A 30-year-old primigravida at 39 weeks of gestation is in active labor. Electronic fetal monitoring reveals a baseline fetal heart rate of 145 bpm with moderate variability. However, smooth, gradual decreases in fetal heart rate are observed that begin after the peak of each uterine contraction and return to baseline only after the contraction has finished. What do these late decelerations indicate, and what is the underlying pathophysiology?
Answer and explanation
Correct answer: A. Uteroplacental insufficiency leading to fetal hypoxemia and chemoreceptor stimulation
- Step 1: Late decelerations are characterized by a gradual symmetrical decrease and return of fetal heart rate with the nadir occurring AFTER the peak of the contraction.
- Step 2: They are a physiological reflex triggered by fetal hypoxemia: uterine contractions transiently reduce intervillous blood flow; when baseline placental perfusion is compromised (uteroplacental insufficiency), critical fetal arterial hypoxemia stimulates carotid chemoreceptors, causing a vagally mediated drop in heart rate.
- Step 3: Persistent late decelerations represent a Category II or III tracing requiring immediate intrauterine resuscitation (oxygen, maternal repositioning, fluid bolus, stopping oxytocin) and evaluation for operative delivery.
Answer
Uteroplacental insufficiency leading to fetal hypoxemia and chemoreceptor stimulation.
Why not the others: 'Umbilical cord compression' produces variable decelerations with abrupt onset; 'Fetal head compression' produces early decelerations that mirror contractions; 'Maternal hypotension after epidural' would produce a prolonged deceleration, and late decelerations indicate hypoxemia, not a normal-oxygen state.
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