Decelerative cardiac responsiveness to acoustical stimulation in the near term fetus.

Human fetal cardiac responses (36-39 weeks gestational age) to brief, repeated vocal stimuli (male or female voice uttering the same sentence), given at 90-95 dB SPL ex utero (around 20-30 dB less in utero) during a state of low fetal heart rate (FHR) variability, were examined using highly conservative statistical criteria taking into account each subject's prestimulus FHR variability. Subjects exposed to either of the two stimuli displayed significantly more decelerative heart rate (HR) changes compared to control subjects receiving no stimulation. The decelerative changes started during the first seconds following the onset of stimulation and reached their amplitude peak within 10 or 20 sec, depending on the subject. The direction--HR acceleration or deceleration--and the amplitude of the response depended on prestimulus HR variability only, not on prestimulus level. No major difference was found between the effects of the two voices. The data are compared to previous studies demonstrating fetal decelerative changes to acoustic stimuli of less than 105 dB SPL. The choice of an objective criterion to define an HR response and the possible orienting response nature of the decelerative change are discussed.