Human fetal responses to sound as a function of stimulus intensity

The effect of stimulus intensity on fetal heart rate (FHR) acceleration and body movement responses was evaluated in term fetuses in a two-stage study. During the first stage (N = 30), a pink noise played at 110 or 105 dB elicited a greater mean peak FHR acceleration than when played at 100 dB. Movement scores indicated that the 110-dB noise elicited more movements than the 105-dB noise which, in turn, elicited more than the 100-dB noise. The FHR acceleration and movement responses elicited on 100-dB sound trials could not be differentiated from spontaneous activity on no-sound control trials. During the second stage (N = 93), the 110-dB pink noise elicited a mean peak FHR acceleration of 14 beats per minute on the first stimulus presentation, replicating the findings of the first stage. Furthermore, the percentage of 110-dB stimulus/control trials in which there was an FHR acceleration greater than ten beats per minute (stage 1: 57%/10%; stage 2: 58%/24%) and movement response (stage 1: 50%/7%; stage 2: 52%/7%) was virtually identical in the two stages. We conclude that fetal response varies as a function of stimulus intensity and that the threshold for response to an air-borne pink noise is between 100-105 dB. These findings have implications for stimulus selection and control procedures in acoustic stimulation testing.