Every month, tissue made up of blood vessels builds up inside the uterus to prepare for ovulation when an egg is released. If a sperm fertilizes the egg, pregnancy happens. If not, the egg breaks apart, and the lining of tissue and blood won't be needed. It flows out of the uterus, through the cervix, into the vag*** and out of the body. This is called menstrual flow or what we call - a period.