Nuchal translucency is used in

Fetuses with trisomy 21 and other chromosomal anomalies often have excess fluid in the subcutaneous tissue behind the fetalSonographically, this appears as an echolucent fluid collection between the soft tissue over the cervical spine and an echogenic line representing the skin edge. This fluid space is called the nuchal translucency (NT).The lucency is thought to represent mesenchymal edema and is often associated with distended jugular lymphatics.The prevailing theory suggests an alteration in lymphangiogenesis and delayed lymphatic development. Other possible etiologies include cardiac failure and abnormal extracellular matrix, but these do not explain the localized and transient nature of the NT. Most likely, the etiology is a complex interaction of factors.NT normally increases with advancing gestational age, and therefore the measurement is compared to crown-rump length (CRL). A thin NT is seen in most normal fetuses.An NT greater than 95% for CRL is considered thickened. An NT greater than 99% does not change significantly with CRL and is approximately 3.5 mm(Ref: Diagnostic Ultrasound by Carol M. Rumack, Stephanie R. Wilson, J. William Charboneau, and Deborah Levine,4thed, page 1190)