Menstruation
Inside the thick part of the uterus are blood vessels and nutrients that a baby will need to grow. Every month in a human female who has gone through puberty, the brain sends signals to the ovaries that tell them to release an ovum (or egg). The ovum then travels through the fallopian tubes (with the expectation of becoming fertilized by a sperm cell) into the uterus. If an ovum is fertilized, it sticks to the wall of the uterus and starts to grow. If the ovum is not fertilized, it does not stick. The uterus then gets rid of the ovum and the extra tissue by releasing it from the body. The tissue and blood flows out of the uterus through the vagina. This is called menstruation or having a period. The bleeding normally lasts about 3-5 days, though some people may bleed longer or have a bit of bleeding between periods.