During the menstrual cycle the lining of the womb, known as the endometrium, gradually thickens and fills with blood as it prepares for the arrival of a fertilised egg. If the egg is not fertilised, the womb lining breaks down and is lost as the monthly period. Sometimes small pieces of endometrium grow on organs outside the uterus. This is called endometriosis. The endometriosis goes through the same monthly changes as the womb lining; sometimes swelling and bleeding into the body cavity, which can cause inflammation and pain. Rubbery bands of scar tissue, called adhesions, may form. These can tie organs and tissues together and affect the normal working of organs.
Endometriosis most commonly occurs on the ovaries, on the tubes that carry eggs from the ovaries to the womb (Fallopian tubes), and on the tissues that hold the womb in place. When it occurs on the ovaries the endometriosis can forms cysts (endometrioma). These are also called chocolate cysts because of the brown fluid they contain. They may not cause any pain and may only be found during an internal examination to check fertility. However if they burst, their contents spill into the body cavity where they can cause severe pain and result in adhesions. Less commonly endometriosis can occur on the bladder or bowel or on other organs.