Endometriosis and ovarian cyst, axial magnetic resonance imaging (MRI) scan through the abdomen of a twenty-nine year old woman. The front of the body is at the top of the image. The dark mass at centre left is a cyst, a fluid-filled sac, of an ovary. The dark blue masses at centre right are endometriomas, or chocolate cysts, a result of endometriosis. Endometriosis is the movement of cells from the lining of the uterus to the abdominal cavity. The cells still respond to the menstrual cycle and so bleed once a month. However, the blood has no way of leaving the body and so causes pain, inflammation and scar tissue, and can lead to infertility.