Endometriosis is a chronic condition that usually affects women in their 30s and 40s, but symptoms can develop at any time while a woman is menstruating. “Endometriosis occurs when endometrial tissue, the tissue of the inner lining of the uterus, starts to grow outside of the uterus,” says Veerle Bossuyt, MD. The displaced tissue can cause irritation and sometimes severe pain, during menstruation and intercourse. The condition can also lead to infertility. Most commonly, endometriosis involves the ovaries, the bowel or the tissue lining the pelvis, and only rarely does it spread beyond the pelvic region.