The 11-14 week scan

Fetal nuchal translucency thickness (NT) at the 11-14 week scan has been co mbined with maternal age to provide an effective method of screening for tr isomy 21;for an invasive testing rate of 5%, about 75% of trisomic pregnanc ies can be identified. When maternal serum free-beta-human chorionic gonado tropin and pregnancy-associated plasma protein-A at 11-14 weeks are also ta ken into account, the detection rate of chromosomal defects is about 90%. I ncreased NT can also identify a high proportion of other chromosomal abnorm alities and is associated with major defects of the heart and great arterie s, and a wide range of skeletal dysplasias and genetic syndromes. Other ben efits of the 11-14 week scan include early diagnosis of major fetal defects and the detection of multiple pregnancies, as well as reliable identificat ion of chorionicity. As with the introduction of any new technology into ro utine clinical practice, it is essential that those undertaking the 11-14 w eek scan are adequately trained and that their results are subjected to rig orous audit.