Nuchal fold thickness
The proposed etiology of increased nuchal thickness is as result of congenital heart disease and lymphatic obstruction.
Although as the pregnancy progresses excess nuchal thickness or even cystic hygromas can resolve, especially toward the third trimester. The risk of karyotypic abnormalities is not reduced, however.
The nuchal thickness is measured on an axial section through the head and the level of the thalami, cavum septi pellucidum, and cerebellar hemispheres (i.e in the same plane that is used to assess posterior fossa structures). One caliper should be placed at the skin, and the other against the outer edge of the bone of the occiput.
A thick nuchal fold is often considered the most sensitive 1 and most specific 3 (best) 2nd trimester marker for Down syndrome 1 with false positive rates as low as 1%.
It is ideally obtained with the long axis of the head at (or as close as possible to) 45% to the transducer)
An abnormal value is one that is >5-6 mm in thickness:
- nuchal thickness should not be measured after 20.6 weeks
- NF >5 mm: Sens 15%, Spec 97% PPV 8%, NPV 99% for trisomy 21 9
- NF >6 mm: Sens 12%, Spec 99% PPV 13%, NPV 99% for trisomy 21 9