Though it can vary from month to month, periods usually start off with a light flow, get heavier, then taper off.
On average, women lose about 2 to 4 tablespoons of blood during each period. In terms of pads or tampons, that means changing them out every 2 hours or more. If you need to change out less than every 2 hours -- or have quarter-size or larger clots -- let your doctor know.
Small clots of blood are common. Your body releases anticoagulants to keep the blood from clotting as it moves through your vagina. But on days when you're bleeding or cramping heavily, it may not be released in time.