Menstruation

Menstruation, periodic vaginal discharge in humans and other mammals, consisting of blood and cells shed from the endometrium, or lining of the uterus (see Reproductive System). Menstruation accompanies a woman's childbearing years, usually beginning between the ages of 10 and 16, at puberty, and most often ceasing between the ages of 45 and 50, at menopause. Menstruation is part of the process that prepares a woman for pregnancy. Each month the lining of the uterus thickens; if pregnancy does not occur, this lining breaks down and is discharged through the vagina. The three to seven days that menstruation lasts is called the menstrual period.

In most women the menstrual cycle is about 28 days, but it can vary considerably even from one month to another. The cycle is initiated by hormones in the blood that stimulate the ovaries (the two female organs that produce ova, or eggs). Each month, hormones cause an egg in one of the two ovaries to mature (to become capable of being fertilized and develop into a fetus). The ovaries also produce hormones of their own, primarily estrogen, which cause the endometrium to thicken. About midway through the menstrual cycle, 14 to 15 days before the next period, the ovary releases the mature egg in a process called ovulation. The egg passes through the fallopian tube to the uterus. If the egg unites with a sperm on its way to the uterus, fertilization occurs and pregnancy ensues.