Nuchal fold is a normal fold of skin seen at the back of the fetal neck on a second-trimester scan. Increased thickness of the nuchal fold is a soft marker associated with multiple fetal anomalies, and is measured on a routine second trimester ultrasound.
The proposed etiology of increased nuchal thickness is the result of hydrops or lymphatic obstruction.
The predominant reason for measuring the nuchal fold is that it is a soft marker for aneuploidy. As an isolated finding, it has a likelihood ratio of 3.25 for Down syndrome9.
Other associations include:
- Turner syndrome
- congenital heart disease
- Klippel-Feil syndrome
- Zellweger syndrome
- Cumming syndrome
- Robert syndrome 7
An abnormally thickened nuchal fold or even a cystic hygroma may resolve, especially toward the third trimester; however, the risk of karyotypic abnormalities is not reduced.
Nuchal fold thickness of > 6 mm is abnormal on a routine morphology ultrasound performed at 18-22 weeks.
The nuchal fold is known to increase throughout the second trimester in a normal pregnancy, and can be measured during a broader window of 14 and 24 weeks when required, with some controversy regarding the normative thresholds at the earlier and later gestations - some authors advocating the use of a normogram10.
Nuchal fold thickness is measured on an axial section through the head at the level of the thalami, cavum septi pellucidi, and cerebellar hemispheres (i.e. in the same plane that is used to assess the posterior fossa structures). One caliper should be placed on the outer edge of the skin, and the other against the outer edge of the occipital bone. The ideal angle of insonation is at approximately 30 degrees to the horizontal. This plane is less likely to produce a false positive thickened NF.