Effects of low-frequency vibration on human term fetuses

Thirteen healthy pregnant women between 37 and 41 weeks' gestational age were studied to examine effects of a 5-second external vibratory stimulus (100 Hz, square wave) on fetal heart rate, fetal breathing, and gross fetal body movement patterns. All fetuses were stimulated during an episode of low fetal heart rate variability (mean minute range less than or equal to 32 milliseconds for greater than or equal to 5 minutes) lasting at least 5 minutes. There was an immediate and sustained increase in long-term FHR variability, number of FHR accelerations, and gross fetal body movements after stimulation. Fetuses made breathing movements more irregularly after vibratory stimulus. We hypothesize that external low-frequency vibratory stimulus, applied during episodes of low fetal heart rate variability, causes a change from a state of quiet sleep to a state of rapid-eye-movement sleep in healthy term fetuses.