Aneuploidy is a condition in which a cell has an incorrect number of chromosomes. Human cells are supposed to have 46 chromosomes. Females have 23 pairs of chromosomes, whereas males have 22 pairs but then a final pair containing an X and a Y (females have two X chromosomes).
In aneuploidy, a cell might have three copies of a particular chromosome -- making 47 chromosomes -- or only one copy of a particular chromosome, making 45 chromosomes in the cell. Any change in the number of chromosomes can affect health. Some aneuploidies can result in a live birth, such as Down syndrome (Trisomy 21), but others are lethal in the first trimester and can never lead to a viable baby.
Environmental influences may be able to cause aneuploidy, but scientists believe that most cases of aneuploidy result from random errors in cell division. Most of the time, even when chromosomal testing after a miscarriage shows that the baby was affected by aneuploidy, the aneuploidy does not recur in future pregnancies.
