DEVELOPMENT OF FETAL HEARING

Previous research has revealed that the human fetus responds to sound, but to date there has been little systematic investigation of the dev elopment of fetal hearing. The development of fetal behavioural respon siveness to pure tone auditory stimuli (100 Hz, 250 Hz, 500 Hz, 1000 H z, and 3000 Hz) was examined from 19 to 35 weeks of gestational age. S timuli were presented by a loudspeaker placed on the maternal abdomen and the fetus's response, a movement, recorded by ultrasound. The fetu s responded first to the 500 Hz tone, where the first response was obs erved at 19 weeks of gestational age. The range of frequencies respond ed to expanded first downwards to lower frequencies, 100 Hz and 250 Hz , and then upwards to higher frequencies, 1000 Hz and 3000 Hz. At 27 w eeks of gestational age, 96% of fetuses responded to the 250 Hz and 50 0 Hz tones but none responded to the 1000 Hz and 3000 Hz tones. Respon siveness to 1000 Hz and 3000 Hz tones was observed in all fetuses at 3 3 and 35 weeks of gestational age, respectively. For all frequencies t here was a large decrease (20-30 dB) in the intensity level required t o elicit a response as the fetus matured. The observed pattern of beha vioural responsiveness reflects underlying maturation of the auditory system. The sensitivity of the fetus to sounds in the low frequency ra nge may promote language acquisition and result in increased susceptib ility to auditory system damage arising from exposure to intense low f requency sounds.