Although a concentric series of barriers buffer the fetus from the outside world--amniotic fluid, embryonic membranes, uterus, and the maternal abdomen--the fetus lives in a stimulating matrix of sound, vibration, and motion. 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 Busnel, Granier-Deferre, and Lecanuet 1992.
Sounds have a surprising impact upon the fetal heart rate: a five second stimulus can cause changes in heart rate and movement which last up to an hour. Some musical sounds can cause changes in metabolism. "Brahm's Lullabye," for example, played six times a day for five minutes in a premature baby nursery produced faster weight gain than voice sounds played on the same schedule (Chapman, 1975).
Researchers in Belfast have demonstrated that reactive listening begins at 16 weeks g.a., two months sooner than other types of measurements indicated. Working with 400 fetuses, researchers in Belfast beamed a pure pulse sound at 250-500 Hz and found behavioral responses at 16 weeks g.a.--clearly seen via ultrasound (Shahidullah and Hepper, 1992). 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.
Talk, sing and read to your baby. You may find in a few more weeks that (s)he responds differently to mom's voice and dad's voice. (S)he will recognize your voices at birth, and turn toward you in preference to a stranger.
(It's what I do for a living)