The uterus has a lining called the Endometrium. Endometriosis is a condition in which the tissue grows on the outside of the uterus, and sometimes on other parts of the body – including the ovaries, the fallopian tubes, and sometimes the bowel as well. The lining outside of the uterus can cause severe pain during periods. For some women, the pain may be so severe that they are incapacitated by it for the entire duration of their period. It’s not just physical pain; the chronic pain associated with endometriosis can also lead to emotional difficulties.