Special Issue: How Kids Grow Do You Hear What I Hear?

She slides into the world with eyes alert, the tiny ridges of her ears living antennae scanning the conversation frequencies in the room. She finds her mother's voice with her ears, and then her eyes. Her cheek feels the sweaty chest, she hears the calming heartbeat that has been her Muzak for nine months, she turns her face, and sucks. Surely the infant could not have learned to recognize her mother's voice, resonate to her heartbeat, and find food in the few short minutes since birth?

Surely not. Once scientists discovered over the last 25 years that newborns can learn and understand, remember and recognize, it was only a short leap to asking when and how those talents bloomed in utero. Life in the womb represents the next frontier for studies of human development, and the early explorations of that frontier--through ultrasound, fiber-optic cameras, miniature microphones--have yielded startling discoveries. Scientists have found hints of consciousness in 7-month-old fetuses and measured brain-wave patterns like those during dreaming in 8-month-olds. They have pushed sentience back to the end of the second trimester and shown that fetuses can learn. Some research has been sensationalized and used to support prenatal "universities" that purport to teach fetuses words, numbers and letters. That is doubly unfortunate, for with no hype at all the fetus can rightly be called a marvel of cognition, consciousness and sentience.

After 28 weeks in utero, the fetus can hear - the rumbling of the mother's intestines and stomach, the whoosh of blood through her arteries. Walls of fat and muscle between fetus and outside world cut the volume by about 30 decibels, says Dr. Denis Querleu of the Hospital of Roubaix, France, who placed a tiny microphone in the uterus. Bass sounds penetrate better than treble, with the result that "it sounds like Lauren Bacall talking from behind a heavy curtain," says psychologist Anthony J. DeCasper of the University of North Carolina. "But the melody of language is still conveyed almost intact." By the third trimester, the fetus can respond to sound. Car horns can make a fetus jump and its heartbeat, pulsing since the fourth week, quicken. Pregnant women have had to flee fortissimi concerts to calm their kicking, punching passengers. "But if we repeat the loud noise many times, there is less cardiac reaction," finds Querleu.

That lack of response, called habituation, is primitive learning. Sea slugs are quite adept at it. Still, it shows that a fetus is retaining the memory of experience and altering its behavior as a result. Some memories inscribed in the brain can even be retrieved after birth. When Peter Hepper of Queen's University in Belfast repeatedly played to 30-week-old fetuses the theme song from a popular soap opera, they relaxed. After birth the babies became "quite alert" when they heard the tune, says Hepper. Nor is hearing limited to music. When DeCasper and associate William Fifer rigged up a nipple and a two-track tape recorder so that one pattern of sucking played the mother's voice while another rhythm produced a stranger's, eight out of 10 newborns sucked to hear Mom.

Voices sound so different in the womb that the baby is almost surely remembering not pitch or intonation but rhythm. When DeCasper and associate Melanie Spence had 16 pregnant women read aloud "The Cat in the Hat" to their abdomens twice a day for the last six and a half weeks of pregnancy, the babies later modified their sucking patterns to make the tape machine play the Dr. Seuss tale, not a verse with a different meter. Says DeCasper, "It's the first direct demonstration that human speech has a discernible effect on the fetus."

Such findings are music to the ears of companies selling "prelearning" tapes and prenatal "universities." In a typical program, expectant parents intone numbers, letters, words and colors like mantras. Although anecdotal reports from delighted parents tell of 1-year-olds who play piano and 14-year-olds with IQs above 235, researchers are skeptical. Says DeCasper, "The promises of extra prenatal stimulation are simply guesses stemming from the unwarranted generalization of selected findings, ill-conceived theories of human development, faulty logic, anecdotal information and a dose of hubris."

Even in its cramped chamber, the fetus can perform like an acrobat. It begins to move spontaneously at seven weeks, to open and close its mouth by the 11th week, to grasp its hands at 12 weeks, to frown and squint and grimace and suck its thumb by the fourth or fifth month. It moves in response "to touch, sound and light, and is becoming more attuned to the world," says psychologist Darwin W. Muir of Queen's University in Ontario. At 12 to 16 weeks, if its foot or hand grazes its body, it responds by jerking back in an uncoordinated way; by 24 weeks, as its nervous system becomes connected, it moves only what was touched. Whether the fetus is tranquil or restless may correlate with its future personality: anecdotal evidence hints that extremely active fetuses will be unusually anxious children. Whatever makes them respond to sound or motion with violent somersaulting rather than a gentle kick may also shape them into shy, fretful children, withdrawn from a world of unwanted stimuli.

By six weeks, the brain is visible and electrically active; by eight, it has the convoluted folds and shape of an adult brain. About 100,000 nerve cells sprout every minute until, by birth, there are 1 billion. Mother's movements stimulate the fetus's balance and motion detectors. Babies deprived of this movement, as when a high-risk woman is confined to bed, may lag behind in sensory-motor development. "Neural pathways that the child will use to think and remember are being laid down in the womb," says Dr. Mortimer Rosen of Columbia University College of Physicians and Surgeons. By the start of the last trimester the brain's neural circuits are as advanced as a newborn's, capable of paying attention and discriminating new from old. When a loudspeaker directs speech syllables at a mother-to-be's abdomen, the fetus's heart slows, a sign of attentiveness. The heartbeat speeds up as the fetus gets bored with the sounds, then slows again if new ones flow into the womb.

Light penetrates the womb, but dimly: the brightest is toned down to a diffuse orange glow. A midterm fetus will try to shield its closed eyes from the shining of a probe. The eyes open at about the 26th week, and from then on will open and close as the fetus sleeps and wakens. After 32 weeks, the fetus spends half its time in REM sleep, the brain state associated with dreaming. If the fetus feels a contraction of the uterus, or any other stimulus, it can be disturbed from REM and enter a period of quiet sleep, its brain activity dampened. In this way, says Dr. Peter Nathanielsz of Cornell University, "these constantly changing features of the fetal environment may play an important role in brain development."

Perhaps the strongest means of communication between mother and fetus is hormonal. Stress hormones cross the placenta and "may affect fetal development, the level of excitability and brain development," says Nathanielsz. They could even be responsible for effects that mothers, and researchers, attribute to sound: if Mom hears a jackhammer, her adrenaline levels may surge. That will be transmitted to the fetus, whose somersaults can then be blamed not on the noise but on the chemical. Because the fetal brain is immature, little short of starvation or exposure to toxic substances is likely to leave an indelible mark.

After nine months, the fetus speaks back. Research on sheep suggests that "the fetus plays a major role in controlling the onset of labor," says Nathanielsz. In response to signals from its brain, the fetus's pituitary gland pumps out the hormone ACTH, which stimulates its adrenal cortex to grow and secrete a hormone which, after several steps, stimulates the uterus to contract. Labor begins. The fetus is about to move from its twilit antechamber into the glare of the outside world.

the embryo, about 1.5 inches long, has all its organs in place and is now called a fetus. Its heart has been beating for a month; limbs, hands and feet have taken shape. A week before, neural cells in the brain began to connect. The fetus moves.

the fetus frowns, moves its lips, turns its head. Its hands grasp, its feet kick. The first signs of hair have appeared. In a female fetus, all 5 million ova have formed.

the fetus has a chance for survival if it is born prematurely. Its eyes have opened and can perceive light. It processes and responds to sound. In a month, its brain will have as many cells as it will have at birth.

the cerebral cortex is well defined. Brain waves have developed patterns like those seen when a newborn sleeps, wakes and dreams.