A diagnosis of endometriosis denotes the presence of endometrial tissue outside the lining of the uterine cavity. Such ectopic tissue is generally confined to the pelvic area, most commonly around the ovaries, uterovesical peritoneum, uterosacral ligaments, and cul-de-sac, but it can appear anywhere in the body.

This ectopic endometrial tissue responds to normal stimulation in the same way the endometrium does. During menstruation, the ectopic tissue bleeds, causing the surrounding tissues to become inflamed. This inflammation causes fibrosis, leading to adhesions that produce pain and cause infertility.

Active endometriosis usually occurs between ages 30 and 40, but may be seen before age 20. Severe symptoms of endometriosis may have an abrupt onset or develop over many years. This disorder usually becomes progressively severe during the menstrual years; after menopause, it tends to subside.