the world and the love of parents and caretakers. Nature and nurture go hand in hand as each sensory interaction adds to the wiring.9 The number of synapses skyrockets during the first three months and beyond, for as long as three years. At birth, an infant has about twenty-five hundred synapses per neuron. By three she has about fifteen thousand synapses per neuron, or some 1,000 trillion synapses—twice the number of an adult brain.10
It's too many, and the brain knows it, as it kick-starts a use-it-or-lose-it mechanism, a lifelong process that begins during the fourth trimester even as new connections are being made. Synapses are refined and pruned to eliminate those brain connections that are not used, and to favor those that get used frequently.11 Coo, cuddle, and comfort a baby, and the synapses responding to loving behavior will endure. Scream, neglect, or strike a baby—events that are read by the brain as toxic stress—and the synapses responding to cruelty and violence will take hold. The brain pathways that are repeatedly used, even as early as the fourth trimester, are protected.
Caregivers’ every interaction serves to support the scaffolding for infants’ developing brains, part of the crucial postfetal development period that acts as a transition in getting them ready for the world. The earliest games of peekaboo form neural connections for vision as faces come close to infant faces and then disappear. The first hushed baby-talk messages begin to wire young brains for the sounds of language, specifically their own native language. Each new neural structure allows for newer layers of increasingly complex structures. Parental games, lullabies, verbal patter, and comforting touches all cause the newborn's brain to vigorously form the connections that in turn increase the number of complex links needed for passing electrochemical messages from brain neighborhood to brain neighborhood.12 All of this biological activity mingles with every sound, touch, sight, taste, and smell that mothers, fathers, and caretakers provide. And since the environment is different for every infant, each newborn begins to be transformed into the irreplaceable baby parents have been waiting for.
THE CHANGE FROM STRANGE NEONATE TO ONE-OF-A-KIND BABY
The change from the newborn that a mother first held in the hospital, or the infant that was first handed to an adoptive parent, to the child that is a unique part of the family doesn't happen in the delivery room. It begins to happen during the outside-the-uterus fourth trimester of development as worldly experiences shape the developing brain. What for nine months was largely under the purview of evolution and genetics now partners increasingly with culture and environment. Brain development becomes a product of a delicate balance between nature and nurture, genes and environment. Most scientists agree that the nature/nurture debate is over, and it's a tie, with each influencing the other. Genetic predispositions, while influencing brain growth, don't altogether dictate it. Non-genetic influences—neighborhood, parents, siblings, extended family, peers, school, and nutrition—are important in shaping who this special infant will become. Both nature and nurture are important.
When a mother cuddles an infant, she affects the formation of neural connections. When a father hums a lullaby, the infant's brain responds by retaining the cells that feel the pleasure of the sound. Touching, comforting, rocking, talking, and singing to babies provide exactly what they need to stimulate their growing brains. As the baby is exposed to her unique surroundings, a remarkable thing happens. The brain activity resulting from environmental influences causes synaptic connections—neuron to neuron—to get stronger. The next time she's exposed to a similar influence, her brain cells respond more quickly and strongly. Meanwhile, those connections that aren't needed fall away. This use-it-or-lose-it model is the basis for each infant's growing individuality.
THE NEWBORN IS PREPARED
With a brain only about one-fourth ready, babies land right smack in the middle of a chaotic and messy real world. The soothing things the growing fetus had in the womb—the peace to sleep, a controlled space for exploring her own movements, the comforting external movements of her mother, the familiar muffled sounds of the household—have been abruptly snatched away. Parents and caregivers help with the transition by paying close attention to comfort. But modern science tells us that, even though the world is confusing to newborns, they've got amazing devices with which to begin sorting it all out, right from the very start.
Despite the newborn's extreme immaturity, he is well prepared. He has at his disposal an arsenal of tools for himself; and some he'll find himself using in response to signals from mother, father, or caregivers.
Survival for an infant in the fourth trimester means being constantly close to a nurturing caregiver—to the soothing touch, sound, odor, and radiated warmth provided by someone who loves and pays close attention. Newborns are naturally built and equipped by evolution to prefer their mothers, though adopted infants have proven that their allegiance changes when it must. That closeness is a vital part of the transition from womb to world. Human babies pick up on movement patterns, breathing sounds, and body heat, all of which begin to regulate hormonal releases—melatonin to help manage the sleep-wake cycle and body temperature, and cortisol to regulate blood pressure, blood sugar, and immune response.
The kinds of behaviors that come naturally to parents and caregivers around the world are just what the baby needs. Rubbing and massaging her back, stomach, or legs keeps the infant warm; stimulates respiration, digestion, and elimination; and calms her down. Mothers naturally hold their babies most often on the left side of their bodies, and babies love feeling the soothing heartbeat. Mothers, fathers, and almost all adults talk in high-pitched voices when they speak to babies, and they look their babies in the eye. They've been doing these things for millions of years—exactly the things that newborns crave.
Just as the colt is born ready to stand, a human baby is born ready to recognize another human face, the smell of her mother's milk, and the familiar sound of her voice. It's precisely because human babies are so extremely neurologically immature at birth that they are exquisitely responsive to the body cues of adults, even to the point of matching the rhythm of breathing when they rest on a person's chest. Fetal life has prepared the newborn to recognize these cues from another loving body, and the familiarity helps to ease the transition of the fourth trimester. Babies have been responding to those instinctive touches, smells, and sounds since the first human put one foot in front of the other.
“We are all preemies at birth, relative to other primates. The baby is highly sensitized to gases the mother gives off,” says Dr. James McKenna, anthropologist and director of the Mother-Baby Behavioral Sleep Laboratory at the University of Notre Dame. “Every baby in the world—put them next to their mothers and they all do the same thing. They root. They breathe differently. The baby is waiting to respond to these kinds of things. They have come off a long evolutionary tree, and they know what to do.”13
Evolution, biology, genetics, and the environment all help to fashion one special baby, far better than anything parents might have imagined. But the deep well of parental love won't be returned in kind. Not yet. Babies need that love, can't thrive without it; but at first, it's all an infant can do to handle the new work of eating, breathing, and regulating her own heartbeat and digestion. She's not yet ready to show any signs of returning the outpouring of love. It can seem like unrequited love, but the demands and frustrations of the first months do not represent a failure of parenting. It's not personal. It's simply biology. Parents have waited for a baby, and they've been handed a mysterious, not-fully-formed neonate. Patience. The baby's brain, from the moment of birth, is beginning to mature, to figure out sleeping, seeing, hearing. It's part of the dance of life—her cries, grimaces, and involuntary smiles encouraging a parental response and paving the way for a two-way attachment.14 In time, she'll begin to respond. And one day soon, she'll smile, a reward making it all worthwhile.
WHETHER THEY KNOW IT OR NOT, PARENTS ARE PREPARED
Sometimes we describe newborns as “half-baked” or “almost finished.” In many ways this is true. Fortunately, nature, evolution, and three trimesters in the womb have prepared
