Pathophysiology
Clinical meaning
The fetal heart rate (FHR) is regulated by the autonomic nervous system. The sympathetic nervous system increases heart rate, while the parasympathetic (vagus nerve) decreases it. Normal baseline is 110-160 bpm. Variability reflects intact CNS function: absent (no fluctuation: ominous), minimal (<5 bpm), moderate (6-25 bpm: reassuring, indicates intact neurological pathway), and marked (>25 bpm). Accelerations (≥15 bpm above baseline for ≥15 seconds) are reassuring and indicate fetal well-being. Decelerations are classified as Early (head compression, mirrors contractions), Late (uteroplacental insufficiency, begins after contraction peak), and Variable (cord compression, abrupt onset/offset with varying shape).