At the time a baby girl is born, her two ovaries already contain all the egg cells she might ever need. Of the million or so eggs in her ovaries, only about 300 to 500 will actually be released (ovulated) during the years between puberty and menopause.

As a girl approaches puberty, her body steps up the production of the hormones that permit ovulation and reproduction. The most important hormones are estrogen, progesterone, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and gonadotropin-releasing hormone (GnRH). The production of each of these hormones increases and decreases in a regular pattern every month. During this cycle, changes in hormone levels cause an egg in one ovary to mature. Each egg develops within a follicle, a hollow, fluidfilled sphere of cells. When an egg is ready for fertilization, the follicle breaks open and the egg floats out and normally is captured by the fimbria. Usually only one egg cell develops each month; if two eggs mature and both are fertilized successfully, the result is fraternal twins. (Identical twins result when one fertilized egg cell divides into two.)

Ovulation usually takes place approximately halfway between menstrual periods, most commonly 14 days before the next menstrual period, although this can vary greatly from woman to woman and from month to month. While the egg is maturing, the increased levels of hormones cause the lining of the uterus to prepare for nurturing a fertilized egg.

Physical and emotional events can have a considerable effect on hormone production and changes in hormone levels can alter the menstrual cycle; as a result, ovulation can occur at an unexpected time. In some instances, ovulation has been known to take place even during menstruation.

Some experts believe that sperm can survive in the reproductive tract for up to 7 days, and an egg cell can be fertilized anytime during the 24 hours after it has left the ovary. No one knows precisely how long eggs and sperm live in the human reproductive system and, therefore, fertilization is considered possible for about an 8-day span in each menstrual cycle. If an egg is unfertilized, it simply is absorbed by the uterus or flushed out with the menstrual blood.