That’s because they grow—and bleed—as if they were still at home in the uterus. “Every time a woman has a period, there are these micro-periods happening,” says Tamer Seckin, MD, a specialist in endometriosis in private practice in New York and the co-founder and medical director of the Endometriosis Foundation of America. The immune system is altered in some way in women with endometriosis so that no matter how much swelling and inflammation the body sends to the pelvic cavity to try to clean away the blood that doesn’t belong, the implanted cells are still able to thrive, acting almost like cancer in many ways. Growths on the ovaries, called endometriomas or chocolate cysts, can permanently damage a woman’s fertility. Cysts may grow on the bowels, bladder, or, more rarely, even infiltrate the lungs.