Prolonged decelerations can be caused by any mechanism which normally may lead to periodic or episodic decelerations, but the return to baseline is delayed because the stimulus or mechanism causing the deceleration is not reversed. This often is associated with hypoxia. Mechanisms which are less likely to resolve spontaneously are therefore more likely to be associated with prolonged decelerations, such as cord compression, profound maternal hypotension or hypoxemia, tetanic uterine contractions, or prolonged head compression associated with the second stage of labor. A FHR above 100 beats/min with good variability is tolerable, but a prolonged deceleration below 100 beats/min calls for immediate efforts at resolution and a drop below 60 beats/min becomes an obstetric emergency since it is almost always associated with fetal hypoxia.

See illustration below for an example of prolonged deceleration. Please click to enlarge.

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