**Core Concept**
During pregnancy, several factors can increase the risk of postpartum hemorrhage (PPH), which is defined as the loss of more than 500 mL of blood after delivery. Understanding the underlying causes and risk factors is crucial for timely intervention and management.

**Why the Correct Answer is Right**
Postpartum hemorrhage is often caused by uterine atony, which is the failure of the uterus to contract after delivery. Other risk factors include retained placental tissue, lacerations, and coagulopathy. However, certain conditions during pregnancy can increase the risk of PPH. For example, multiple gestations, polyhydramnios, and placenta previa can all contribute to an increased risk of PPH. Additionally, conditions that cause uterine distension, such as multiple fibroids or a large uterus, can also increase the risk of PPH.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because multiple gestations, such as twins or triplets, increase the risk of PPH due to uterine distension and the potential for multiple placental sites.
* **Option B:** This option is incorrect because polyhydramnios, or excessive amniotic fluid, can cause uterine distension and increase the risk of PPH.
* **Option C:** This option is incorrect because placenta previa, a condition where the placenta covers the cervix, can cause bleeding during pregnancy and increase the risk of PPH.

**Clinical Pearl / High-Yield Fact**
It's essential to identify risk factors for PPH during pregnancy and to have a plan in place for management, including the availability of blood products and surgical backup.

**Correct Answer: D**