## Core Concept
Postpartum hemorrhage (PPH) is a leading cause of maternal morbidity and mortality worldwide. It is defined as excessive bleeding following the birth of a baby, typically considered to be more than 500 mL after vaginal delivery or more than 1000 mL after cesarean section. The causes of PPH can be categorized into four main areas: uterine atony, lacerations, retained placental tissue, and coagulopathy.

## Why the Correct Answer is Right
Uterine atony is the most common cause of postpartum hemorrhage, accounting for approximately 80-90% of cases. It occurs when the uterus fails to contract down effectively after delivery, leading to excessive bleeding from the site where the placenta was attached. This condition can be due to various factors, including overdistension of the uterus (as in multiple gestations or polyhydramnios), prolonged labor, and the use of certain medications like magnesium sulfate.

## Why Each Wrong Option is Incorrect
- **Option A:** Lacerations, although a significant cause of postpartum hemorrhage, are less common than uterine atony. They can occur during delivery and may not always be immediately apparent.
- **Option C:** Retained placental tissue can cause PPH but is less common than uterine atony. It happens when parts of the placenta are left behind in the uterus after delivery.
- **Option D:** Coagulopathy, or disorders of blood clotting, can lead to PPH but is a less frequent cause compared to uterine atony. It may be due to pre-existing conditions or complications during pregnancy.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the risk factors for uterine atony, and thus PPH, include uterine overdistension (e.g., multiple gestations, polyhydramnios), prolonged labor, and certain medications (e.g., magnesium sulfate). Early identification of these risk factors and proactive management can significantly reduce the incidence and severity of PPH.

## Correct Answer: B. Uterine Atony.