The intrauterine sound environment of the human fetus during labor.

Ten pregnant women near term with medical indications for internal monitoring during labor were studied to quantify the intrauterine sound environment. The intrauterine background noise was measured with a miniaturized hydrophone placed transcervically at the level of the fetal neck under ultrasonographic guidance. In eight women no cardiovascular noise was audible and the intrauterine noise consisted predominantly of low-frequency noise (less than 100 Hz) from 60 to 85 dB. Above 100 Hz, the sound pressure level was less than 60 dB, decreasing to 40 dB above 500 Hz. In only two women maternal cardiovascular sounds were audible during uterine relaxation and disappeared during uterine contraction. Maternal bowel sounds and maternal vocalization were well above the intrauterine background noise. Because of this relative quietness, we hypothesized that the fetal acoustic environment could be altered significantly during antenatal vibratory acoustic stimulation.