Endometriosis
A complex condition in which endometrial tissue (which normally lines the uterine cavity) has implanted outside the uterine cavity, usually in the pelvis.
This abnormal tissue can cause pain or infertility. Pain is often felt during menstruation, but it can become chronic. In severe cases, it can cause pain during sexual intercourse or bowel movements.
Further information: Endometriosis and The World Endometriosis Research Foundation (WERF)