## **Core Concept**
The incidence of preterm delivery is significantly higher in twin pregnancies compared to singleton pregnancies. This is due to the increased uterine distension and the associated physiological changes that can lead to cervical shortening and uterine contractions. Preterm delivery is a major concern in twin pregnancies as it is a leading cause of neonatal morbidity and mortality.

## **Why the Correct Answer is Right**
The correct answer, **50%**, reflects the substantially increased risk of preterm birth in twin pregnancies. Studies have shown that nearly half of all twin pregnancies result in preterm delivery, which is significantly higher than the approximately 10% risk seen in singleton pregnancies. This increased risk is attributed to the mechanical factors such as uterine overdistension and possibly underlying cervical insufficiency.

## **Why Each Wrong Option is Incorrect**
- **Option A: 10%** - This is incorrect because it represents the preterm birth rate for singleton pregnancies, not twin pregnancies.
- **Option B: 20%** - This underestimates the risk of preterm delivery in twin pregnancies.
- **Option D: 80%** - While preterm delivery is common in twin pregnancies, 80% is higher than the generally reported incidence.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the risk of preterm labor in twin pregnancies can be somewhat mitigated through careful prenatal monitoring, including regular ultrasound assessments for cervical length, and the use of progesterone supplementation in select cases. However, even with these interventions, the baseline risk remains elevated.

## **Correct Answer: C. 50%.**