Growth in Infancy

In infancy, growth is so rapid and the consequences of neglect are so severe that gains are closely monitored. Medical checkups, including measurement of height, weight, and head circumference, occur often in developed nations because these measurements provide the first clues as to whether an infant is progressing as expected—or not.

Body Size

Newborns lose several ounces in the first three days and then gain an ounce a day for months. Birthweight typically doubles by 4 months and triples by a year. An average 7-pound newborn will be 21 pounds at 12 months (9,525 grams, up from 3,175 grams at birth). Height increases too: A typical baby grows 10 inches (24 centimeters) in a year.

Physical growth then slows, but not by much. Most 24-month-old children weigh almost 28 pounds (13 kilograms) and have added another 4 inches (10 centimeters) or so. Typically, 2-year-olds are half their adult height and about one-fifth their adult weight, four times heavier than they were at birth (see Figure 5.1).

Eat and Sleep The rate of increasing weight in the first weeks of life makes it obvious why new babies need to be fed day and night.

Each of these numbers is a norm, which is an average, or standard, for a particular population. The “particular population” for the norms just cited is United States infants in about 1970.

At each well-baby checkup (monthly at first), a doctor or nurse measures growth and compares it to that baby’s previous numbers. Often measurements are expressed as a percentile, from 0 to 100. Percentiles indicate where an individual ranks on a particular measure. Percentiles are often used for school achievement; here they are used for an infant’s rank on weight and height compared to other babies of the same age. Thus, weight at the 30th percentile means that 30 percent of all babies weigh less and 70 percent weigh more.

For a baby who has always been average (50th percentile), something is amiss if the percentile changes a lot, either up or down. If an average baby suddenly grows more slowly, that could be the first sign of a medical condition called failure to thrive. If weight gain is much above the norm, that may be a warning for later obesity.

Abnormal growth in either direction was once blamed on parents. For small babies, it was thought that parents made feeding stressful, leading to “nonorganic failure to thrive.” Now dozens of medical conditions have been discovered that cause failure to thrive. Pediatricians consider it “outmoded” to blame parents (Jaffe, 2011, p. 100).

Same Boy, Much Changed All three photos show Conor: first at 3 months, then at 12 months, and finally at 24 months. Note the rapid growth in the first two years, especially apparent in the changing proportions of the head compared to the body and use of the legs.

CECILIA VARAS

Brain Growth

Prenatal and postnatal brain growth (measured by head circumference) affects later cognition (Gilles & Nelson, 2012). If teething or a stuffed-up nose temporarily slows weight gain, nature slows growth of the body but not the brain, a phenomenon called head-sparing. Sadly, prolonged malnutrition eventually affects the brain, as explained later.

From two weeks after conception to two years after birth, the brain grows more rapidly than any other organ, being about 25 percent of adult weight at birth and almost 75 percent at age 2 (see Figure 5.2). Over the same period, head circumference increases from about 14 to 19 inches.

Growing Up Two-year-olds are totally dependent on adults, but they have already reached half their adult height and three-fourths of their adult brain size.

The brain develops lifelong; it is mentioned in every chapter of this book. We begin with the basics—neurons, axons, dendrites, neurotransmitters, synapses, and the cortex, especially the prefrontal cortex.

Communication within the central nervous system (CNS)—the brain and spinal cord—begins with nerve cells, called neurons, which proliferate in the last half of fetal life. The brain continues to grow rapidly after birth, protected by the skull, which has two “soft spots” (fontenelles) to allow it to squeeze through the vagina. Even so, the newborn head is proportionally the biggest part of the body, which is why birthing the head takes hours; only a few moments are needed for birth of the rest of the body.

The newborn brain has billions of neurons, about 70 percent of them in the cortex, the brain’s six outer layers (see Figure 5.3). Most thinking, feeling, and sensing occur in the cortex (Johnson, 2010).

The Developing Cortex The infant’s cortex consists of four to six thin layers of tissue that cover the rest of the brain. The cortex contains virtually all the neurons that make conscious thought possible. Some areas, such as those devoted to the senses, mature relatively early. Others, such as the prefrontal cortex, mature quite late.

The final part of the brain to mature is the prefrontal cortex, the area for anticipation, planning, and impulse control. It is not, as once thought, “functionally silent during most of infancy” (Grossmann, 2013, p. 303). Nonetheless, many of the connections between emotions and thought have not yet formed. The prefrontal cortex gradually becomes more efficient over the next two decades (Wahlstrom et al., 2010). [Lifespan Link: Major discussion of the growth of the prefrontal cortex is in Chapter 14.]

All areas of the brain specialize, becoming fully functioning at different ages. Some regions deep within the skull maintain breathing and heartbeat, and thus are able to sustain life by seven months after conception. Some areas in the mid-brain underlie emotions and impulses. These functions are apparent in the first year of life and are shared with many other animals, although emotional regulation and impulse control continue to develop throughout childhood. Finally, some regions in the cortex allow perception and cognition, first cognition of exciting, social interactions and later cognition of more abstract thoughts (Grossmann, 2013). These areas reach final maturation in adulthood.

Examples of this specialization are the areas of the cortex: a visual cortex, an auditory cortex, and an area dedicated to the sense of touch for each body part—including each finger of a person and each whisker of a rat (Barnett et al., 2006). The senses require maturation and learning, but all are present at birth. Humans have a much larger frontal cortex relative to body size than any other animal: That is why people can plan and create better than any mouse, whale, or chimpanzee.

Within and between areas of the central nervous system, neurons are connected to other neurons by intricate networks of nerve fibers called axons and dendrites. Each neuron has a single axon and numerous dendrites, which spread out like the branches of a tree. The axon of one neuron meets the dendrites of other neurons at intersections called synapses, which are critical communication links within the brain.

To be more specific, neurons communicate by sending electrochemical impulses through their axons to synapses, to be picked up by the dendrites of other neurons. The dendrites bring messages to the cell bodies of their neurons, which, in turn, convey the messages via their axons to the dendrites of other neurons (see Visualizing Development, p. 131).

Axons and dendrites do not quite touch at synapses. Instead, the electrical impulses in axons typically cause the release of chemicals called neurotransmitters, which carry information from the axon of the sending neuron, across a synaptic gap, to the dendrites of the receiving neuron (see Figure 5.4).

How Two Neurons Communicate The infant brain contains billions of neurons, each with one axon and many dendrites. Every electrochemical message causes thousands of neurons to fire, each transmitting the message across the synapse to neighboring neurons. This electron micrograph shows neurons greatly magnified, with their tangled but highly organized and well-coordinated sets of dendrites and axons.

CNRI/SCIENCE SOURCE

VISUALIZING DEVELOPMENT: Nature, Nurture, and the Brain

Nature, Nurture, and the Brain

The mechanics of neurological functioning are varied and complex; neuroscientists hypothesize, experiment, and discover more each day. Brain development begins with genes and other biological elements, but hundreds of epigenetic factors affect brain development from the first to the final minutes of life. Particularly important in human development are experiences: plasticity means that dendrites form or atrophy is response to nutrients and events. The effects of early nurturing experiences are lifelong, as proven many times in mice; research on humans suggest similar effects.

SOURCES & CREDITS LISTED ON P. SC-1

NATURE

The link between one neuron and another is shown in this simplified diagram. The infant brain contains billions of neurons, each with one axon and many dendrites. Every electrochemical message to or from the brain causes thousands of neurons to fire simultaneously, each transmitting the message across the synapse to neighboring neurons.

NURTURE

In experiments with baby mice, those that were frequently licked and nuzzled by their mothers exhibited very different behaviors from those that were neglected.

PHOTO: RUBBERBALL/NICOLE HILL/ALAMY

PHOTO: ANYAIVANOVA/ISTOCK/THINKSTOCK

THE BRAIN

Social scientists now believe that every aspect of early life affects brain patterns: Each experience activates and prunes neurons, such that the firing patterns from one axon to another dendrite reflect the past. When a mother mouse licks her newborn babies, that reduces methylation of a gene (called Nr3cl), allowing increased serotonin to be released by the hypothalamus. That serotonin not only increases momentary pleasure (mice love being licked) but also starts a chain of epigenetic responses that reduce stress hormones from many parts of the brain and body, including the adrenal glands. The effects are lifelong, as proven many times in mice. In humans, a mother’s gentle stroking and cuddling of her newborn seem to affect the baby similarly.

At birth, the brain contains at least 100 billion neurons, more than a person needs. However, the newborn’s brain has far fewer dendrites and synapses than the person will eventually possess. During the first months and years, rapid growth and refinement in axons, dendrites, and synapses occurs, especially in the cortex. Dendrite growth is the major reason that brain weight triples from birth to age 2 (Johnson, 2010).

An estimated fivefold increase in the number of dendrites in the cortex occurs in the 24 months after birth, with about 100 trillion synapses being present at age 2. According to one expert, “40,000 new synapses are formed every second in the infant’s brain” (Schore & McIntosh, 2011, p. 502).

This extensive postnatal brain growth is highly unusual for mammals. It occurs in humans because heads cannot grow enough before birth to contain the brain networks needed to sustain human development. Although prenatal brain development is remarkable, it is limited because the human pelvis is relatively small, so the baby’s head must be much smaller than an adult’s head in order to make birth possible. For that reason, unlike other mammals, humans must nurture and protect their offspring for more than a decade while the child’s brain continues to develop (Konner, 2010).

Early dendrite growth is called transient exuberance: exuberant because it is so rapid and transient because some of it is temporary. The expansive growth of dendrites is followed by pruning. Just as a gardener might prune a rose bush by cutting away some growth to enable more, or more beautiful, roses to bloom, unused brain connections atrophy and die, enabling the brain to develop in accord with the sociocultural context (Stiles & Jernigan, 2010).

The details of brain structure and growth depend on genes and maturation but even more on experience (Stiles & Jernigan, 2010). Some dendrites wither away because they are never used—that is, no experiences have caused them to send a message to other neurons. Expansion and pruning of dendrites occur for every aspect of early experience, from noticing musical rhythms to understanding emotions (Scott et al., 2007).

Electric Excitement Are those stripes exciting? Researchers at University of California, Berkeley, know that the neurons in this eight-week-old boy’s brain will show changes in brain activity as the patterns on the screen change. Electrodes on his head map those changes as he undergoes a color vision test. Every month of life up until age 2 shows increased electrical excitement in an infants’ brain.

LAWRENCE MIGDALE/SCIENCE SOURCE

Strangely enough, this loss of dendrites increases brainpower. The space between neurons in human brains, for instance—especially in regions for advanced, abstract thought—is far greater than the space in chimpanzee brains (Miller, 2010). The densely packed neurons of chimps make them less intelligent than people, probably because humans have more space for dendrite formation. This spacing allows more complex thinking, as well as learning that is specific to the particular culture each baby is born into.

Some children with intellectual disabilities have “a persistent failure of normal synapse pruning” (Irwin et al., 2002, p. 194). That makes thinking and learning difficult. One sign of autism is rapid brain growth, suggesting too little pruning (Hazlett et al., 2011). Indeed, studies of brain development in autistic children suggest that their brains develop normally for the first six months, but then synapses do not develop as they should, so that by the second year of life symptoms of autism become evident (Landa et al., 2013).

Yet just as too little pruning creates problems, so does too much pruning. Brain sculpting is attuned to experience: The appropriate links in the brain need to be established, protected, and strengthened while inappropriate ones are eliminated. One group of scientists speculates that “lack of normative experiences may lead to overpruning of neurons and synapses, both of which may lead to reduction of brain activity” (Moulson et al., 2009, p. 1051).

Another group suggests that infants who are often hungry, or hurt, or neglected may develop brains that compensate—and cannot be reprogrammed even if circumstances change. The hungry baby becomes the obese adult, the abused child rejects attention, and so on, always with the interaction of nature and nurture (van IJzendoorn et al., 2012).

Most infants develop well within their culture, and head-sparing usually ensures that baby brains are sufficiently nourished. For normal brain development, a baby could hear French or Farsi, or see emotions displayed dramatically or subtly (e.g., throwing oneself to the floor or merely pursing the lips, a cultural difference). However, infant brains do not develop well without essential experiences that all humans need.

Those essential experiences are called expectant because the brain requires them to develop normally and therefore “expects” them to be provided in much the same way the stomach expects food. That contrasts with dependent experiences, which vary from family to family.

Experience-dependent brain development is variable because circumstances vary: For example, a baby’s main caregiver could be a biological or adoptive mother or father, or a grandparent, or a hired baby nurse. Happy and successful babies have been raised by each of those types of caregivers, although obviously the children are affected by their particular caregiver, whomever that might be. All that is experience-dependent.

Experience-expectant brain development occurs because of circumstances that all human babies should have. For example, every baby needs at least one steady caregiver: Without that stability the brain might not develop normal emotional responses. [Lifespan Link: Neglected children are discussed in Chapter 8.]

Another essential experience that every infant brain expects is sensory stimulation. Playing with a young baby, allowing varied sights, sounds, touches, and movements (arm waving in the early months, walking later on) are all fodder for brain connections. Severe lack of stimulation (e.g., a baby who never hears speech) stunts the brain. As one review of early brain development explains, “enrichment and deprivation studies provide powerful evidence of…widespread effects of experience on the complexity and function of the developing system” (Stiles & Jernigan, 2010, p. 345).

This does not mean that babies require spinning, buzzing, multitextured, and multicolored toys. In fact, such toys may be a waste of money. Infants are fascinated by simple objects and facial expressions. Fortunately, although elaborate infant toys are not needed, there is no evidence that they harm the brain; babies prevent overstimulation by ignoring them. A simple application of what has been learned about the prefrontal cortex is that hundreds of objects, from the very simple to the quite complex, can capture an infant’s attention.

However, because the prefrontal cortex is underdeveloped in infancy, the brain is not yet under thoughtful control. For instance, it is useless to insist that an infant stop crying: Babies are too immature to decide to stop crying, as adults do. If adults do not understand this fact, the results can be tragic.

If a frustrated caregiver shakes a crying baby sharply and quickly, that can cause shaken baby syndrome, a life-threatening condition. Because the brain is still developing, shaking stops the crying because blood vessels in the brain rupture and neural connections break. Shaken baby syndrome is an example of abusive head trauma (Christian et al., 2009). Death is the worst consequence; lifelong intellectual impairment is the more likely one.

The fact that infant brains respond to their circumstances suggests that waiting for evidence of child mistreatment is waiting too long. In the first months of life babies adjust to their world, becoming withdrawn and quiet if their caregivers are depressed, or becoming loud and demanding if that is the only way they can get fed. In both these circumstances, they learn destructive habits. Thus, since development is dynamic and interactive, caregivers need help from the start, not when harmful systems are established (Tronick & Beeghly, 2011).

The word systems is crucial here. Almost every baby experiences something stressful—a caregiver yelling, or a fall off the bed, or a painful stomachache. Fortunately, self-righting—an inborn drive to remedy deficits—is built into the human system. For example, infants with no toys develop their brains by using whatever objects are available, and infants with neglectful mothers may bond with a father, grandparent, or stranger who provides daily affection and stimulation. Human brains are designed to grow and adapt; plasticity is apparent from the beginning (Tomalski & Johnson, 2010). It is the patterns, not the moments, of neglect or maltreatment that harm the brain.

A VIEW FROM SCIENCE

Face Recognition

Unless you have prosopagnosia (face blindness, relatively uncommon), the fusiform face area of your brain is astonishingly adept at face recognition. This area is primed among newborns, who are quicker to recognize a face they have just seen once than older children and adults (Zeifman, 2013). However, every face is fascinating early in life: Babies stare at pictures of monkey faces and photos of human ones, at drawings and toys with faces, as well as at live faces.

Soon, experiences refine perception (De Heering et al., 2010). By 3 months, babies smile more readily at familiar people and are more accurate at differentiating faces from their own ethnic group (called the own-race effect). Babies are not prejudiced: The own-race effect results from limited multiethnic experience. Indeed, children of one ethnicity, adopted and raised exclusively among people of another ethnicity, recognize differences among people of their adopted group more readily than differences among people of their biological group.

The importance of early experience is confirmed by two studies. From 6 to 9 months, infants were repeatedly shown a book with pictures of six monkey faces, each with a name written on the page (see photo). One-third of the parents read the names while showing the pictures; another one-third said only “monkey” as they turned each page; the final one-third simply turned the pages with no labeling.

At 9 months, infants in all three groups viewed pictures of six unfamiliar monkeys. The infants who had heard names of monkeys were better at distinguishing one new monkey from another than were the infants who saw the same picture book but did not hear each monkey’s name (Scott & Monesson, 2010).

Many children and adults do not notice the individuality of newborns. Some even claim that “all babies look alike.” However, the second interesting study found that 3-year-olds with younger siblings were much better at recognizing differences between photos of unfamiliar newborns than were 3-year-olds with no younger brothers or sisters (Cassia et al., 2009). This finding shows again that experience matters, contributing to development of dendrites in the fusiform face area.

Iona Is Not Flora If you heard that Dario was not Louis or Boris, would you stare at unfamiliar monkey faces more closely in the future? For 6-month-olds, the answer is yes.

REPRINTED FROM NEUROPSYCHOLOGIA, 48.SCOTT, L. ET AL. EXPERIENCE-DEPENDENT NEURAL SPECIALIZATION DURING INFANCY 1857©1861. COPYRIGHT 2010 WITH PERMIS SION FROM ELSEVIER.

Sleep

One consequence of brain maturation is the ability to sleep through the night. Newborns cannot do this. Normally, they sleep 15 to 17 hours a day, in one- to three-hour segments. Hours of sleep decrease rapidly with maturity: The norm per day for the first 2 months is 14¼ hours; for the next 3 months, 13¼ hours; for 6 to 17 months, 12¾ hours.

Wide variation is particularly apparent in the early weeks. As reported by parents (who might exaggerate), one new baby in 20 sleeps 9 hours or fewer per day and one in 20 sleeps 19 hours or more (Sadeh et al., 2009).

Sleep specifics vary not only because of biology (age and genes) but also because of the social environment. Full-term newborns sleep more than low-birthweight babies, who are hungry every two hours. Babies who are fed cow’s milk and cereal sleep more soundly—easier for parents but not ideal for the baby. Such feeding practices arise from the interaction between infant and culture. The social context also has a direct effect: If parents respond to predawn cries with food and play, babies wake up early each morning (Sadeh et al., 2009).

Over the first months, the relative amount of time in various stages of sleep changes. Babies born preterm may always seem to be dozing. About half of the sleep of full-term newborns is REM (rapid eye movement) sleep, with closed lids but flickering eyes and rapid brain waves, indicating dreaming. REM sleep declines over the early weeks, as does “transitional sleep,” the dozing, half-awake stage. At 3 or 4 months, quiet sleep (also called slow-wave sleep) increases, as does time alert and wide awake.

Overall, 25 percent of children under age 3 have sleeping problems, according to parents surveyed in an Internet study of more than 5,000 North Americans (Sadeh et al., 2009). Sleep problems are more troubling for parents than for infants. This does not render the problems insignificant, however; overtired parents may be less patient and responsive (Bayer et al., 2007).

One problem for parents is that advice about where infants should sleep varies. Some suggest that infants should sleep beside their parents—who must immediately respond to every cry (Nicholson & Parker, 2009). Others advise that infants need their own room, should be allowed to “cry it out” so they will not be spoiled, and can learn to soothe themselves. Both sets of advice make sense, as the following explains.

OPPOSING PERSPECTIVES

Where Should Babies Sleep?

co-sleeping A custom in which parents and their children (usually infants) sleep together in the same room.

Traditionally, most middle-class U.S. infants slept in cribs in their own rooms; it was feared that they would be traumatized by their parents’ sexual interactions. By contrast, most infants in Asia, Africa, and Latin America slept near their parents, a practice called co-sleeping. People in those cultures believed that parent—child separation at night was cruel.

Even today, at baby’s bedtime, Asian and African mothers worry more about separation, whereas European and North American mothers worry more about lack of privacy. A 19-nation survey found that parents act on these fears: The extremes were 82 percent of babies in Vietnam sleeping with their parents compared with 6 percent in New Zealand (Mindell et al., 2010) (see Figure 5.5).

Awake at Night Why the disparity between Asian and non-Asian rates of co-sleeping? It may be that Western parents use a variety of gadgets and objects—monitors, night lights, pacifiers, cuddle cloths, sound machines—to accomplish the same things Asian parents do by having their infant next to them.

Source: Mindell et al., 2010.

This difference in practice may seem related to income, since low-SES families are less likely to have an extra room. But even wealthy Japanese families often co-sleep. By contrast, many poor North American families find a separate space for their children at night. Co-sleeping results from culture and custom, not merely income (Kohyama et al., 2011).

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The argument for co-sleeping is that it makes it easier to respond to a baby in the middle of the night, especially if he or she is hungry or scared. When parents opt for co-sleeping, they are less exhausted since they can reach over to feed or comfort their baby. Breast-feeding, often done every hour or two at first, is easier if it does not require walking to another room.

Yet the argument against co-sleeping rests on a chilling statistic: Sudden infant death is more common when babies sleep beside their parents (Gettler & McKenna, 2010; Ruys et al., 2007). (Sudden infant death syndrome [SIDS] is discussed at the end of this chapter.) Many young parents occasionally go to sleep after drinking or drugging. If their baby is beside them, bed-sharing (not merely co-sleeping), is dangerous.

One reason for opposite practices is that adults are affected by their own early experiences. This phenomenon is called ghosts in the nursery because new parents bring decades-old memories into the bedrooms of their children. Those ghosts can encourage either co-sleeping or separate rooms.

For example, compared with Israeli adults who had slept near their parents as infants, those who had slept communally with other infants (as sometimes occurred on kibbutzim) were more likely to interpret their own infants’ nighttime cries as distress, requiring comfort (Tikotzky et al., 2010). That is how a ghost affects current behavior: If parents think their crying babies are frightened, lonely, and distressed, they want to respond. Quick responses are easier with co-sleeping.

But remember that infants learn from their earliest experiences. If babies become accustomed to bed-sharing, they will crawl into their parents’ bed long past infancy. Parents might lose sleep for years because they wanted more sleep when their babies were small.

Developmentalists hesitate to declare any particular pattern best because the issue is “tricky and complex” (Gettler & McKenna, 2010, p. 77). Sleeping alone may encourage independence—a trait appreciated in some cultures, abhorred in others. Past experiences (ghosts in the nursery) affect us all: Should some ghosts be welcomed and others banned?

Especially for New Parents You are aware of cultural differences in sleeping practices, which raises a very practical issue: Should your newborn sleep in bed with you?

Response for New Parents: From the psychological and cultural perspectives, babies can sleep anywhere as long as the parents can hear them if they cry. The main consideration is safety: Infants should not sleep on a mattress that is too soft, nor beside an adult who is drunk or drugged. Otherwise, each family should decide for itself.

Infant at Risk? Sleeping in the parents’ bed is a risk factor for SIDS in the United States, but don’t worry about this Japanese girl. In Japan, 97 percent of infants sleep next to their parents, yet infant mortality is only 3 per 1,000—compared with 7 per 1,000 in the United States. Is this bed, or this mother, or this sleeping position protective?

YAGI STUDIO/GETTY IMAGES.