How much blood loss usually occurs during childbirth?

Depends. For a vaginal delivery the average blood loss is around 500 cc. That is a little more than is usually given when someone donates a unit of blood. For a cesarean delivery the average loss is about twice as much (1000 cc). Pregnancy causes a woman's blood volume to expand about 2000 cc to 2500 cc, so there is adequate volume to accomodate even greater blood loss with delivery.
About 1/2 quart. Normally, we estimate blood loss at about 500 cc, or 1/2 liter, which is a little more than half a quart. This includes the blood that is contained in the placenta (afterbirth). However, postpartum hemorrhage (heavy bleeding) is not uncommon, and requires agressive intervention in the hospital to prevent complications such as shock. Sometimes surgery and/or transfusions are needed. Dr. Mike.