Comprehensive and Detailed Explanation From NCC-Aligned Sources:

Vagal stimulation is part of the parasympathetic nervous system, which causes:

Slowing of the fetal heart rate (FHR)

Rapid but temporary changes in HR

Seen with head compression, scalp stimulation, or fetal movement

NICHD/NCC physiology explains:

Vagus nerve activation → acetylcholine release → slowed SA node firing → decrease in FHR

This mechanism is responsible for early decelerations during labor due to head compression.

Why the incorrect answers are wrong:

B. Increased cardiac contractility → sympathetic effect, not vagal.

C. Increased fetal blood pressure → also a sympathetic effect.

Correct answer: A. Decreased fetal heart rate