The endometrium is the tissue that lines the uterus. During a woman's normal menstrual cycle, this tissue builds up and then is shed if the woman does not become pregnant. Women with endometriosis develop excess tissue that looks and acts like endometrial tissue, but it is located outside the uterus, usually on other reproductive organs inside the pelvis or in the abdominal cavity.
Each month, this misplaced tissue responds to the hormonal changes of the menstrual cycle by building up and breaking down just as the endometrium does, resulting in small amounts of internal bleeding and often, pain. Unlike menstrual fluid from the uterus which is shed by the body, blood from the misplaced tissue has nowhere to go, and often causes surrounding tissue to become inflamed or swollen. This process can produce scar tissue around the area which may develop into lesions or growths.
In some cases, particularly when an ovary is involved, the blood can become embedded in the tissue where it is located, forming blood blisters that may become surrounded by a fibrous cyst.