The purpose of the present literature review is to assess the screening value of 
trisomy 21 by measurement of fetal nuchal translucency (NT) thickness in the 
first trimester. NT is a subcutaneous translucency between the skin and the soft 
tissues overlying the cervical spine, which disappears in the second trimester. 
Ultrasound examination was used to image a sagittal section of the fetus to 
measure the maximum thickness of the subcutaneous translucency. NT is 
physiological for a measurement < 3 mm but the incidence of chromosomal 
abnormalities (essentially trisomies 21, 18 and 13) increases when NT > or = 3 
mm. Differential diagnoses include cystic hygroma and fetal hydrops. For 
screening purposes, a cut-off threshold value of > or = 3 mm, with a 
standardized technique, gave a sensitivity > or = 50%, a false positive rate < 
5% and a positive predictive value > 1%. In the chromosomally normal group, 
prognosis was good, but incidence of structural defects and fetal loss 
increased, with a sharp rise in these complications for fetal translucency 
thickness > or = 5 mm.