Know the reason causing Postpartum hemorrhage?

When the baby is delivered, the uterus contracts and pushes the placenta out. After delivery of placenta these contractions help in pressurizing the blood vessels in the area where placenta was attached. These blood vessels bleed freely if the uterus does not contract strongly which is the most common cause for postpartum hemorrhage. 

Question: Name any one other cause of postpartum hemorrhage?

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