FHR Late Decelerations — Pattern Recognition
A deceleration that mirrors the contraction but arrives late tells you the placenta is failing in real time — and the timing is everything.
Core Concept
Late decelerations are symmetrical, gradual drops in fetal heart rate that begin after the peak of a uterine contraction and return to baseline after the contraction ends. The hallmark is the offset in timing: the nadir of the deceleration occurs after the acme of the contraction. This lag reflects uteroplacental insufficiency — the placenta cannot deliver adequate oxygen during the stress of contractions, so the fetal response is delayed. A single late decel may appear benign, but a repetitive pattern (occurring with ≥50% of contractions over a 20-minute window) is always concerning and classified as Category II or III depending on variability. Late decels look uniform and smooth, shaped like a shallow U that mirrors the contraction curve but is shifted to the right on the tracing. The onset-to-nadir takes ≥30 seconds, distinguishing them from variable decelerations which drop abruptly. Crucially, late decelerations with absent or minimal variability represent the most ominous combination on a fetal monitor strip — Category III — because it suggests the fetus has lost the compensatory reserve to maintain beat-to-beat variation.
Watch Out For
Don't confuse late decels with early decels: early decels mirror the contraction exactly (nadir aligns with acme) and are benign head compression. Late decels are shifted right — the nadir comes after the contraction peaks. Students mistake a single late decel for a reassuring strip; repetitive lates are never normal. Late decels are gradual (onset to nadir ≥30 seconds); abrupt drops suggest variable decelerations from cord compression, not placental insufficiency.
Clinical Pearl
Think 'late to the party, late to leave.' The decel starts late, bottoms out late, and recovers late — always after the contraction. That delay means the placenta, not the cord.
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