Menstruation, specifically, is the monthly loss of blood (period), representing shedding of uterine (womb) endometrium, in women of reproductive age. In more general terms, menstruation is the whole monthly cycle of hormone, structural, and functional changes in such women, punctuated by menstrual blood loss. After each period, the endometrium starts to proliferate and thicken under the influence of follicle-stimulating hormone and estrogens. In midcycle a burst of luteinizing hormone secretion, initiated by the hypothalamus, causes release of an ovum (egg) from an ovarian follicle (ovulation). More progesterone is then secreted and the endometrium is prepared for implantation of a fertilized egg. If the egg is not fertilized, pregnancy does not ensue and blood-vessel changes occur leading to the shedding of the endometrium and some blood; these are lost through the vagina for several days, sometimes with pain or colic. The cycle then restarts. During the menstrual cycle, changes in the breasts (see mammary glands, body temperature, fluid balance, and mood occur, the manifestations varying from person to person. Cyclic patterns are established at puberty (menarche) and end in middle life (45–50) at the menopause.
Disorders of menstruation include heavy, irregular, or missed periods; bleeding between periods or after menopause, and excessively painful periods.