In women who are not pregnant, Progesterone prepares the endometrium, or lining of the uterus, for implantation by the fertilized egg. Progesterone Levels are lowest during menstruation, and remain low up until ovulation. This is the point approximately midway through a normal menstrual cycle when an egg is released from one follicle in the ovary. After releasing the egg, the follicle is known as the corpus luteum, and begins producing increasing levels of Progesterone. This Progesterone acts on the endometrium to make it thicker and richer in blood supply and nutrients to prepare for implantation by the fertilized egg. If the egg is not fertilized, Progesterone Levels drop rapidly, causing most of the endometrium to break down and leaving only a thin layer of tissue. The broken-down endometrium is shed during a woman