ABSTRACT
Down syndrome (DS) is the most frequent severe chromosomal anomaly in live born infants, with a frequency of 1 in about 600 births. Its name originates from the British physician Langdon Down who first described the syndrome in 1866 (1). The association of DS with maternal age was known from the beginning of the 20th century. Since the 1970s, maternal age-based screening for DS has been introduced in most developed countries. Invasive diagnostic procedures (amniocentesis and, chorion biopsies) were offered to all pregnant women above a certain age threshold. Every country has variably set, from 35 years onward, the age limit for offering DS screening, the choice in cutoff depending on public health, economic, and/or social argumentations. Disadvantages of a maternal age-based selection are (i) the high screen-positive rate-due to the increasing number of women postponing reproduction to a later phase in life, (ii) the low positive predictive value, and (iii) the unfavorable ratio between detected DS cases and iatrogenic abortions caused by the invasive diagnostic procedure. In fact, for each detected DS case one healthy baby is lost as a consequence of the invasive procedures (2). Moreover, as the majority of babies are still born from younger mothers, a screening strategy based on maternal age only leads to the detection of about one-third of all DS cases. Over the last decade, the need for a safer and more efficient screening strategy has been a major challenge for researchers and health policy makers. In his Observation of an Ethnic Classification of Idiots, Down described the typical features of affected individuals as ‘‘ . . . their skin appears to be too large for their bodies, the nose is small and the face is flat . . . ’’ One hundred and thirty years later, these features have been proposed as, at the moment, the best available strategies for an early ultrasound-based screening for DS. In this chapter, the various aspects and implications of first-trimester screening for chromosomal anomalies by nuchal translucency (NT) measurement will be discussed.
