Development of Hearing in a Human Fetus
Some mothers-to-be surround their babies with Mozart streaming from headphones strapped around their bellies trying to foster an appreciation of classical music while the child is still in the womb. Yet, others laugh and dismiss this idea as nonsense. How well can a fetus hear the noises around it? Developmental researches have conducted some sophisticated studies in order to develop a timeline of hearing development in the womb.
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Formation of the Ears
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The most important requirement for fetal hearing is the ears. The ears of a fetus develop during the fetal period---more specifically, between the 13th and 16th week after conception. If the mother experiences any terotogens (toxins such as drugs, alcohol, infections, or chemicals) during this period, they might adversely affect the baby's hearing.
First Response to Sound
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A 1998 study demonstrated that five to six months after gestation, the fetus will first respond to sounds. Researchers can detect these responses through changes in the baby's heart rate. At this point, sounds of medium intensity provoke a change in the heart rate. Other studies have indicated that a fetus can hear a tone of 500 Hz as early as the 19th week.
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Sounds in the Womb
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Others studies done in 1996 and 2001 used tiny microphones placed in the uterus near the fetus' head to describe the sound environment there. These studies found that the usual sound level in the womb is about 75 decibels. This is what you would hear if you were listening to outside sounds while driving in the car with the windows rolled down. The fetus also hears the sounds of air going through the mother's stomach and her heartbeat.
Mother's Voice
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The fetus hears it's mothers voice better than all of these other sounds because her voice is transmitted through vibrations in her body. Developmental researchers have conducted several interesting experiments on fetuses and the effect of their mothers voices. In 1995, for instance, Fifer and Moon demonstrated that newborns not only prefer the sound of their mothers' voices, they prefer the voices passed through a noise filter designed to make them sound like they would in the womb over their mothers' normal voices.
Cat-in-the-Hat Study
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In another famous study, DeCasper and Spence used a sucking apparatus to detect changes in a baby's sucking rates while listening to different sounds. The researchers had 16 pregnant women read a passage from "The Cat in the Hat" out loud for a total of 3.5 hours while they were pregnant. When their babies were two to three days old, they responded more favorably (sucked at a higher rate) to "The Cat in the Hat" than to other stories. This demonstrates that a fetus not only has highly developed hearing in the womb, but already demonstrates auditory preference.
Weeks Before Birth
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These hearing preferences and abilities on the part of the fetus become more prominent in the weeks before birth. By this time, the fetus will respond to small differences in musical notes and prefer its native language.
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