Nuchal thickness refers to increased soft tissue thickening on the posterior aspect of the neck measured in the early 2nd trimester between 16 and 20 weeks gestation. Beyond this gestation the measurement is unreliable as a marker for chromosomal abnormalities. It is also referred to as nuchal edema or nuchal skinfold thickness. It is a different measurement to that of nuchal translucency in the first trimester and it would appear that very few cases of increased nuchal translucency progress to nuchal thickening in the 2nd trimester. A measurement of 6mm or more is generally taken to indicate increased risk of aneuploidy. An association has been found with chromosomal defects, mainly trisomy 21 but also other trisomies, deletions, triploidy and Turner’s Syndrome. Early studies found it to be a poor and unreliable screening test for Down syndrome. However, recent work suggests it is useful as a ultrasonographic marker for aneuploidy, present in about 40% of cases of Trisomy 21. It may be the most sensitive and specific of all the second trimester ultrasonographic markers for Down Syndrome. Some groups have proposed the use of this marker in combination with others, such as femur length, to develop risk scoring systems. Each additional marker seen on ultrasound raises the risk of Down still further. Recently studies combining increased nuchal thickness, maternal age and urinary beta-HCG core fragment have been used to screen for Down with 80% sensitivity. Spontaneous resolution of nuchal thickening may occur after 20 weeks even in a chromosomally abnormal fetus, so resolution cannot be interpreted as reassuring.
The scanning plane used in the assessment of nuchal thickness is crucial. In the 2nd trimester a modified axial or transverse plane of the fetal head (suboccipitobregmatic) is obtained, with the transducer angled posteriorly in order to simultaneously visualize the cavum septum pellucidum, cerebral peduncles, cerebellar hemispheres and the cisterna magna. In this plane measurement of nuchal thickness is obtained from the outer limit of the occipital bone to the outer aspect of the skin edge. A search should be made for additional ultrasonographic markers of fetal aneuploidy, such as reduced femur and humeral length, pyelectasis, hypoplasia of the fifth digit middle phalanx, small ear, echogenic bowel and echogenic ventricular foci as well as anomalies such as cardiac defects, omphalocoele, hydrops, and duodenal atresia.
Nuchal thickening may be part of the spectrum of cystic hygroma. Most encephalocoeles are located in the occipital area. An underlying bony defect is the rule, but may be difficult to demonstrate when small. Tumors such as teratomas are likely to have a solid or mixed appearance. The twin sac of a blighted ovum may have a similar appearance. Cystic lymphangioma has a more complex appearance than nuchal thickening and is both cystic and solid.
Increased thickening of the nuchal skin greater than 6mm in the second trimester.