Response of fetal heart rate to uterine contractions

The heart rate variability of fetuses under stress from maternal uterine contractions conveys critical information to clinicians and also provides theoretical clues about heart rate regulatory mechanisms. According to the polyvagal theory, the deceleration of fetal heart rate under stress is caused by the withdrawal of vagal tone. Recovery is mediated by its reestablishment. An implication of this mechanism is that the respiratory sinus arrhythmia (RSA) is elevated after the deceleration and subsequent recovery. Findings from our clinical data on fetuses support this theory. The data also indicate that in the instance of a late deceleration the RSA may not be elevated after recovery.