Why does endometriosis cause pain?

Endometriotic deposits in the pelvis and other abnormal sites undergo changes like the normally sited endometrium within the womb; ectopic deposits thicken and then breakdown on a cyclical basis in synchrony with the womb lining. Unlike the womb lining, however, blood from deposits outside the womb have no path for getting out of the body and accumulate within the pelvic cavity. These bleeding sites also trigger a local inflammatory response. All these changes cause pain. Because these events that cause pain occur in relation to the menstrual cycle, pain with endometriosis is often cyclical and may intensify at the time of the menses. Pain can also come about from the scarring (known as adhesions) caused by repeated episodes of bleeding from these endometriosis sites that cause the pelvic organs to become stuck to one another.