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Nuchal translucency

Nuchal translucency is a finding during a specific period in the late first trimester and early second trimester (11.3-13.6 weeks) and should not be confused with nuchal thickness which is measured 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 lucency requires careful attention to technique.

Associations

Nuchal translucency can being associated with a number of anomalies, including:

Radiographic assessment

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

Technique

Use of correct technique is essential:

  • the fetus should be sagittal in the imaging plane (the vertebral column should be facing the bottom of the screen)
  • 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)
  • ideally only the head and upper thorax should be included in the measurement 
  • the level of magnification should be appropriate (fetus should occupy most of the image): enabling 1 mm changes in measurement possible
  • 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
Assessment
  • only values obtained when CRL values are between 45-84 mm are considered valid
  • 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
  • normal value is usually less than ~2.2-2.8 mm in thickness, 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
Interpretation

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

Correlation with serum markers

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

  • maternal B-HCG
  • alpha-fetoprotein (AFP)
  • pregnancy-associated plasma protein A (PAPP-A)
  • oestriol/estriol

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.

Further work up

If abnormal, further work-up is carried out which includes:

Treatment and prognosis

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

Differential diagnosis


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