Research has shown that the average period blood loss is 40-60 mls (8-12 teaspoonfuls). Some women may complain of excess loss when they are only losing less than 10mls.
Some women losing up to 400 mls do not realize that they have a problem (500mls is about a pint, the amount given when someone donates blood), which for a menstrual cycle, is very heavy. A physician should be contacted if this is the case.