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 weeks 3 days to 13 weeks 6 days).

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 generalized fetal abnormality. Measurement of the nuchal translucency requires specific and standardized assessment and 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 the correct technique is essential:

  • the fetus must be in midsagittal imaging plane (the vertebral column should be facing the bottom of the screen); the 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
  • the fetal head should not be extended or flexed
  • the 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 "+" calipers should be used for measurement
    • only the lucency is measured (again differing from nuchal thickness)
    • the calipers are put inside the hyperechoic edges
  • the widest part of the translucency should be measured
  • values obtained when the CRL is between 45 and 84 mm (11 weeks 3 days to 13 weeks 6 days) 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:

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

The rate of aneuploidy is directly proportional to the value of nuchal translucency 9:

  • < 2 mm have a risk < 1%.
  • 3.4 mm have a risk of 7%
  • 3.5-4.4 mm have a risk of 20%
  • 5.5-6.4 mm have a risk of 50%
  • ≥8.5 mm have a risk of 75%

In the majority of fetuses with trisomy 21, the nuchal translucency thickness was < 4.5 mm, while with trisomies 13 or 18 it was 4.5-8.4 mm, and in those with Turner syndrome it was 8.5 mm or more 9.

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 NT and screening test results show an increased risk of less than 1 in 300, further workup may be carried out based on the patient's desire after counseling. Further investigations include:

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
  • normal outcome: the change is proportionate to NT value 12
    • 3.5-4.4 mm have a chance of 70%
    • 4.5-5.4 mm have a chance of 50%
    • 5.5-6.4 mm have a chance of 30%
    • >6mm have a chance of 15%
Ultrasound - obstetric
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Article information

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

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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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