The endometrium is the lining of the womb that builds up every month in preparation for pregnancy. If there is no pregnancy the lining is shed (your period) and the process starts again. Endometriosis is a condition where, for reasons that we do not know, the endometrial cells start growing outside the womb, typically in the pelvis. When they bleed internally they can lead to a lot of pain, especially during sex and around the time of the period. An endometrioma (often called a chocolate cyst as the contents look like liquid chocolate) is a cyst on the ovary caused by endometriosis. In addition to pain and bleeding, endometriosis can lead to problems with fertility.