## **Core Concept**
Postpartum hemorrhage (PPH) is a leading cause of maternal morbidity and mortality worldwide. It is defined as excessive bleeding following the delivery of the 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**
The most common cause of postpartum hemorrhage is **uterine atony**, which refers to the failure of the uterus to contract down effectively after delivery. This failure to contract prevents the uterus from compressing the blood vessels at the placental site, leading to continued bleeding. Uterine atony can be caused by various factors, including prolonged labor, overdistension of the uterus (as in multiple gestations or polyhydramnios), and the use of certain medications like magnesium sulfate.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Lacerations can indeed cause postpartum hemorrhage but are not the most common cause. Lacerations can range from minor vaginal tears to more severe extensions into the cervix or perineum.
- **Option B:** Retained placental tissue can cause PPH by preventing the uterus from contracting down properly. However, it is less common than uterine atony as a cause.
- **Option C:** Coagulopathy, or a disorder of blood clotting, can lead to PPH but is a less frequent cause compared to uterine atony.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the risk factors for uterine atony, and thus PPH, include prolonged labor, uterine overdistension, and certain medications. Early identification of these risk factors and proactive management, such as the administration of uterotonic agents like oxytocin immediately postpartum, can significantly reduce the risk of PPH.

## **Correct Answer:** D. Uterine Atony