Although theories abound, just why some women develop endometriosis and others don’t remains a mystery. Or why endometrosis produces painful or heavy periods in some and no symptoms in others. Or why some women with endometriosis are infertile and others are not. What doctors do agree on is what endometriosis is. Pieces of the lining of the uterus called the endometrium find their way outside the uterus and into the pelvis, where they implant themselves and grow.