[Fetal nuchal translucency thickness and risk for chromosomal defects]

In normal pregnancy fetal nuchal translucency (NT) thickness increases with gestation, in trisomy 21 and other major chromosomal defects fetal NT is increased. The risk for trisomies can be derived by multiplying the a priori maternal age and gestation related risk by a likelihood ratio, which depends on the degree of deviation in fetal NT measurement from the normal median for that crown-rump length. In a pregnant population with a mean maternal age of 28 years, using the risk cut-off of 1 in 300 to define the screen positive group would detect about 80% of trisomy 21 fetuses for a false positive rate of 5%. It was estimated that, for a 5.4% false positive rate, 90% of trisomy 21 fetuses could be detected. Prospective studies have demonstrated that such results are achievable by screening with fetal NT and maternal serum free b-hCG and PAPP-A in the first-trimester. It is therefore essential that, in screening, attention is paid to the provision of high quality sonographic and biochemical services for early diagnosis of chromosomal defects.