ABSTRACT
The natural frequency of chromosomal abnormalities at birth is around 6 cases per 1000 births among populations without any form of prenatal screening. The aneuploides represent the most frequent of these, with Down syndrome being the most common with a historical birth prevalence of 1 in 800. The other common autosomal trisomies include Edward’s syndrome (trisomy 18) and Patau’s syndrome (trisomy 13), occurring with historical birth incidences of 1 in 6500 and 1 in 12,500, respectively. The other group of aneuploides include the sex aneuploidies, such as Turner’s syndrome (45x), Klinefelter syndrome (47xxy), and those with 47xyy, and the types I and II versions of triploidy.
