The normal human menstrual cycle is 28 days, but no woman is always precisely regular, and cycles as short as 21 days or as long as 35 days are not abnormal. Menstruation is the shedding of the uterine lining, the endometrium.

The menstrual cycle begins with about five days of menstruation, followed by a proliferative phase during which ovulation occurs. The proliferative phase lasts to about the fourteenth day, and then a secretory phase begins.

At the end of menstruation, just at the beginning of the proliferative phase, the endometrium is thin, and the ovary is dormant. With the production of gonadotropic hormones from the pituitary gland, an ovarian follicle (occasionally more than one) ripens in one of the ovaries. This ovarian follicle contains the ovum. Cells surrounding the ovum multiply, and secrete an estrogenic hormone, estradiol This hormone causes multiple changes to occur in the endometrium, so that it becomes thicker and more vascular.

At about mid-cycle ovulation occurs: The ovum is released from the follicle and received into the fallopian tube, down which it is carried to the uterus.

If the ovum is fertilized during the proliferative phase, it reaches the uterine cavity at a time when the endometrium is in the secretory phase, and the ovum embeds itself in the endometrium and starts its growth. If the ovum is not fertilized the endometrium breaks down and menstruation occurs. The disintegrating endometrium is shed, together with some blood. The endometrium contains plasmin, an enzyme that dissolves blood clots, so that the menstrual discharge is normally fluid. The total blood loss does not generally exceed 1.69 oz (50 ml).