## Core Concept
The **Stallworthy sign**, also known as the **Stallworthy's sign**, is a clinical sign used in obstetrics. It is associated with **placenta accreta**, a condition where the placenta grows too deeply into the uterine wall.

## Why the Correct Answer is Right
A positive Stallworthy's sign is suggestive of **placenta accreta**. This condition is characterized by the abnormal adherence of the placenta to the uterus due to a defect in the decidua basalis, leading to the placenta invading the myometrium. The sign is elicited by attempting to separate the placenta manually during delivery; if the placenta cannot be separated easily, it indicates abnormal adherence.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not specified, but conditions like placenta previa or uterine atony do not directly relate to the difficulty in separating the placenta manually.
- **Option B:** Similarly, this option is unspecified but likely refers to other obstetric conditions not directly related to the adherence of the placenta to the uterine wall.
- **Option C:** This could potentially refer to other placental abnormalities, but only placenta accreta is directly associated with a positive Stallworthy's sign.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that **placenta accreta** is a serious condition that can lead to severe postpartum hemorrhage. A high index of suspicion is crucial, especially in women with a history of uterine surgery, such as cesarean sections. The presence of a positive Stallworthy's sign necessitates careful management to prevent life-threatening complications.

## Correct Answer Line
**Correct Answer: D. Placenta accreta.**