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-park-ride 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.5 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.6 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.7 (Only recently has research begun to show that important areas of the forebrain continue to produce new neurons into adulthood.)8 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.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