To better understand endometriosis, its symptoms, its consequences and how to diagnose and treat it, we must explain what the endometrium is, which is where this disease gets its name. The endometrium is a thin layer that coats the inside of the uterus and transforms during each menstrual cycle in order to eventually receive an embryo. At the beginning of each cycle, this layer (also known as the endometrial mucus) becomes thicker and changes so that when fertilization occurs, it can accommodate a pregnancy. These cells only exist in the female body. Men do not have endometrial cells in their bodies, since they do not have uteri and cannot become pregnant. In each cycle that does not result in a pregnancy, the feminine body releases this unused layer. As it detaches, it causes bleeding which is called menstruation. This endometrial mucus that is meant to accommodate a pregnancy should only be found in one part of the body: inside the uterus. However, 10% of women are diagnosed with having endometrial cells in other areas of their bodies. Ectopic endometrium, that is, when it is found outside of the uterus, is a condition known as "endometriosis".