Fetal Heart Rate Testing: Periodic Changes

Introduction

Description: Periodic changes in the fetal heart rate in conjunction with uterine contractions may occur. These may indicate fetal stress when they are persistent or become progressively deeper or longer lasting. Recurrent decelerations are defined as occurring with 50% or more of contractions during a 20-minute period. In the United States, decelerations in the fetal heart rate are classified by their relationship to uterine activity: early, late, and variable. The shape of the deceleration is also significant in the classification. Accelerations higher than the baseline often accompany fetal movement and are reassuring.
Prevalence: Mild and transient periodic decelerations are not uncommon during the course of normal labor. Accelerations are documented in virtually all normal labors.
Predominant Age: Reproductive age.
Genetics: No genetic pattern.

Accelerations
- Physiologic response to fetal activity or external stimuli (acoustic stimulation, scalp stimulation). Compensatory accelerations also occur following variable decelerations. These changes reflect an intact neurohormonal cardiovascular control system
Early decelerations (mirror the timing, and sometime the amplitude, of the contraction wave)
- Physiologic response to head compression; dural stimulation mediated via the vagus nerve (“diving reflex”). These changes are not associated with hypoxia, acidemia, or low Apgar scores
Variable decelerations (variable relationship to the contraction wave)
- Compensatory response to intermittent obstruction of umbilical blood flow
Late decelerations (begin well after the uterine contraction wave and persist after it begin resolution)
- Decreased fetal oxygenation with reflex bradycardia or myocardial depression. This type of deceleration suggests the greatest fetal stress despite the relatively modest change in heart rate
Risk Factors: Early—occiput posterior position, cephalopelvic disproportion. Variable—low amniotic fluid volume, cord prolapse, abnormal lie. Late—placental aging, reduced placental perfusion (maternal disease, vascular spasm, medications, partial placental separation).

Accelerations
- Abrupt increase in fetal heart rate that reaches a maximum within 30 seconds.
Early decelerations
- Shallow U-shaped, with gradual onset and resolution, generally (10–30 beats/min), that reaches a nadir at the peak of uterine activity; rarely associated with heart rates below 100–110 beats/min.
Variable decelerations
- Slowing with abrupt onset and return, frequently associated with accelerations before, after, or both; variable in depth and duration but coincide with the compression of the umbilical cord during contraction.
Late decelerations
- U-shaped, with gradual onset and resolution, generally shallow (10–30 beats/min), and reaches a nadir after the peak of uterine activity; often associated with decreased variability. Generally, not diagnosed unless present with more than 50% of contractions.

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