Menstruation

Menstruation, periodic vaginal discharge in humans and other mammals, consisting of blood and cells shed from the lining of the uterus (endometrium). It 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 (see Reproductive System).

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 the ova, or eggs). Each month, hormones cause an egg in one of the two ovaries to mature, that is, to become capable of being fertilized and develop into a foetus. The ovaries also produce hormones of their own, primarily oestrogen and progesterone, 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.

The three to five days the egg takes to reach the uterus after being released by the ovary is known as the woman's fertile period. If fertilization does occur, the fertilized egg attaches itself to the enriched uterine lining and pregnancy becomes established. Menstruation does not occur during pregnancy, and a missed period is often the first sign of pregnancy that a woman notices. If fertilization does not occur, the lining of the uterus does not receive the hormones it needs to continue the thickening process. Thus, the uterine lining breaks down and is discharged from the body during menstruation.

Many women experience premenstrual pain and discomfort. Tenderness of the breasts and a tendency to retain fluid (bloat) are common one to seven days before each period. Some women also experience premenstrual tension in the form of headache, irritability, nervousness, fatigue, crying spells, and depression with no apparent cause (premenstrual stress, or PMS). A few women also experience severe menstrual cramps (dysmenorrhoea) during the first day or two of the period. Although premenstrual symptoms and discomfort during menstruation were once thought to be of psychological origin, research now indicates that hormonal and chemical changes are responsible. New medications are effective in treating these problems. Amenorrhoea, the absence of periods, has both physical and psychological causes.