All the egg cells (oocytes) a woman will ever have are already present before she is born.

When a female fetus is still 4 months away from being born, the number of her oocytes peaks at about 6 million. This declines to about 2 million at birth. The number continues to drop until only about 200,000 are present when she starts her menses and the beginning of her reproductive life.

On average, women menstruate for 40 years and ovulate 13 times per year. This means only about 500 eggs (1 in 400), actually come to full development. Every month several eggs start to develop, but early in the cycle one is selected to be dominant. The others die. The factors controlling this process remain poorly understood.

Each egg is surrounded by hormone producing cells that form a unit called a follicle. Menopause marks the time when the follicles are depleted and the ovaries stop functioning. It is usually a gradual process that evolves over several years. (In contrast, sperm are continuously created throughout the reproductive life of men.)

Women who are older when become pregnant have a higher risk of having a baby with a genetic disorder. As egg cells age, they accumulate defects in their genetic mechanisms.