Menstruation can come from the nipple. Young children do not have periods, but older children and adults may. Menstruation consists of blood mixed with mucus and fluid from the uterus. The uterus sits inside the abdomen and is like a small balloon. About once a month, the walls of the uterus are covered with a soft mucous membrane so that a baby can grow there. If there is no baby, the mucous membrane is not needed, but it is emptied through the nipple as menstruation.