Endometriosis is the abnormal growth of cells (endometrial cells) similar to those that form the inside or lining the tissue of the uterus, but in a location outside of the uterus. Endometrial cells are the lining cells of the uterus and are cells that are shed each month during menstruation.
Endometriosis occurs when cells like the lining of the uterus grow in or on tissues outside the uterus; these areas 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. Endometrial implants, while they can cause problems, are benign (not cancerous).
Endometriosis affects women in their reproductive years. The exact prevalence of endometriosis is not known, since many women may have the condition and have no symptoms. Endometriosis is estimated to affect over one million women (estimates range from 3% to 18% of women) in the United States. It is one of the leading causes of pelvic pain and reasons for laparoscopic surgery and hysterectomy in this country. Estimates suggest that 20% to 50% of women being treated for infertility have endometriosis, and up to 80% of women with chronic pelvic pain may be affected.