When a girl reaches puberty, her ovaries (small egg-containing organs that are located on both sides of the uterus) make hormones that cause menstrual periods. The pituitary gland (located in the brain) releases chemical messengers called gonadotropins (FSH and LH) that “tell” the ovaries to release a mature egg once a month. This egg travels towards the uterus. If the egg isn’t fertilized by a sperm, the thick bloody lining that builds up in the uterus passes out of the body through the vagina as blood (a “period”, or “menstrual period”) two weeks later. This whole process is called menstruation.