**Core Concept**
Ergometrine is a synthetic ergot alkaloid used to induce sustained uterine contractions, primarily in the third stage of labor to prevent postpartum hemorrhage. Its mechanism of action involves stimulating the 5-HT2B receptors in the smooth muscle of the uterus, leading to sustained contractions.

**Why the Correct Answer is Right**
Ergometrine's action on the 5-HT2B receptors causes intense and sustained uterine contractions, which can compromise fetal oxygenation and lead to fetal hypoxia. This is because the increased uterine tone reduces blood flow to the placenta, resulting in decreased oxygen and nutrient delivery to the fetus. This risk of fetal hypoxia is particularly concerning during the initiation of labor, when the fetus is already vulnerable to changes in maternal-fetal circulation.

**Why Each Wrong Option is Incorrect**
**Option A:** Ergometrine's onset of action is not a reason for its avoidance in labor initiation; its rapid onset is actually a desirable property for preventing postpartum hemorrhage.
**Option C:** While ergometrine can cause vasoconstriction and increase blood pressure, this is not the primary reason it is avoided in labor initiation. The risk of fetal hypoxia is a more significant concern.
**Option D:** Ergometrine does not act on D2 receptors to cause vomiting; its action on 5-HT2B receptors is responsible for its uterotonic effects.

**Clinical Pearl / High-Yield Fact**
Remember that ergot alkaloids like ergometrine can cause intense uterine contractions, which can compromise fetal oxygenation. This is a classic exam trap – the correct answer may seem counterintuitive, but it is based on the underlying pharmacology of ergot alkaloids.

**✓ Correct Answer: B. Fetal hypoxia**