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Sense of Hearing in the Womb
The womb is not a silent place. There's blood whooshing through the mother's vessels, gurgling and rumbling from her stomach and intestines, and the tones of her voice and the voices of others.

A baby's ears begin to form at around eight weeks and become structurally complete at about 24 weeks. But as early as 18 weeks, the bones of the inner ear and the nerve endings from the brain have developed enough for your baby to hear sounds such as your heartbeat and blood moving through the umbilical cord. He may even be startled by loud noises! For the rest of the pregnancy, sound serves as a baby's major information channel.

By week 25, your baby begins to hear your voice — and your partner's — and may even recognize those voices as early as week 27. Sounds may be muffled, though, because the ears are still covered with vernix, the thick waxy coating that protects the skin from becoming chapped by the amniotic fluid.

A fetus's movements or body patterns may change in response to sounds. Many pregnant women report a fetal jerk or sudden kick just after a door slams or a car backfires. A baby's heart rate often slows down when its mother is speaking, suggesting that he not only hears and recognizes the sound, but is calmed by it.



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Researchers cannot easily investigate language perception in the womb, however. So they study newborn babies' reactions to sounds that mimic the muffled language that penetrates the womb. In this technique, newborn babies listen to filtered recordings of a woman (the baby's mother or another mother) speaking, while sucking on a pacifier that is attached to a pressure transducer. Filtering erases the crisp edges of words, while leaving intact other features such as rhythm, melody, pitch, and intonation-- similar to what a fetus hears in the womb. "It's kind of like listening to a stereo next door," says William Fifer, an associate professor of developmental psychobiology at Columbia University. "You hear a lot of bass, but not the crisp, clear high frequencies."

Using this technique, Fifer and his colleagues found that newborns suck harder on the pacifier when listening to filtered recordings of their own mother's voice in comparison to another mother's. The newborns thus recognize and prefer their own mother's voice, concludes Fifer.

In further studies, Jusczyk and postdoc Thierry Nazzi found that newborns prefer filtered recordings of their own native language over that of a foreign language. Babies like what they know, says Jusczyk. Newborns, he says, apparently learn the rhythm of their native language and of their mother's voice while in the womb.



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