An acceleration is a visually abrupt increase in the fetal heart rate (FHR) above the baseline with onset to peak of the acceleration less than 30 seconds .


For fetuses > 32 weeks gestational age the peak heart rate must be> 15 beats per minute (BPM) above the baseline and must last for> 15 seconds but less than 2 minutes from the initial change in heart rate to the time of return of the fetal heart rate to the baseline. For fetuses < 32 weeks of gestation the accelerations must have a peak heart rate > 10 beat and a duration of > 10 seconds but less than 2 minutes .An acceleration >  2 minutes but less than 10 minutes in duration is called a prolonged acceleration.   An acceleration lasting > 10 minutes is a baseline change. Accelerations may be further categorized as episodic or periodic. Episodic accelerations occur independent of uterine contractions.  Periodic accelerations are associated with uterine contractions.  Accelerations may be present or absent in an otherwise normal category I fetal heart rate tracing.[1].
Accelerations are usually, but not always, associated with either spontaneous fetal activity, stimulation, or uterine activity [2-4]. Accelerations have also been attributed to partial umbilical cord occlusion caused by a baroreceptor-mediated response to decreased venous return [5].The presence of either spontaneous or stimulated FHR accelerations reliably predicts the absence of fetal metabolic acidemia at the time they are observed provided the heart rate being recorded is truly the fetus and not the mother [1,6].  Periodic accelerations caused by partial umbilical cord compression are not as reassuring as episodic accelerations since continued cord compression may cause progression to a category II or category III tracing [10]. A rise in the fetal heart rate provoked by fetal scalp stimulation during a prolonged deceleration is of uncertain significance since the rise in heart rate is not by definition an acceleration.
The tracing below shows accelerations assocated with maternal pushing.


Although the accelerations in Figure 1  might be attributed to partial umbilical cord occlusion, fetuses typically show decelerations with pushing while mothers show accelerations in their heart rate when pushing. The heart rate could, therefore, be either fetal or maternal. In this situation (as well as in other situations where signal ambiguity might occur such as with a low baseline FHR or maternal tachycardia) placing a pulse oximeter on the mother,  and comparing the maternal heart rate (MHR) with FHR (Figure 2)  may help to clarify the source of the heart rate pattern being traced [7-9].


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