Menstruation usually begins between 10 and 16 years of age, depending on a variety of factors, including the young woman's general health, nutritional status, and body weight relative to height. Menstruation continues approximately once a month until a woman is about 45 to 50 years of age, again depending on health and other influences. The end of a woman's ability to menstruate is called menopause and it marks the completion of a woman's childbearing years. The average length of the menstrual cycle is 28 days, but ranges from 21 to 40 days. The length of the cycle may also vary for a woman during different phases of life, and even from one month to the next depending on a variety of factors, including the woman's physical, emotional, and nutritional health.
Menstruation is part of the regular process that prepares a woman's body each month for pregnancy .
This cycle involves several phases that are controlled by the interactions of hormones secreted by the hypothalamus, anterior pituitary gland, and ovaries. At the beginning of the cycle, the cell lining of the uterus begins to develop and thicken. This lining will serve as the anchor for the developing fetus if the woman is impregnated. Hormones signal an ovum or egg in the ovary to begin developing. Soon, an ovum is released from the woman's ovary and begins to move through the Fallopian tube toward the uterus.