Aneuploidy

One possible result of a genetic test is aneuploidy, which is a defect in the number of chromosomes occurring during conception. One possibility is trisomy, in which three copies of a chromosome are formed during conception. This means the individual has 47 chromosomes instead of 46. The risk of this occurring increases with the mother's age, whereas it most often affects chromosomes number 21 (Down syndrome), 13 (Patau syndrome) and 18 (Edwards syndrome).