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Obstetric Imaging
Demonstration data

Nuchal Translucency (NT)

Interprets first-trimester nuchal translucency thickness and confirms measurement validity against the crown-rump length window.

Nuchal translucency is the maximal thickness of the subcutaneous fluid collection behind the fetal neck, measured in a strict mid-sagittal plane between 11+0 and 13+6 weeks. It is a standardised first-trimester marker for aneuploidy and, when increased with a normal karyotype, for cardiac and other structural anomalies. Valid measurement requires the crown-rump length to fall within 45–84 mm.

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📐 Formula

NT measured 11+0 to 13+6 weeks with CRL 45–84 mm; increased if >3.0–3.5 mm or >99th centile

When to Use

  • First-trimester combined screening for trisomy 21, 18 and 13.
  • Risk stratification for congenital heart disease when NT is increased with a normal karyotype.
  • As one input to a formal risk algorithm alongside maternal age and serum markers.

How It Works

  • Confirm CRL is 45–84 mm (gestational age 11+0 to 13+6 weeks); measurements outside this window are not valid.
  • Obtain a mid-sagittal view with the fetus neutral and the image magnified so the head and upper thorax fill the frame.
  • Place calipers on the inner borders of the nuchal space at its widest point (on-to-on technique).

Interpreting the Result

  • NT above the 99th centile is approximately ≥3.5 mm and confers a substantially increased risk of aneuploidy and cardiac defects.
  • NT >3.0 mm is a common practical threshold for increased risk and further assessment.
  • NT thickness rises with CRL, so risk is ideally expressed as a centile or multiple of the median rather than an absolute cut-off.

Limitations & Pitfalls

  • A measurement outside the 45–84 mm CRL window is invalid.
  • NT is a screening marker, not diagnostic; a normal NT does not exclude aneuploidy and an increased NT can occur with a normal outcome.
  • Accuracy depends on strict technique and operator credentialing (e.g. Fetal Medicine Foundation certification).

References

  • Nicolaides KH. Nuchal translucency and other first-trimester sonographic markers of chromosomal abnormalities. Am J Obstet Gynecol. 2004;191(1):45-67.
  • Fetal Medicine Foundation. The 11-13 weeks scan.
  • ISUOG Practice Guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2013;41(1):102-113.

⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.