Adenomyosis (pronounced “add-en-o-my-OH-sis”) is when tissue similar to the lining of your uterus (endometrium) starts to grow into the muscle wall of your uterus (myometrium). It causes your uterus to thicken and enlarge — sometimes, up to double or triple its usual size. Adenomyosis can cause painful periods, heavy or prolonged menstrual bleeding with clotting and abdominal/pelvic pain.