The 11-14 weeks scan - KH Nicolaides, NJ Sebire, RJM Snijders, AP Souka

Chapter 3

INCREASED NUCHAL TRANSLUCENCY WITH NORMAL KARYOTYPE

Increased fetal nuchal translucency (NT) thickness is a common phenotypic expression of trisomy 21 and other chromosomal abnormalities, but it is also associated with fetal death and a wide range of fetal malformations, deformations, dysgeneses, and genetic syndromes. This chapter reviews the outcome of chromosomally normal fetuses with increased NT. On the basis of these data it is possible to estimate, for each NT group, the chances of intrauterine survival and delivery of a healthy baby with no major defects. These data are useful in counselling parents of pregnancies with increased fetal NT and in planning the appropriate follow-up investigations.

In normal fetuses NT thickness increases with fetal crown-rump length (CRL). The median and 95th centile of NT at a CRL of 45 mm are 1.2, and 2.1 mm and the respective values at CRL of 84 mm are 1.9 and 2.7 mm (Snijders et al 1998). The 99th centile does not change significantly with CRL and it is about 3.5 mm. Increased NT, refers to a measurement above the 95th centile and the term is used irrespective of whether the collection of fluid is septated or not and whether it is confined to the neck or envelopes the whole fetus. After 14 weeks, increased NT usually resolves but in some cases it evolves into nuchal edema or cystic hygromas.