TUESDAY, Feb. 10, 2004 (HealthDayNews) -- Human fetuses can recognize their mothers' voices and react to music, studies show. But what do unborn babies actually hear from inside the womb?

A new animal experiment may provide a clue. University of Florida researchers implanted a tiny electronic device in the inner ear of a pregnant ewe's fetus to capture the signals the ear sends to the brain.

Sound passing through a pregnant sheep's uterus to her fetus is believed to roughly mimic the sound transmitted through a woman's womb to her unborn child, explains study leader Kenneth J. Gerhardt, a professor of communication sciences and disorders.

Assuming similar acoustic environments, the findings suggest fetuses hear mostly low-frequency sounds, such as vowel sounds. High-pitch sounds, such as consonants, are much more difficult to hear.

"What we think the human fetus is probably detecting would be the rhythm or melody of speech," Gerhardt says.

Fetuses probably hear their mothers' voices but are not likely to detect differences in words, he says.

As for music, "They won't hear violins, they won't hear high notes on a piano, but they will hear the bass."

The study appears in a recent issue of Audiology and Neuro Otology.

The researchers tested fetal hearing by playing sentences to a pregnant ewe and making recordings from microphones placed near the mother's abdomen, in her uterus, at the head of the fetus and within the fetal inner ear.

Next, human volunteers who were seated at computer stations listened to the recordings and typed in their responses to fill-in-the-blank questions based on what they heard. The questions were designed to test the intelligibility of the recorded sentences.

Intelligibility scores were about 26 percent lower for speech recorded from the fetal inner ear compared with recordings outside of the mother, near her abdomen.

In a statement, Anthony DeCasper, a professor of developmental psychology at the University of North Carolina-Greensboro, says the findings are noteworthy because the sounds were recorded as a sheep fetus would hear them. The results bolster what researchers have previously concluded about what the human fetus may hear.

"The way I put it is, the way the mother's voice would sound in utero would be like Lauren Bacall speaking from behind a heavy curtain," he says.

Janet DiPietro, a developmental psychologist and professor of population and family health sciences at Johns Hopkins Bloomberg School of Public Health, says the fetus actually gets a double dose of mother's voice: through sounds picked up from the air when she speaks and through vibrations in the body produced by her vocal cords.

Sheep don't talk, so what this study doesn't show is how distinguishable the mother's spoken word is when it is transmitted through the body, she notes.

What a fetus hears is particularly relevant to the current debate about playing music to unborn children. The theory that it will boost a child's intelligence has fueled a market for prenatal CDs and speakers to be placed on the mom's abdomen.

"There are lots of reasons not to do that," DiPietro insists. First, there is no research to show that it increases IQ. In addition, it can disrupt the unborn child's sleep. What's more, people tend to turn up the volume, creating a concern for the fetus' safety, she explains.

As Gerhardt sees it, "The normal daily lives of the mother and family are probably more important than artificially stimulating the fetus with either music or speech," he reasons.

More information

Learn more about babies, children and language development at the University of Wisconsin or Dartmouth College.

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