(1) Approximately three in every four fetuses with Down syndrome have increased nuchal translucency (NT), which is a larger than normal build-up of fluid at the back of the neck. (2) The ultrasound measurement of NT between 11 and 14 weeks' gestation, in combination with the mother's age and the levels of placental biochemical markers in her blood, can be used to detect approximately 84% of fetuses with Down syndrome. (3) The accuracy of NT measurement is affected by fetal position, measurement technique, the type of risk-calculation software used, and the sonographer's experience and technical expertise. (4) A rigorous standardization and quality assurance system for NT measurement is needed before any test using NT ultrasound is offered universally. The cost of establishing such a program is unknown.