Final answer:
Excessive postpartum hemorrhage is commonly caused by uterine atony, where the uterus fails to contract after delivery, or by retained placental fragments, both of which can lead to significant blood loss that needs immediate medical attention.
Step-by-step explanation:
The most common cause of excessive postpartum hemorrhage is uterine atony, where the uterus fails to contract effectively after delivery. If uterine contractions are inadequate, the blood vessels that were attached to the placenta can continue to bleed. Ensuring that the placenta is expelled completely and intact is crucial because any retained placental fragments can prevent the uterus from contracting properly, which can also lead to hemorrhage. Postpartum hemorrhage requires prompt medical intervention to restore blood volume and prevent further complications.