Traditionally it has been believed that a woman is born with all of the eggs she will ever have. They sit in the ovaries in a state of suspended animation. They start out numbering around a million, and about 30 die off every day.
As a woman ages, the chromosomes in a woman’s eggs change. Chromosomes ride on railroad-like tracks called a spindle. As women age, those tracks deteriorate, derailing the separation of the chromosomes. By the time a woman is 38, doctors can see abnormalities in the spindles of about 86 percent of the eggs. Derailing the separation process can produce Down syndrome, miscarriage or stillbirth. This aging of the eggs can also make them less likely to fertilize.
Some recent studies have suggested that female mice may have certain germ-line stem cells in their ovaries that are able to produce new eggs throughout the female’s cycle. The same results have not been seen in humans, and much more study is needed before any real conclusions can be drawn.