Endometriosis is the growth of cells similar to those that form the inside of the uterus (endometrial cells), but in a location outside of the uterus. Endometrial cells are the same cells that are shed each month during menstruation.
The cells of endometriosis attach themselves to tissue outside the uterus and are called endometriosis implants.
These implants are most commonly found on the ovaries, the Fallopian tubes, outer surfaces of the uterus or intestines, and on the surface lining of the pelvic cavity. They can also be found in the vagina, cervix, and bladder, although less commonly than other locations in the pelvis.
Rarely, endometriosis implants can occur outside the pelvis, on the liver, in old surgery scars, and even in or around the lung or brain