Nuchal translucency

Nuchal translucency is the normal fluid-filled subcutaneous space identified at the back of the fetal neck during the late first trimester and early second trimester (11.3-13.6 weeks).

It should not be confused with the nuchal fold, which is seen in the second trimester.  

Increased nuchal translucency is thought to be related to dilated lymphatic channels and is considered a nonspecific sign of more generalised fetal abnormality. Measurement of the nuchal translucency requires careful attention to technique.

Thickening of the nuchal translucency can be associated with a number of anomalies, including:

Nuchal lucency is measured on a sagittal image through the fetal neck.

Use of correct technique is essential:

  • the fetus must be in midsagittal imaging plane (the vertebral column should be facing the bottom of the screen); following structures must be seen to confirm correct mid sagittal position:
    • two tiny parallel echogenic lines 
      • tip of the nose
      • nasal bone (if not absent)
    • hard palate
    • diencephalon 
  • magnification so that only fetal head and upper thorax included in the image: enabling 1 mm changes in measurement possible
  • fetal head should not be extended or flexed
  • fetus should be floating free of the uterine wall i.e. amniotic fluid should be seen between its back and the uterus; this is to not mistakenly measure the distance to the amniotic membrane or uterine wall
  • the "+" callipers should be used for measurement
    • only the lucency is measured (again differing from nuchal thickness)
    • the callipers are put inside the hyperechoic edges
  • the widest part of the translucency should be measured
  • values obtained when CRL is between 45-84 mm (11.3-13.6 weeks) may be used for combined first-trimester screening
  • the lucent region is generally not septated
  • the thickness rather than the appearance (morphology) is considered to be directly related to the incidence of chromosomal and other anomalies
  • a value of less than 2.2-2.8 mm in thickness is not associated with increased risk. However, it is maternal age-dependent and needs to be matched to exact gestational age and crown-rump length (CRL)

Nuchal translucency cannot be adequately assessed if there is:

  • unfavourable fetal lie
  • unfavourable gestational age: CRL <45 or >84 mm

The rate of aneuploidy when the nuchal translucency is <2 mm is less than 1%.

To increase the clinical accuracy of nuchal lucency, it can be correlated with serum markers such as:

The combination of nuchal translucency thickness, PAPP-A, and hCG detects 87% of cases of trisomy 21 at 11 weeks, 85% at 12 weeks, and 82% at 13 weeks, with a 5% false positive rate 8.

If abnormal and screening test results show increased risk of less than 1 in 300, further workup may be carried out based on patient's desire after counselling and which includes:

As the second-trimester approaches, the region of nuchal translucency might either:

  • regress
    • if chromosomally normal, a large proportion of fetuses will have a normal outcome
    • spontaneous regression does not, however, mean a normal karyotype
  • evolve into
Ultrasound - obstetric
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Article information

rID: 1742
System: Obstetrics
Synonyms or Alternate Spellings:
  • Nuchal lucency
  • Nuchal translucency assessment

Cases and figures

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    Case 1: nuchal translucency measurement
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    Case 2: normal - correct measurement
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    Case 3: normal - correct measurement
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