The name endometriosis comes from the word "endometrium," the tissue that lines the inside of the uterus. If a woman is not pregnant, this tissue builds up and is shed each month as menstrual flow at the end of each cycle. In endometriosis, endometrial tissue is found outside the uterus.
Endometrial tissue residing outside the uterus responds to the menstrual cycle in a way that is similar to the way endometrium usually responds in the uterus. At the end of every cycle, when hormones cause the uterus to shed its endometrial lining, endometrial tissue growing outside the uterus will also bleed.
Tissues surrounding the affected areas of endometriosis become inflamed. This inflammation may produce scar tissue around the area of endometriosis. These endometrial tissue sites may develop into what are called lesions or growths.
Infrequently, endometrial growths are found on the intestines or in the rectum, on the bladder, vagina cervix, and vulva (external genitals), or in abdominal surgery scars. Very rarely, endometrial growths have been found outside the abdomen, in the thigh, arm, or lung.
Endometriosis can cause infertility by three main mechanisms: the anatomical distortion of the pelvic organs which may interfere with normal fertilization of the egg by the sperm, ovarian function compromise as a result of adhesions or cysts (endometriomas) on the ovaries, and inflammatory processes which cause harm to the implanting embryo.