Where does menstrual blood come from?

The uterus: The uterus, which is also termed the womb, is the place in which a fertilized egg implants and a fetus develops. To accomplish this it grows a blood rich lining, ready for this process each month. If no egg implants, the excess is shed as a discharge through the vagina. This keeps companies that make tampons rich, and is a signal as to whether a woman might be pregnant.