The Mannion IUD stops endometrial hyperplasia, which is usually caused by a localized increase in estrogen. In addition to being a contraceptive, the IUD has a number of additional effects based on the steady release of some progesterone, which is relatively safe as it is concentrated in the endometrium and rarely enters the bloodstream. If progesterone acts for a long time, estrogen-induced endometrial hyperplasia is suppressed. The menstrual cycle is related to the levels of estrogen and progesterone. There are cyclical changes in estrogen and progesterone, and menstruation also belongs to the cyclical changes. When estrogen is elevated, endometrial hyperplasia occurs, and when progesterone is elevated, endometrial atrophy and shedding occurs to form menstruation, and a stable menstrual cycle is formed by the weekly changes of estrogen and progesterone. If there is an imbalance between estrogen and progesterone, such as a persistent effect of estrogen without elevated progesterone or insufficient progesterone, the endometrium is in a proliferative state for a long period of time without shedding or incomplete shedding, endometrial hyperplasia will result. If the endometrial hyperplasia lasts too long, there is usually a risk of cancer. With polycystic ovary syndrome (PCOS), where patients ovulate less and have less progesterone, the risk of endometrial cancer can be much higher than in the general population when this happens over a long period of time into the late 40s.