Endometriosis is when the endometrial cells that are formed inside of the uterus start to grow somewhere on the outside of it. These cells are the exact same cells that women get rid of once a month during their menstruation cycle. Endometriosis cells will latch on to the tissue that surrounds the outside of the uterus. Doctors refer to them as endometriosis implants. 

 These implants can be found on the ovaries, the outer surfaces of the uterus, outer surfaces of intestines, the surface lining of the pelvic cavity, and the Fallopian tubes. They may also be located in the bladder, cervix, or vagina. However, they are only found here on rare occasions.  

 It is not likely that many of these implants are located on the outside of the pelvis, in or close to the brain, lung, liver, or also on old surgery scars. While these endometriosis implants can cause problems they are not at all cancerous.  

 Health experts do not know the cause of endometriosis. Some believe that the endometrial tissue is dropped in unusual spots when the menstrual flow backs up into the Fallopian tubes and the abdominal cavity.