Nuchal Translucency Calculator

Estimate gestational age from crown-rump length (CRL) and check a first-trimester nuchal translucency (NT) measurement against standard screening cutoffs.

Quick Facts

Screening window
CRL 45-84mm
Roughly 11 weeks 0 days to 13 weeks 6 days - the only window NT reference ranges are validated for.
Key cutoff
NT ≥ 3.5mm
Widely used threshold for "markedly increased," prompting genetic counseling and fetal echocardiography.
One marker, not a diagnosis
Combined with PAPP-A / hCG
A formal first-trimester risk estimate pairs NT with maternal serum markers, not NT alone.

Your Results

Calculated
Gestational age
-
From CRL, Robinson-Fleming formula
NT-to-CRL ratio
-
Measured NT relative to fetal size
NT screening band
-
Vs. standard 2.5mm / 3.5mm cutoffs
Maternal age background rate
-
Age-only population rate, not adjusted for NT

Ready

Enter the CRL, measured NT, and maternal age, then calculate.

About the nuchal translucency calculator

Nuchal translucency (NT) is the small pocket of fluid normally seen at the back of a fetus's neck on a first-trimester ultrasound. This calculator does two things with your measurements: it dates the pregnancy from the crown-rump length (CRL) using the Robinson-Fleming formula, and it places your measured NT (in millimeters) against the standard screening cutoffs used to flag a measurement for further follow-up.

How the calculations work

  • Gestational age from CRL: GA (days) = 8.052 x sqrt(CRL in mm x 1.037) + 23.73 - the Robinson-Fleming formula, a long-standing first-trimester ultrasound dating equation.
  • NT-to-CRL ratio: (NT / CRL) x 100, a simple proportion some sonographers use as a secondary sanity check that the NT reading is reasonable for the fetal size.
  • Screening band: NT is compared to the widely used round-number cutoffs of 2.5mm and 3.5mm to sort the measurement into "within typical range," "above typical range," or "markedly increased."

Putting the result in context

NT screening is only validated for a CRL between 45mm and 84mm - about 11 weeks 0 days to 13 weeks 6 days. Outside that window the reference ranges do not apply, so this calculator restricts inputs to that range. An NT result above the typical range is a prompt to combine it with maternal serum markers (PAPP-A, free beta-hCG) for a formal first-trimester risk estimate - not a diagnosis on its own.

When to consult a professional

This tool performs the dating and cutoff-comparison arithmetic only. It does not calculate a personalized chromosomal-abnormality risk (that requires a certified first-trimester combined screening program using PAPP-A and free beta-hCG alongside NT) and it is not a substitute for your ultrasound report or your care provider's interpretation. Discuss any result that concerns you with your obstetric provider or a genetic counselor.

Frequently Asked Questions

What is nuchal translucency and when is it measured?
Nuchal translucency (NT) is the fluid-filled space at the back of a developing fetus's neck, measured by ultrasound. It is measured once, when the crown-rump length (CRL) is between 45mm and 84mm - roughly 11 weeks 0 days to 13 weeks 6 days of pregnancy - because the measurement technique and reference ranges are only validated in that window.
How is gestational age calculated from CRL?
This calculator uses the Robinson-Fleming formula: gestational age in days equals 8.052 times the square root of (CRL in mm times 1.037), plus 23.73. It is a long-standing first-trimester ultrasound dating formula that converts a single CRL measurement into an estimated gestational age.
What do the NT cutoffs of 2.5mm and 3.5mm mean?
An NT below 2.5mm is within the typical range seen in most pregnancies at this stage. Between 2.5mm and 3.4mm is above that range, and guidelines commonly recommend combining it with maternal serum markers (PAPP-A and free beta-hCG) for a formal first-trimester risk estimate. 3.5mm or higher is considered markedly increased and is usually followed by genetic counseling, diagnostic testing options, and a fetal echocardiogram, since it is linked to a higher chance of chromosomal differences and heart defects.
Does a normal NT measurement rule out chromosomal conditions?
No single measurement can rule a condition in or out on its own. A normal NT lowers the likelihood but does not eliminate it. This calculator only reports where your CRL-based dating and NT measurement fall relative to standard screening cutoffs - it is not a diagnosis and does not replace combined first-trimester screening or diagnostic testing.

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