Endometriosis is defined as the presence of normal tissue in an abnormal place. The endometrium (lining) of the uterus spreads to the pelvis through the tubes and settles most commonly in the pelvis. Like the lining of the uterus, the endometrium grows under the influence of the major female hormone oestrogen. The most common sites in the pelvis are on and below the ovaries, and deep in the pelvis behind the uterus, called the Pouch of Douglas. Here the endometriosis grows on the ligaments behind the uterus and on the vagina and rectum. It also may grow on the bladder, appendix, abdominal wall and even sometimes in the upper abdomen.
retrograde menstruation which causes most endometriosis
the most common sites of endometriosis