ABSTRACT
Down syndrome (DS) is named after John Langdon Down, the first scientist to describe the pattern of congenital anomalies characterizing individuals with trisomy 21. The overall incidence in an unselected population at 20 weeks is about 1/500, even though this figure has suddenly plunged due to the introduction in clinical practice of the cell-free DNA screening test for aneuploidies. DS is due to trisomy of chromosome 21; this is a free trisomy in 95% of cases, while the remaining 5% are represented by Robertsonian translocations and mosaicisms of variable degree. It is well established that the occurrence of the trisomy has a positive correlation with advancing maternal age, especially after 35 years of age. Molecular biologic assays have demonstrated that, in 95% of cases, the trisomy is the result of a nondisjunction during maternal meiosis (meiosis I in 75–80% of cases). In the remaining 5% of cases, it is due to a paternal error, most frequently during meiosis II.
