Researchers think this may come from cues they have learned in prenatal life. Many studies now confirm that voices reach the womb, rather than being overwhelmed by the background noise created by the mother and placenta. Intonation patterns of pitch, stress, and rhythm, as well as music, reach the fetus without significant distortion. A mother's voice is particularly powerful because it is transmitted to the womb through her own body reaching the fetus in a stronger form than outside sounds. For a comprehensive review of fetal audition, see Busnell, Granier-Deferre, and Lecanuet 1992.
Some musical sounds can cause changes in metabolism.
Researchers in Belfast have demonstrated that reactive listening begins at 16 weeks g.a., two months sooner than other types of measurements indicated. This is especially significant because reactive listening begins eight weeks before the ear is structurally complete at about 24 weeks. These findings indicate the complexity of hearing, lending support to the idea that receptive hearing begins with the skin and skeletal framework, skin being a multireceptor organ integrating input from vibrations, thermo receptors, and pain receptors. This primal listening system is then amplified with vestibular and cochlear information as it becomes available. With responsive listening proven at 16 weeks, hearing is clearly a major information channel operating for about 24 weeks before birth.