Endometriosis is a condition where cells of the endometrium are abnormally located outside the uterine cavity, commonly in the pelvis behind the uterus and ovaries. The endometrium is tissue that lines the inner surface of the uterus and respond to ovarian hormones. If pregnancy does not occur the lining is shed as a bleed which is commonly known as a ‘period’.

Endometriosis may be caused by backflow of menstrual blood through the fallopian tubes into the abdomen. During each period, this tissue outside the uterus bleeds in the same way as the lining of the uterus. Unlike normal uterine bleeding, the areas of endometriosis have no access to the outside and bleed into the pelvis. This leads to inflammation, pain and scar tissue. Endometrial tissue is commonly found in the ovary where it can form cysts, called chocolate cysts because of the brown colour and thick consistency of the trapped blood.