Endometriosis
The reason for endometriosis is unknown, but it occurs when endometrial tissue grows outside the uterine cavity. It can cause painful symptoms that can get worse with hormone fluctuations. The lining of your uterus is called the endometrium. Endometriosis is characterized by the growth of endometrial-like tissue on the ovaries, intestines, and pelvic tissues. Endometrial-like tissue rarely spreads beyond the pelvis, but it is not impossible. An endometrial implant is tissue that looks like endometrium and grows outside your uterus.