Fetuses hear drums, not violins, study finds

Unborn children hear low pitches


Published: Friday, January 23, 2004 at 6:01 a.m.
Last Modified: Thursday, January 22, 2004 at 11:43 p.m.
Just what does an unborn child hear? A new University of Florida study indicates that high-frequency sounds aren't heard by the fetus, but a low-pitched, melodic sound can be music to the fetal ear.
UF researchers performed a series of experiments in a pregnant ewe to record exactly what sounds reached the ear of the fetus.
The results confirm previous findings that an unborn child is most likely to hear low-frequency sounds.
That means they hear vowels rather than consonants and are more sensitive to the melodic parts of speech than to pitch, according to Ken Gerhardt, a UF professor of communication sciences and disorders.
As for music, he said, "They're not going to hear violins, but they will hear the drums."
Gerhardt, who is also an associate dean of the graduate school, led the study reported in the journal Audiology and Neuro-Otology.
Gerhardt began the studies of fetal hearing in response to inquiries from law enforcement agencies and the U.S. Navy. The organizations were concerned that the noise of gunfire or the rumble of a ship's engine could affect the hearing of babies being carried by pregnant women in their service.
Gerhardt and Robert Abrams, UF professor emeritus of obstetrics and gynecology, chose sheep for their experiments because earlier research had shown that properties of sound transmission in sheep and pregnant women were similar.
Their experiments indicate that the mother's womb will dampen all but the loudest sounds. Even a high-decibel rock concert is probably not loud enough to threaten fetal hearing development, according to Gerhardt.
The pair implanted a tiny electronic "pickup" inside the inner ear of a fetal lamb, then played 64 recorded sentences on a loudspeaker placed near the mother sheep in open air. Another microphone was placed inside the uterus of the ewe, where it was expected that its mother's movement, breathing and digestive processes would garble whatever the fetus would "hear."
In fact, because the ears of an unborn child are filled with fluid, much of the outside noise is transmitted to their inner ear through vibrations in the skull.
When the recorded sounds were played back to 30 human listeners to determine how much the fetus actually heard, the listeners understood all of the sentences recorded in the open air, about 70 percent of what was recorded in the womb and about 30 percent of the sentences recorded in the fetal sheep's inner ear.
Anthony DeCasper, a professor of developmental psychology at the University of North Carolina-Greensboro, said the results of the new study are noteworthy because they were obtained from the inner ear, which probably registered sounds exactly as the sheep fetus would hear them.
"The way I put it is, the way a mother's voice would sound in utero would be like Lauren Bacall speaking from behind a heavy curtain," DeCasper said.
The research could have implications for the care of premature babies, Gerhardt and DeCasper agree. Such studies raise awareness - and questions - about how the sounds premature babies are exposed to in the neonatal intensive care unit influence their growth and development.
Diane Chun can be reached at 374-5041 or chund@gvillesun.com.

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