A baby girl is born with egg cells (oocytes) in her ovaries. By the 5th month of pregnancy, the ovaries of a female fetus contain about 7 million oocytes. Most of the oocytes gradually waste away, leaving about 1 to 2 million present at birth. No oocytes develop after birth. At puberty, only about 300,000—more than enough for a lifetime of fertility—remain.

Only a small percentage of oocytes mature into eggs. The many thousands of oocytes that do not mature degenerate. Degeneration progresses more rapidly in the 10 to 15 years before menopause. All are gone by menopause. (Menopause is defined as 1 year after the last menstrual period.)

Only about 400 eggs are released during a woman's reproductive life, usually one during each menstrual cycle. Until released, an egg remains dormant in its follicle—suspended in the middle of a cell division. Thus, the egg is one of the longest-lived cells in the body.

Because a dormant egg cannot repair itself as cells usually do, the opportunity for damage increases as a woman ages. A chromosomal or genetic abnormality is thus more likely when a woman conceives a baby later in life.