Why is aneuploidy more common as women get older?
The cells that will eventually become eggs each have 46 chromosomes. These ‘pre-eggs’ are stored in the female ovaries from birth and women do not make more during their lifetime. After ovulation each month, one of the pre-eggs divides in half, giving one chromosome from each chromosome pair to the mature egg, for a total of 23 chromosomes. As women get older their pre-eggs get older, too, and the process of chromosome division does not work as well. Thus, as a woman ages, the mature eggs released at ovulation have a higher chance of dividing abnormally and containing extra or missing chromosomes instead of the normal number of 23. The process of abnormal division is called chromosome non-disjunction.
