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As printed in Hearing Health, volume 19:2,
Summer 2003
By Lynne Werner, Ph.D.
Parents and expectant parents may receive all manner
of advice about how to stimulate their infants, even
during pregnancy, to promote rapid development. Sound
is one of the few sorts of stimulation that can actually
be delivered in utero. As a result, there are plenty
of products that market the use of speech, music or
other sounds prenatally as beneficial to the auditory
system both during pregnancy and after birth.
But the quantity and type of sound that typically
reaches the fetus and its effect is not known. Our best
guess is that under normal circumstances, the mother
produces nearly all of the sound reaching the fetus,
including speech, because her body reduces sound from
the environment while it produces noises that mask others.
The human ear begins to function around the start of
the third trimester of pregnancy. It is several weeks
later, however, near the seventh month of pregnancy
before the ear is mature enough to respond to sounds
that are not very loud. Sound reaches the fetal inner
ear through the fluid and tissue that make up the womb
and the fetal skull, not air. Thus, even if intense
enough to reach the fetus and the ear is able to separate
it from background noise, speech or music will be different
in the womb.
Despite the limitations on hearing in utero, fetuses
hear maternal speech and remember something of what
is heard after they are born. We know this because we
can measure an infant’s responses to two different
human voices using a pacifier attached to a pressure-sensitive
gauge that indicates when the baby is sucking more in
response to one voice over another.
Studies using this or similar methods show that: a
baby will recognize Mom’s but not Dad’s
voice within a short time after birth; if a mother reads
a rhythmic story to her fetus in the last weeks of pregnancy,
her newborn will recognize the story even if another
woman reads it; and a newborn can discriminate between
spoken languages, for example French vs. English, as
long as their native language is one of them.
Does the fact that infants learn about sounds before
they are born mean that stimulating the fetus with more
sounds will be better for development? First, it is
clear that specific sorts of prenatal stimulation are
not necessary for normal development. Adopted newborns
can and do have strong emotional bonds with their new
parents and have no difficulty learning a language that
was not spoken by their birth mother. Second, there
is evidence from animal studies that suggests that stimulating
a fetus at a period of development when it would not
normally receive that stimulation actually retards some
aspects of postnatal development. Finally, studies claiming
to show that infants and children are more intelligent
or better adjusted if they are exposed to certain types
of music, i.e., “the Mozart effect,” have
not been substantiated by subsequent research. Likewise,
there are no data that show children who are deaf or
hard-of-hearing are at a disadvantage because they did
not receive in utero stimulation of this kind.
Perhaps the best conclusion we can draw from our studies
of fetal experience with sound and speech is that it
actually serves to fine-tune the basic circuitry of
the auditory system and makes infants more responsive
to speech after birth. You might say that it gets them
into a language-learning mode.
Lynne Werner, Ph.D., is professor
of speech and hearing sciences at the Univ. of Washington,
Seattle.
Related article:
The Ear In Utero,
An Engineering Masterpiece
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