Down syndrome, trisomy 21 is the most common chro¬mosome abnormality and occurs in approximately 1 in every 650 births. It alone accounts for 1 in 20 congenital cardiac malformations. In virtually all individuals with Down syndrome 47 chromosomes are found with an extra copy of chromosome 21 (trisomy 21), although in approximately 3% of individuals it may originate from an extra copy of all or part of the long arm of chromosome 21 translocated to another chromosome. The recurrence risk is approximately 1% after a child with trisomy 21, and the recurrence is higher if one of the parents carries a translocation.
Increasing maternal age increases the incidence of Down syndrome and prenatal screening has led to a reduction in its postnatal frequency. The risk rises steeply after age 35 reaching 4% for women older than 44.