Endometriosis is a disease in which patches of endometrial tissue, which normally is found only in the uterine lining (endometrium), grow outside the uterus.

Usually, endometriosis is confined to the lining of the abdominal cavity or the surface of abdominal organs. The misplaced endometrial tissue (endometrial implant) commonly adheres to the ovaries and the ligaments that support the uterus. Less commonly, it adheres to the outer surface of the small and large intestines, the ureters (tubes leading from the kidneys to the bladder), the bladder, the vagina, surgical scars in the abdomen, or the lining of the chest cavity. Rarely, endometrial tissue is found in the lungs.

Because the misplaced endometrial tissue responds to the same hormones that the uterus responds to, it may bleed during the menstrual period, often causing cramps, pain, irritation, and the formation of scar tissue. As the disease progresses, adhesions (fibrous bands that connect normally unconnected structures) may form. The misplaced endometrial tissue and adhesions can block or interfere with the functioning of organs. Rarely, adhesions block the Intestine.

Because endometriosis depends on hormonal cycles, and pregnancy temporarily interrupts those cycles, many women find their symptoms improve during pregnancy. In some cases, the improvement may be permanent, presumably because the break from cycles of growth, bleeding, and scarring finally allows the implants to heal and be shed. In other cases, however, the relief is only temporary, and once the hormonal cycles return to normal, the symptoms of endometriosis recur.