A female human has all the oocytes they will ever have at birth. During the 30-35-year maturation period, about 400 of these oocytes will have time to mature. Normally, each ovary releases an ovum in alternating months. Oocytes do not mature or get released during pregnancy. Diseases, the use of birth control pills together with major mental stress can affect ovulation.

The first menstrual periods come at an average age of 11 to 13. A period happens because of the changes in the body's hormonal balance. At this point, the girl has grown enough mass for the period to start, averaging 47 kilograms. Generally, menstruation is quite irregular in its early stages.

The menstrual cycle lasts about 28 days and is calculated to begin on the first day of the menstrual period.

An average menstrual cycle progresses as follows:

• Days 1 to 4: Menstrual bleeding begins. This happens when the lining of the uterus, the endometrium, is shed.
• Day 5: A new ovum begins to mature in the ovarian follicle. This is affected by the hormone secreted by the pituitary gland.
• Days 5 to 14: The follicle grows and produces estrogen, which causes important mucosal regeneration.
• Day 14: Ovulation occurs. An ovum detaches from the erupted follicle and travels to the fallopian tube. The onset of the follicle is caused by another hormone (LH) secreted by the pituitary gland.
• Days 15 to 20: A corpus luteum is formed at the site of a follicle, or sac, that has matured and released its ovum. It is made up of lutein cells, which secrete progesterone. This thickens the uterus.
• Days 21 to 28: If fertilization has not occurred, the corpus luteum becomes inactive. Decreased progesterone production triggers mucosal degeneration.

A female human stops menstruating between the ages of 45 and 55. At first, the menstrual cycle becomes irregular, and gradually the menstruation stops completely. The name menopause comes from a period of change that lasts for a couple of years.