Nuchal translucency (NT) is the sonographic appearance of a subcutaneous collection of fluid behind the fetal neck. The measurement of fetal NT thickness at the 11-14-week scan has been combined with maternal age to provide an effective method of screening for trisomy 21; for an invasive testing rate of 5%, about 75% of trisomic pregnancies can be identified. When maternal serum free-beta human chorionic gonadotrophin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) at 11-14 weeks are also taken into account, the detection rate of chromosomal defects is about 90%. Increased NT can also identify a high proportion of other chromosomal abnormalities and is associated with major defects of the heart and great arteries, and a wide range of skeletal dysplasias and genetic syndromes. In monochorionic twins, discordancy for increased NT is an early marker of twin-to-twin transfusion syndrome (TTTS). As with the introduction of any new technology into routine clinical practice, it is essential that those undertaking the 11-14-week scan are adequately trained and their results are subjected to rigorous audit.
Copyright 2002 John Wiley & Sons, Ltd.