Menstruation is a flow of blood that occurs about once a month in most women from approximately ages twelve to forty-eight. Although menstruation is a normal part of the female reproductive cycle, it is a subject of considerable misunderstanding and taboo. In ancient times, a menstruating woman was regarded as unclean and liable to pollute foods she handled, or as contagious and liable to cause illness or even death in others. In the modern era, menstruation is sometimes seen as a physical and emotional handicap that makes women "inferior" to men. Whether menstruation is called "the curse," "the monthlies," or "being on the rag," it is often referred to in negative terms.

The menstrual cycle is traditionally described as starting with the first day of menstrual flow (cycle day one) and ending the day before the next menstruation begins. The length of the cycle varies, normally ranging from twenty-one to forty days and averaging about twenty-eight days. Very few women are so regular that they can accurately and consistently predict the length of each cycle on the basis of their past pattern of cycle lengths.

The menstrual cycle consists of three phases, which we will describe in terms of an "average" twenty-eight-day cycle. The follicular phase is the first portion of the menstrual cycle. Ovarian follicles, oval arrangements of cells around a young egg, begin to mature as FSH stimulates them. At the start of this phase, estrogen and progesterone levels are quite low and the uterus sheds its lining, resulting in three to six days of menstrual flow. Menstrual flow consists of a small amount of blood combined with tiny bits of tissue from the lining of the uterus, and the entire amount is usually only 2 or 3 ounces (4 to 6 tablespoons).

Midway in the follicular phase (around cycle days seven to ten), estrogen output from the ovaries increases, which acts with FSH to prepare the developing follicle for ovulation. Estrogen also causes the lining of the uterus to thicken, or proliferate, because of growth of glands, connective tissue, and blood vessels. Just before ovulation, estrogen levels reach a broad peak, which acts on the hypothalamus to trigger the surge of LH and FSH released from the pituitary a day or two afterward.