OK, here's an article I found from the University of Virginia (USA).

Postpartum Hemorrhage
What is postpartum hemorrhage?

Postpartum hemorrhage is excessive bleeding following the birth of a baby. About 4 percent of women have postpartum hemorrhage and it is more likely with a cesarean birth. Hemorrhage may occur before or after the placenta is delivered. The average amount of blood loss after the birth of a single baby in vaginal delivery is about 500 ml (or about a half of a quart). The average amount of blood loss for a cesarean birth is approximately 1,000 ml (or one quart). Most postpartum hemorrhage occurs right after delivery, but it can occur later as well.
What causes postpartum hemorrhage?

Once a baby is delivered, the uterus normally continues to contract (tightening of uterine muscles) and expels the placenta. After the placenta is delivered, these contractions help compress the bleeding vessels in the area where the placenta was attached. If the uterus does not contract strongly enough, called uterine atony, these blood vessels bleed freely and hemorrhage occurs. This is the most common cause of postpartum hemorrhage. If small pieces of the placenta remain attached, bleeding is also likely. It is estimated that as much as 600 ml (more than a quart) of blood flows through the placenta each minute in a full-term pregnancy.

Some women are at greater risk for postpartum hemorrhage than others. Conditions that may increase the risk for postpartum hemorrhage include the following:

* placental abruption - the early detachment of the placenta from the uterus.

* placenta previa - the placenta covers or is near the cervical opening.

* overdistended uterus - excessive enlargement of the uterus due to too much amniotic fluid or a large baby, especially with birthweight over 4,000 grams (8.8 pounds).

* multiple pregnancy - more than one placenta and overdistention of the uterus.

* pregnancy-induced hypertension (PIH) - high blood pressure of pregnancy.

* having many previous births

* prolonged labor

* infection

* obesity

* medications to induce labor

* medications to stop contractions (for preterm labor)

* use of forceps or vacuum-assisted delivery

* general anesthesia

Postpartum hemorrhage may also be due to other factors including the following:

* tear in the cervix or vaginal tissues

* tear in a uterine blood vessel

* bleeding into a concealed tissue area or space in the pelvis which develops into a hematoma, usually in the vulva or vaginal area

* blood clotting disorders such as disseminated intravascular coagulation

* placenta accreta - the placenta is abnormally attached to the inside of the uterus (a condition that occurs in one in 2,500 births and is more common if the placenta is attached over a prior cesarean scar).

* placenta increta - the placental tissues invade the muscle of the uterus.

* placenta percreta - the placental tissues go all the way into the uterine muscle and may break through (rupture).

Although an uncommon event (one in 2,000 deliveries), uterine rupture can be life threatening for the mother. Conditions that may increase the risk of uterine rupture include surgery to remove fibroid (benign) tumors and a prior cesarean scar in the upper part (fundus) of the uterus. It can also occur before delivery and place the fetus at risk as well.