High Quality Content by WIKIPEDIA articles! 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.This generally happens when cytokinesis starts occurring while karyokinesis is still under the process. 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.