Fetal heart rate decelerations are decreases from the fetal heart rate baseline, quantified by depth in beats per minute from the baseline and in duration by minutes and seconds. Decelerations are defined based on onset (gradual or abrupt) and timing (periodic vs episodic). Decelerations are recurrent if they occur with at least 50% of contractions within a 20-minute period, or intermittent if they occur with less than 50% of contractions within a 20-minute period.
An early deceleration is defined as:
- Gradual onset (onset to nadir ≥ 30 seconds)
- Associated with a uterine contraction
- Onset, nadir, and recovery occur at the same time as the beginning, peak, and end of the contraction
Early decelerations are thought to be associated with a fetal autonomic response to altered intracranial pressure or cerebral blood flow, resulting from fetal head compression during labor. They have no known relationship to fetal oxygenation, and are considered clinically benign.
A variable deceleration is defined as:
- Abrupt onset (onset to nadir < 30 seconds)
- Decrease from baseline to nadir at least 15 bpm
- Duration of at least 15 seconds and < 2 minutes from onset to return to baseline
- Not always associated with uterine contractions, but if they are, can be intermittent or recurrent
Variable decelerations reflect a temporary interruption of fetal oxygenation due to transient mechanical compression of the umbilical cord. Occasional cord compression has shown to have little to no effect on fetal wellbeing; however, recurrent disruption of fetal oxygenation may lead to fetal compromise including hypoxemia, hypoxia, metabolic acidosis, and metabolic acidemia. Other fetal heart rate changes may then include a rising baseline, minimal or absent variability, absent accelerations, and a slow return to baseline after decelerations.
A late deceleration is defined as:
- Gradual onset (onset to nadir ≥ 30 seconds)
- Associated with a uterine contraction
- Onset, nadir, and recovery occur after the beginning, peak, and end of the contraction
- Can be intermittent or recurrent
A late deceleration is a fetal response to transient hypoxemia during a uterine contraction, reflecting transient interruption of fetal oxygenation and uteroplacental insufficiency. Recurrent or sustained disruption of fetal oxygenation can progress to metabolic acidemia, in which case the late deceleration may reflect direct myocardial depression. In the case of direct myocardial depression, other fetal heart rate changes would be expected, such as fetal tachycardia, absent variability, and absent accelerations.
A prolonged deceleration is defined as:
- Decrease in fetal heart rate at least 15 bpm below the baseline
- ≥ 2 minutes from onset to return to baseline
While a prolonged deceleration lasting more than 10 minutes is, by definition, considered as a baseline change, this event should never be interpreted as a benign baseline change; rather, a prolonged decel (typically to a FHR less than 110bpm) lasting close to or over 10 minutes is an emergent situation requiring immediate intervention.
A prolonged deceleration reflects an interruption in fetal oxygenation at one or more points along the oxygen pathway. Sustained disruption of fetal oxygenation can lead to fetal hypoxemia, hypoxia, metabolic acidosis, and metabolic acidemia. Continued tissue hypoxia and acidosis may result in failure of peripheral vascular muscle contraction and fetal hypotension, cascading into compromised coronary blood flow, myocardial hypoxia, direct myocardial depression, and a fetal bradycardia. Such profound compromise may cause the fetal heart to stop beating altogether.
The frequency, duration, and types of decelerations present in a fetal heart rate tracing should all be considered when assessing fetal well being and determining appropriate interventions. Decelerations should be interpreted within the context of the entire clinical picture, including maternal and fetal conditions, labor progress, and the progression of the fetal heart rate tracing over time.
