Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders (birth defects). Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes do not separate properly between the two cells. Chromosome abnormalities occur in 1 of 160 live births. Most cases of aneuploidy result in termination of the developing fetus, but there can be cases of live birth; the most common extra chromosomes among live births are 21, 18 and 13.
Different species have different numbers of normal chromosomes and thus the term "aneuploidy" refers to the chromosome number being different for that species.
Every cell in the human body, apart from enucleated red blood cells and the haploid gametes, has 23 pairs of chromosomes (for a total of 46).
