Fetal heart rate response to acoustic stimulation in relation to fetal development and hearing impairment

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Abstract
In twelve healthy pregnant women the fetal heart rate response to sound stimulation was tested every second week between the 22nd and the 34th week of the pregnancy. Habituation to the acoustic stimuli used was also investigated at the last examination. A high risk material for hearing impairment comprised 31 pregnant women. The stimuli were presented by a vibrator placed on the maternal abdomen over the fetal head. As stimuli pure tones were used in the form of pulses of one second at a frequency of 3000 Hz and with an onset and a decay time chosen not to cause audible clicks to a normal ear. The intensity of the tone pulse was set at a level calculated to correspond at the position of the fetal head to 110 dB sound pressure level in free field. Roughly 50% of the fetuses tested started to react with an increased heart rate in the 24th week of the development. Four weeks later all tested subjects responded to the test stimuli. Habituation of the motor ‘startle’ response was shown but the fetus heart rate change was consistent over 20 consecutive stimulations one min in between each. In 31 high risk subjects the responses from three fetuses must be considered as pathologic. These three children exhibited impaired auditory tests after birth and severe hearing impairment at the age of three years. All the others disclosed normal hearing at birth as well as at three years of age. Repeated significant increases in fetal heart rate following acoustic stimulation may appear as an indication of normal function of the hearing organ. Several consecutive acoustic stimulations without any fetal responses in heartbeat rate is an indication of a possible hearing impairment.

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