FHR Accelerations & Early Decelerations
Accelerations and early decelerations both signal a reassuring fetal status — but only if you can distinguish them from the deceleration patterns that demand immediate action.
Core Concept
Accelerations are transient rises in fetal heart rate above baseline, reflecting an intact autonomic nervous system. At ≥32 weeks gestation, an acceleration is defined as a rise of ≥15 bpm lasting ≥15 seconds (the "15x15 rule"). Before 32 weeks, the threshold drops to ≥10 bpm for ≥10 seconds. Two accelerations in a 20-minute window define a reactive nonstress test (NST), confirming fetal well-being. Accelerations commonly occur with fetal movement, stimulation, or contractions and are the single most reassuring finding on a fetal monitor strip. Early decelerations are gradual, uniform dips in FHR that mirror contractions — they start when the contraction starts, reach their nadir at the peak of the contraction, and recover as the contraction ends. The cause is fetal head compression during descent, which triggers a vagal response. Early decels are benign, require no intervention, and are most common during active labor and pushing. The key visual feature is symmetry: the deceleration is a mirror image of the contraction curve.
Watch Out For
Don't confuse early decelerations (mirror contractions, head compression, benign) with late decelerations (begin after contraction peak, signal uteroplacental insufficiency, require intervention). Students mistake variable decelerations for early decels — variables are abrupt, jagged drops unrelated to contraction timing, caused by cord compression. An acceleration that lasts ≥10 minutes is reclassified as a baseline change, not a prolonged acceleration.
Clinical Pearl
Early decels = Head compression = mirror image of contractions. Think "Early = hEad" — if it mirrors the contraction perfectly, the head is just getting squeezed on the way down.
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