According to WebMD, a woman typically loses between 4 and 12 teaspoons of blood during her period. Though the average period lasts three to five days, it is considered normal for a period to run between two and seven days.
WebMD reports that the color and thickness of menstrual blood varies. Usually, the blood is bright red at the start of a period and darkens as the period progress and the blood becomes older. Also, blood clots sometimes show up during periods. Natural anticoagulants generally keep blood from clotting while it is flowing. However, if the flow is heavy and fast, the anticoagulants do not have time to work, and clots appear.