement for it.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 25). Nuchal Translucency Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/health/medical-reference/nuchal-translucency-calculator/

MLA

Tools Directory Editorial Team. “Nuchal Translucency Calculator.” Tools Directory, 25 Oct. 2026, https://tools.milunir.com/health/medical-reference/nuchal-translucency-calculator/. Accessed 27 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Nuchal Translucency Calculator.” Tools Directory. Last modified October 25, 2026. Accessed October 27, 2026. https://tools.milunir.com/health/medical-reference/nuchal-translucency-calculator/.

Link<a href="https://tools.milunir.com/health/medical-reference/nuchal-translucency-calculator/">Nuchal Translucency Calculator – Tools Directory</a>