Animation of endometriosis, a condition in which the cells like those of the endometrium (red), the tissue lining the inside of the uterus (yellow), start to grow in other locations, typically on the organs around the uterus. These tissues follow the normal menstrual cycle and hence bleed monthly. The condition can cause pelvic pain, painful periods, pain during intercourse and on urination, and infertility. Treatment included oestrogen-blocking hormones or other drugs, and painkillers for the pain. Surgery can remove the growths and help to prevent infertility by protecting the ovaries.