Sound stimulation available to the sheep foetus

Work on the uterine sound environment has been carried out by implanting a hydrophone on the neck of the foetus in two pregnant ewes a few weeks before term. Stimulation with sounds of known amplitude and the use of calibrated microphones and a frequency analyser enabled us to assess (1) the amount of attenuation of sounds of different frequency when they had passed through the maternal tissues and amniotic fluid to become available to the foetus, (2) the absolute level of sound entering the amnion from within the mother and (3) the recognizability of external sound such as speech, when recorded from within the uterus. One puzzling feature of the results has been our inability to hear those sounds which are commonly believed to constitute a predominant stimulus for the foetus: those produced by the maternal cardiovascular system.