The growing tissue then begins to release more hormones to keep everything in the right condition. How- ever, if fertilization does not occur, the continued preparations in the uterus are unnecessary. The body realizes this near the end of the four- week period of the menstrual cycle. At this time, the level of LH decreases, the corpus luteum breaks down, and the level of progesterone decreases. Without this hormone, the uterine lining no longer remains intact. Instead, the lining breaks down. The built-up tissue along with some blood and the unfertilized egg are released from the body. This flow of blood, called menstruation, lasts for about four days. Then.
