Endometriosis is a disorder in which tissue similar to the tissue that forms the lining of your uterus grows outside of your uterine cavity. The lining of your uterus is called the endometrium. Endometriosis occurs when endometrial tissue grows on your ovaries, bowel, and tissues lining your pelvis. The most common symptoms are pain and menstrual irregularities. Effective treatments, such as hormones and excision surgery, are available. If left untreated, endometriosis can (however does not always) result in a range of symptoms, including Dysmenorrhoea (pain during menstruation) Pelvic pain. Infertility (the inability to become pregnant) or subfertility (a reduced ability to become pregnant).