What causes early decelerations in fetal heart rate?

Early decelerations in fetal heart rate are primarily caused by fetal head compression. This phenomenon occurs during labor when the fetal head descends into the birth canal and exerts pressure on the cervix, leading to an increase in parasympathetic nervous system activity. This increased vagal tone can cause a transient decrease in the fetal heart rate, which reflects the physiological response of the autonomic nervous system to the compression.

These decelerations are typically seen early in contraction cycles, beginning and returning to baseline in conjunction with uterine contractions, indicating a normal response to labor rather than a pathological process. This is in contrast to other factors such as placental insufficiency, maternal hypotension, or uterine hypertonicity, which can lead to variable or late decelerations, indicating potential compromise to fetal well-being. Understanding this mechanism is crucial for clinicians monitoring fetal heart rate patterns during labor.