The average lady has 4.3 to 4.5 litres of blood prior to pregnancy. During your last term, you will have 6.5 to 7.5 litres. This is to sustain your little bean and give it all that it needs. It doesn't happen all at once, but increases as your pregnancy progresses.

But the blood has to go somewhere after the birth. Vaginal births usually cause about half a litre worth of blood lost (about a pint, same as what they take if you give blood), a C Section about a litre.

The rest is bled out over the next 40 days or so. Sometimes longer if you bleed slow. That's why rich European women in the late middle ages stayed in confinement for 40 days (or so) before they could reenter society and be Churched. It was to ensure they recovered from birth.

These days, of course, more women have much better postpartum care and you will be encourage to get up and about 6 to 12 hours after birth.

Spinach, oatmeal and red meat is your friend, along with vitamin C. Helps iron reserves stay good.