Abdominal vibration alters sleep state in fetal sheep.

Many pregnant women are exposed to low frequency sounds and vibrations while at work or play. How the fetus is affected by these physical stimuli is not clearly documented. We recorded behavioral state and intrauterine sound pressure levels in eight fetal sheep previously instrumented with electrocortical, electroocular and neck electromyographic leads, and with a miniature hydrophone. Data were collected before, during and after 30 min of abdominal vibration using a belt vibrator commonly used by humans in weight reduction programs. A sudden shift in behavioral state occurred at the onset of vibration. There was a decrease in non-rapid eye movement sleep (P < 0.02) and an increase in time during which the sleep state could not be determined (P < 0.03). A 63% increase in number of epochs spent in this indeterminate period was evident during vibration. During the post-stimulus period, percentage of time spent in rapid eye movement sleep decreased as compared to the pre-stimulus period (P < 0.04), and non-rapid eye movement sleep increased as compared to the stimulus period (P < 0.01). Vibration-induced intrauterine sound pressures ranged from 131-142 dB at the fundamental frequency of 19 Hz and were 20-40 dB lower at the overtones which appeared in the spectrum between 35-200 Hz. Results indicated that environmental vibration can disrupt fetal behavioral state, induce abnormal state changes, and alter distribution of sleep states.