Obstetric Imaging
Live dataNuchal 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.
📐 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.