The menstrual cycle is governed by hormonal changes. In the follicular phase, an increase amount of estrogen occur, follicles begun developing, and one or two become dominant. After some time, the Lutenizing Hormone increases, causing the follicle to release an ovocyte (ovulation). Progesterone is then produced in large amount resulting in the change of the uterine lining for preparation of the implantation of an embryo to establish pregnancy. If implantation does not happen, Progesterone & Estrogen decreases. The hormone drop causes the uterus to shed its lining in a process termed menstruation