The menstrual cycle is the cyclical nature of the reproductive life of a sexually mature female. One ovum develops and matures within a grautian follicle in one of the ovaries. When mature, the follicle ruptures to release the egg, which passes down the fallopian tube to the uterus. The ruptured follicle becomes a temporary endocrine gland, called the corpus luteum which secretes the hormone progesterone. Under the influence of progesterone, the uterus wall (endometrium) thickens and its blood supply increases in readiness for the implantation of a fertilized egg. If the egg is not fertilized and there is no pregnancy, the thickened endometrium is shed along with a flow of blood through the vagina during menstruation.

Heavy periods may be caused by a hormonal imbalance, endometriosis, a pelvic infection, use of an IDU, or uterine growths such as fibroids. Excessive bleeding may signal other irregularities like lack of ovulation, low levels of progesterone, or an excess of progesterone. Heavy periods can cause iron deficiency anemia.