[Down syndrome screening by ultrasound]

The nuchal translucency is the usual subcutaneous space, between the skin and the soft tissues covering the fetus'neck spine, observed by ultrasound during the first trimester of pregnancy. It increases with gestational age. The association between nuchal translucency thickness and risk of Down syndrome was firstly observed in high risk populations as an old maternal age or a history of a previous child affected by Down syndrome. However, these studies are retrospective and only report the issue among pregnancies associated with an abnormal nuchal translucency thickness. Furthermore, as the performance of a screening test is correlated to the prevalence of the disease in the studied population, such results can not be extrapolated to the general population. To date, eleven prospective studies have been published, assessing the performance of the nuchal translucency measurement in a general population, including about 125,000 patients. The global sensitivity of this screening was 70% for a false positive rate of 5%. For an adjusted risk on the maternal age, the detection rate increased up to 77%. Although nuchal translucency measurement is an early and performing screening tool, uncertainties remain nevertheless about its reproducibility in the general population. In order to correctly measure the nuchal translucency, a previous training course is necessary to guarantee adequate and reproducible measurements.