The Fourth Trimester

Understanding, Protecting, and Nurturing an Infant through the First Three Months

UNIVERSITY OF CALIFORNIA PRESS

Contents

Evolution and the Primitive Brain of a Newborn

Why Infants Arrive Unfinished

Blame Lucy. In the throes of labor contractions and delivery, remember that it was this 3.2-million-year-old human ancestor who first had the big idea to stand up and walk on two feet. Lucy is considered by many scientists to be the mother of humankind, and her skeletal remains, discovered in 1974, provided scientific evidence of one of the first upright walkers in our family tree. From that point onward, the human ability to walk on two feet would demand some reworking of the adult pelvis and a major overhaul of the birth canal. Those evolving alterations would, in all descendant female hominids leading up to Homo sapiens, introduce a host of inefficient twists and turns, making the process long and painful for mothers and a brutal challenge for babies.

Lucy's hypothetical offspring, no longer able to survive with the limited brainpower required to climb a tree or flee from danger on four legs, needed more time to grow a bigger brain in order to outwit predators. But a maximum of forty weeks' gestation is all that biology allowed our ancient ancestors and modern babies. They would need a fourth trimester of intense development experienced outside the uterus while remaining physically and emotionally bound to their mothers, just as they had been for the previous nine months. The tight fit through that circuitous birth canal set absolute limits on how much brain development could occur during pregnancy. The additional brain growth required to keep the species thriving would have to happen outside the uterus, an astonishing amount of it occurring during the fourth trimester.

Modern human infants are at the receiving end of millions of years of evolutionary progress, but the tradeoff for upright walking has been immature brain development at birth.

Walking on two feet has been a mixed blessing. Standing upright altered the entire skeletal structure, and the changes came with physical costs for males and females: flat feet, aching backs, and stiff necks. "Ultimately, every part of the human body had to change to adapt to bipedalism," says Dr. Wenda Trevathan, evolutionary anthropologist at the University of New Mexico.

Women and infants have suffered especially onerous consequences of upright posture. What was once a straight shot down a roomy birth canal in our four-legged ancestors has evolved into something akin to an amusement park ride through the modern female pelvis.

Once humans had only two feet for mobility, they could no longer climb as well as, or run as fast as, the local lions and tigers and bears. Since then, they've had to rely on brainpower to escape predators. The pressure for increased intelligence was on, and our ancient forebears began to grow brains far larger than ever before. In fact, the human brain had to get bigger, or our species would have died out. Those outsized brains, and resulting intelligence, began to change the world.

As the human brain has grown in size during evolution, the additional brain growth needed for survival has had to take place after birth because there simply is no extra room in the birth canal for a bigger head. After roughly nine months in the uterus, emerge babies must, ready or not. Most aspects of brain development are delayed until after birth. "And that means the baby is a little more unfinished, if you will," says Dr. Trevathan. "The evolutionary compromise is that about 75 percent of human brain development takes place after birth." That's in contrast with the rest of the animal kingdom. Most animals are born with their brains about half developed, but today human infants are born with only 26 percent to 29 percent of their brains developed. And so, with a brain only about a quarter of its necessary size, the newborn needs a fourth trimester of development, with comforts similar to those enjoyed in the womb preparing him for life in the world.

THE BIRTH RIDE

The evolutionary compromise between the need for a large brain and the confines of a narrow birth canal continues with modern-day infants. They arrive extremely neurologically immature and completely dependent on adults.

Leading, in most cases, with a head that can accommodate roughly a quarter of the brain mass she'll eventually require, the fetus is forced to negotiate a series of turns aligned with the widest parts of the pelvis. The entrance of the birth canal is widest from side to side. About halfway through, the orientation shifts about ninety degrees, and the fetus must turn her large head to make it through. So the infant starts her journey facing her mother's side. Midway she must shift her head to face her mother's back. As the fetus's head turns from facing her mother's side to facing her back, she goes through a series of rotations as she passes through the birth canal. Once the head has emerged, the shoulders must shift, so the baby turns her head to the side, rotating her shoulders so they, too, can make the tight squeeze between pubic bones and tailbone.

The average infant head is ten centimeters from front to back. It's little wonder that childbirth hurts, considering that the average woman's pelvic opening is thirteen centimeters at its largest point and ten centimeters at its smallest point.

The quadruped ancestors of modern humans, with larger birth canals and smaller brains, once might have given birth in solitude—like chimpanzees, orangutans, and gorillas can. However, because of the revised size and position of the human female pelvis, women need midwifelike assistance to give birth. If the mother reached down to assist her own baby's birth, she would risk injuring her baby by bending his back against the natural curve of the spine.

As a result, not only did human bodies change with upright walking, but society also had to change in ways that could accommodate the demands placed on the mother by the baby. First, mothers couldn't deliver in solitude. Once here, babies could not cling with hands and feet, so mothers had to use one arm to hold them and, often, the other arm to quiet them when danger lurked. With hands occupied, human females needed help. They needed fathers to stick around. One of the profound consequences of evolution, including the amusement-parkride aspect of birth, is that it has forced humans to be interdependent and social. New mothers need help in birthing their babies—whether from an obstetrician, a midwife, a father, or an unlucky cab driver—and then they need help in bringing them up. In our modern society, that help often comes from a traditional source: fathers. But it also comes from gay or straight partners, adoptive mothers and fathers, foster families, grandparents and other family members, and loving caretakers of all sorts.

FOURTH TRIMESTER BRAIN DEVELOPMENT

Scientists now know that the brain continues to change and grow, allowing for a lifelong ability to reorganize neural pathways based on new experiences. That ability is called neuroplasticity. But while recent discoveries suggest that new neurons are produced throughout life, it doesn't happen nearly as rapidly as it does during the nine months spent in the womb. Some 100 billion neurons form during pregnancy. At birth, all those neurons are as yet incapable of communicating with each other.

But nature has made sure that the neural circuits responsible for basic body functions are up and running at birth. Infants arrive with the most basic and primitive operating equipment, under the control of the lower parts of the brain. During gestation, the basic architecture of the brain is laid down, beginning development soon after conception. That prenatal architecture eventually includes the brain stem, or lower part of the brain, regulating the central nervous system and cardiac and respiratory functions; the thalamus, two bulb-shaped masses above the brain stem that process and relay sensory information; and the cerebellum, which coordinates motor movement. Those parts direct the infant to kick, grasp, cry, sleep, root, suck, swallow, keep a heartbeat going, and manage a circulatory system. It's all primitive or immature, and the higher centers, those in charge of emotions, intelligence, planning, and motor responses, are still waiting to be formed, influenced by love, conversation, comforting touch, faces, movement, sound—in short, the world he was born into.

The work begins almost immediately. Each newborn is busy developing neural connections by laying down a network of dendrites, branched projections that receive signals of communication and pass them on with the aid of neurochemicals. The connections formed are called synapses. During the first three years of human life, there is an unprecedented pattern of rapid synapse formation. In fact, babies develop so many synapses there simply isn't room for them all, and those that aren't used go by the wayside. The ones that remain get more efficient at providing the information we need.

This is how it works. Neurons are cells specializing in sending and receiving signals. A neuron in the eye gets its signal from light; in the ear, from sound vibrations; in the nose and tongue, from molecules that bind to them; and on the skin, signals come from touch. A message travels, via electrical signal, from neuron to neuron to the part of the brain specializing in, say, seeing, tasting, or moving. Then the output side kicks in, sending an outgoing signal to the retina, or the tongue, or a muscle, complete with instructions on how to move, extend, or contract. So even as the brain is constructing a branchlike communication network, it is also beginning to pare down the number of neurons in the brain in order to ease overload, making experience key to wiring an infant's brain.

During that time, an infant's brain experiences sporadic bursts of activity that are known as exuberant periods. At the peak of one of these periods, the brain is creating 2 million new synapses every second, researchers estimate. These bursts of development happen at various times in different areas of the brain during the first months of life and continue, though at a slower pace, through adolescence. During infancy, the new connections allow for color vision, the ability to grasp, and a strong attachment to parents. Each baby is sculpting a brain that is becoming truly human and uniquely his own.

Neuroscience has become adept at studying the tiny but interconnected cells of the brain using brain-imaging technology. Going well beyond earlier scientific tools—such as observation, autopsies, x-rays, and EEGs—CT scans, functional MRIs, and PET scans create three-dimensional images of the brain and allow scientists to analyze its chemical composition, its electrical transmissions, and the blood flow through the brain. Through the use of such technology, we now know that when babies are born, they come equipped with more neurons than they'll ever need, and some, but not many, synapses.

The neurons are the raw material of the brain, and heredity determines their number. (Only recently has research begun to show that important areas of the forebrain continue to produce new neurons into adulthood.) But the infant brain is in a remarkably unfinished state, with its billions of neurons that are unable to communicate with each other. Those connections only begin to be formed as the baby experiences the world and the love of parents and caretakers. Nature and nurture go hand in hand as each sensory interaction adds to the wiring. 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.

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. 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. 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. Nongenetic 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.