ABSTRACT
First trimester ultrasound offers the advantages of identifying and measuring small subcutaneous collections of fluid behind the fetal back and neck (1), later known as nuchal translucency (NT) (Fig. 1) (2-4). The size of NT increases with gestational age (GA) and fetal crown-rump length (CRL) (5-8). The translucent area disappears after 14 weeks’ GA, when the subcutaneous tissue becomes more echogenic. NT is therefore a transient phenomenon (9). This translucent area in the fetal neck can be considerably enlarged (Fig. 2), which is defined as NT above the 95th percentile for the GA (5-7). An NT >95th percentile is strongly associated with fetal chromosomal abnormalities (2-4,10-24), although some centers report less significant results (25,26). Currently, it is possible to estimate the individual risk of chromosomal abnormalities in a fetus by combining maternal age with NT thickness (4).
