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Nuchal translucency

Nuchal translucency is a measurement performed during a specific period in the first trimester (11.3-13.6 weeks) and should not be confused with nuchal thickness which is measured in the second trimester.  

An increased nuchal translucency is thought to relate to dilated lymphatic channels and is considered a generic marker of 'badness'

Associations

It can being associated with a number of anomlaies including:

Radiographic assessment

Nuchal lucency is measured on a sagittal section through the fetus.

Technique
Use of  the correct technique is essential 
  • the fetus should be transverse (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)
  • only the lucency is measured (again differing from nuchal thickness)
  • 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 1mm changes in measurement possible
  • the " + " calipers should be used for measurement
  • the widest part of the measurement should be taken
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
  • a normal value is usually less than roughly 2.5-3.0 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

Detection rates for aneupliodic anomalies with nuchal translucency alone approaches 80-90% with a false positive rate of ~5%.

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 feto protein (AFP)
  • pregnancy associated plasma protein A (PAPP-A)
  • oestriol
Further work up

If abnormal, further work up is carried out which includes

Natural course - progression

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 a:

Differential considerations for an increased nuchal translucency


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