Intrauterine sound levels: intrapartum assessment with an intrauterine microphone.

The complex mechanisms responsible for fetal hearing are in place and functional by 26 weeks of gestation, but little is known about the acoustic milieu of the amniotic cavity. We placed an electrically isolated microphone in the uterus of nine term gravid volunteers after amniorrhexis. Baseline levels of intrauterine sound were 72 to 88 db. Transabdominal vibroacoustic stimulation with an artificial larynx produced peak mixed frequency sound levels of 91 to 111 db. We conclude that the term fetus in labor is exposed to physiologic sound levels higher than we had anticipated; the application of a quantifiable sound stimulus to the maternal abdominal wall results in a small increment in intrauterine sound; and within the limits specified, experimental fetal acoustic stimulation should pose no major risks.