Down Syndrome

Down syndrome is a genetic condition caused by an extra copy of chromosome 21 material. It is the most common chromosomal condition and a common genetic cause of intellectual disability. Birth prevalence is approximately 1 per 1,000 live births overall, although it varies with the maternal-age distribution of the population. Risk increases with advancing maternal age; nevertheless, most affected infants are born to younger mothers because younger women account for more total births. Cause: - Full trisomy 21 from meiotic nondisjunction accounts for roughly 95% of cases. - Translocation accounts for about 3%, most often a Robertsonian translocation involving chromosome 21 material. - Mosaicism accounts for about 2%, with trisomy 21 present in only some cell lines; clinical effects vary and are not reliably predicted from the proportion of mosaic cells. The condition may be suspected prenatally or after birth. A cfDNA result is a screening result, not a diagnosis; a screen-positive result requires confirmation with chorionic villus sampling or amniocentesis before an irreversible decision is made.