Endometriosis occurs when the tissues that normally line the uterus, grow elsewhere in the body.
Endometriosis can be especially painful during a woman's period. The pain can vary from mild to severe, depending on the woman. Infertility can also be caused by endometriosis. In many cases, endometriosis leads to infertility by causing pelvic adhesions, abnormal ovulation, and interference with the ability of the fallopian tube to pick up the egg from the ovary. It can also be very painful.