A brief overview

The uterine, or menstrual cycle is a series of changes that the uterine endometruim goes through each month. The body releases hormones, estrogen and progesterone, which account for the changes that take place on a daily basis. Estrogen increase the thickness of the endometrium (the inside lining of the uterus) which causes growth of the endometrium (Johnson, 2003). In turn this growth is accompanied by increased blood flow. This leads to the females monthly bleeding. The average menstrual cycle is 28 days, but can vary anywhere from 23-35 days.

Reference:
Johnson, L. (Ed.). (2003). Essential medical physiology (3rd ed.). San Diego: Elsevier.