Nuchal fold thickness
The proposed etiology of increased nuchal thickness is as result of congenital heart disease and lymphatic obstruction.
- congenital heart disease
- Klippel-Feil syndrome
- Zellweger syndrome
- Cumming syndrome
- Robert syndrome 7
Although as the pregnancy progresses excess nuchal thickness or even cystic hygromas can resolve, especially toward the third trimester. However, the risk of karyotypic abnormalities is not reduced.
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 on 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 and most specific 2nd trimester marker for Down syndrome with false positive rates as low as 1% 1,3.
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: sensitivity 15%, specificity 97%, PPV 8%, NPV 99% for trisomy 21 9
- NF >6 mm: sensitivity 12%, specificity 99%, PPV 13%, NPV 99% for trisomy 21 9