Infancy: Body and Mind

This is Unit Three, part one, on the infant's growing body and brain in the first two years. Unit Two closed on the emotional weeks after birth. Unit Three follows the infant from those weeks through growth, sleep, feeding, and the fastest brain development of a lifetime. Carry one question through this part: why do the first two years matter so much? Growth, sleep and feeding all bear on it. At the entry that puts the brain's first two years together, the answer is that experience decides which connections stay. By the end you will be able to read a growth chart, and say when a drop matters. To state the safe-sleep guidance and the evidence behind it. To describe the four processes of the brain's first two years. To explain plasticity, and why it cuts both ways. And to say which newborn reflexes fade, which stay, and why their timing matters.

Five entries on growth: the patterns it follows, the first year's numbers, and the chart that tracks them.

The patterns infant growth follows.

HGD 0301 Patterns of Early Growth

Infant growth follows the patterns set before birth: cephalocaudal, head first and legs last; proximodistal, centre outward; and mass-to-specific, gross before fine. So a baby reaches with the whole arm months before grasping with the fingers. The patterns predict order, not exact timing.

Weight in the first year.

HGD 0302 Weight in the First Year

The average US newborn weighs about seven and a half pounds, loses five to eight percent in the first week, and regains it by two weeks. Birth weight doubles by about five months, and a twelve-month-old averages twenty-two pounds. The first week's loss is expected, not a warning sign.

Length, and the size of the head.

HGD 0303 Length and Head Size

Infants grow about an inch a month in the first year. At birth the head is about one-quarter of body length, against one-eighth in adults, so heavy that a newborn cannot hold it up. Head circumference is positively correlated with cognitive functioning.

How growth is tracked.

HGD 0304 Growth Charts

Growth charts plot a child's weight, length and head size as percentiles by age and sex, and the CDC recommends the WHO charts through age two. A baby who drops across two percentile lines prompts a look for causes; a steady low line may reflect small parents.

Growth that falls well below the norm.

HGD 0305 Failure to Thrive

Failure to thrive is growth significantly below the norm for a child's age, most commonly caused by too few calories: an infant falls off the growth chart because feeding is not meeting needs. Other causes include medical conditions, food insecurity, abuse or neglect. Left unresolved, it can cause developmental delay.

Six entries on sleep: how much, what kind, how night and day separate, and what the evidence says about safe sleep.

How much infants sleep.

HGD 0306 How Much Infants Sleep

Newborns need about fourteen to seventeen hours of sleep a day, two-year-olds eleven to fourteen, and adults seven to nine. About half of a baby's sleep in the first year is REM, against a fifth to a quarter in adults. Sleep is a biopsychosocial process, shaped by biology, psychology and social setting at once.

The two kinds of sleep.

HGD 0307 REM and NREM Sleep

REM sleep, called active sleep, brings eye movement and waking-like brain waves, and it is linked to dreaming, memory consolidation, emotional processing and brain development. NREM sleep has three stages, the third the deepest. A sleep cycle lasts about sixty minutes at two or three, and ninety in adults.

How night and day separate.

HGD 0308 Night and Day

Newborns do not separate night from day. Daytime sleep falls by about three months, night waking drops around six months, and by twelve months most sleep comes at night, though some waking continues into the second year. Fragmented sleep is treated as a problem mainly where night waking and shared beds are less typical.

Sleep across cultures.

HGD 0309 Sleep and Culture

Co-sleeping, infants sleeping with caregivers, is common worldwide, in as many as seventy percent of families, while separate sleep is typical in the US and UK. Swaddling is used in rural Guatemala, and Mayan mothers hold their children to sleep without lullabies. US co-sleeping, regular or occasional, has risen to about half of families over two decades.

Sudden infant death.

HGD 0310 SUID and SIDS

Sudden unexpected infant death is an unexpected death under one year, often in sleep. It has three kinds: accidental suffocation, unknown causes, and sudden infant death syndrome, named when investigation finds no cause. One leading hypothesis links SIDS to abnormalities in the brainstem area that regulates breathing.

What reduced these deaths, and the evidence.

HGD 0311 Safe Sleep and the Evidence

Placing infants on their backs for sleep is the most effective step for reducing the risk of SIDS and other sleep-related deaths, according to the National Institutes of Health. After back-sleeping recommendations and the Back to Sleep campaigns that began in nineteen ninety-four, SIDS rates fell by forty-two to ninety-two percent in the countries that adopted them. The American Academy of Pediatrics summarises its guidance as alone, on the back, in a crib, with room sharing for the first six months. A common worry is that back sleeping makes babies choke; it does not. The course reports this guidance and evidence; it does not advise families.

Three entries on feeding: milk and formula, the move to solid food, and how tastes form.

Human milk, and formula.

HGD 0312 Human Milk and Formula

The WHO, the AAP and UNICEF recommend starting breastfeeding within an hour of birth and feeding only human milk for the first six months. Human milk carries antibodies against respiratory, ear and gut infections, vital where water is unsafe. Formula is a healthy option where breastfeeding is not advised, possible or chosen.

The move to solid food.

HGD 0313 Complementary Feeding

Complementary feeding, adding solid foods alongside milk, usually begins around six months, when an infant can sit up and hold the head steady. Newborns feed every one to three hours; many nine- to twelve-month-olds eat three meals a day. Responsive feeding, following hunger and fullness cues, is linked to building trust.

How tastes form.

HGD 0314 Food Preferences

Newborns prefer sweet tastes and reject bitter ones, and by six months a preference for sweetness tracks how much sweetness the infant has met. A new vegetable may need offering six to fifteen times before any real preference shows. Breastfed children tend to be less picky, likely because of the variety in the nursing parent's diet.

Five entries on building the brain, from the reflexes of the brainstem to the four processes of the first two years. The opening question is answered here.

Where reflexes come from.

HGD 0315 Reflexes and the Brainstem

A reflex is an involuntary movement originating in the brainstem, and most newborn reflexes later give way to movements the child controls, as the grasp reflex gives way, around six months, to deliberate grasping. The brain begins as a tube about three millimetres long and holds an estimated one hundred billion neurons at birth.

The brain's cells.

HGD 0316 Neurons

A neuron is a nerve cell with a cell body, an axon that carries its message, and dendrites that receive messages from other neurons. Myelin, a fatty coating on the axon, speeds transmission. Neurogenesis, the formation of new neurons, begins before birth.

The connections between neurons.

HGD 0317 Synapses

A synapse is the gap where one neuron passes a message to another through neurotransmitters, and forming these connections is called synaptogenesis. By the age of two there are about fifteen thousand synapses per neuron, and toddlers may have twice as many synapses as adults. The surplus is trimmed over the years that follow.

Pruning, and myelination.

HGD 0318 Pruning and Myelination

Synaptic pruning removes unused connections into the teenage years: connections that are used strengthen, and those that are not fall away. Myelination, coating axons in myelin, is fastest in the first two years, when white matter, the myelinated axon bundles, increases fastest, and it continues across life.

The payoff: the first two years together.

HGD 0319 The Brain's First Two Years

The brain's first two years combine four processes, and they answer this part's question. Neurogenesis begins before birth, and a newborn arrives with an estimated one hundred billion neurons. Synaptogenesis connects them at an extraordinary rate. Synaptic pruning removes the connections that are not used, into adolescence. Myelination speeds signals, fastest in these two years. More connections is not better in itself: the brain keeps the connections experience strengthens and discards the rest, which is one reason early experience matters. By two the brain looks structurally like an adult's, but the prefrontal cortex keeps developing into early adulthood.

Four entries on the brain's regions and on its capacity to change.

The four lobes of the cortex.

HGD 0320 The Lobes

The cortex has four lobes: frontal, for thinking, planning and judgment; parietal, for touch and space; temporal, for hearing and language; and occipital, for vision. A baby turning toward a voice uses the temporal lobe, and reaching for a toy the parietal. The prefrontal cortex, at the front of the frontal lobe, develops into early adulthood.

A sensitive period in vision.

HGD 0321 A Sensitive Period for Vision

The visual cortex develops in a burst at two to four months, and most visual areas are developed by twelve months. Infants born with cataracts see detail much better when the cataracts are removed soon after birth. Visual input during the early months shapes how the visual system develops, a sensitive period in action.

How the two hemispheres specialise.

HGD 0322 Lateralization

Lateralization is the settling of particular functions in one hemisphere of the brain, and each hemisphere controls the opposite side of the body. Language comes to rest mainly in the left hemisphere as the right's role declines. It is gradual: the shift continues through childhood.

The brain's capacity to change.

HGD 0323 Plasticity

Plasticity is the brain's capacity to change physically and chemically with experience and to compensate for injury, and it is greatest in early childhood. Young children learn new speech sounds faster than adults and recover well after epilepsy surgery. The same openness makes the infant brain more vulnerable to harm, the other side of plasticity.

Two entries on threats to infant health: what infant mortality measures, and what malnutrition does.

A measure of a society's health.

HGD 0324 Infant Mortality

Infant mortality, death before age one, marks a society's health: about five point four per thousand US births in twenty twenty, against Slovenia's one point five. Within the US, rates run from two point three for Chinese American infants to eleven point two for Black infants, disparities tied to economic and community inequality and to access to care.

What malnutrition does.

HGD 0325 Malnutrition

Undernutrition is responsible for about forty-five percent of child deaths worldwide, and severe malnutrition reduces neurons, synapses, dendrite growth and myelination. Marasmus is starvation from too few calories and too little protein; kwashiorkor is too little protein after weaning, with loss of appetite and a swollen abdomen. Both can delay development permanently.

Two entries to close the part: the newborn's reflexes, and which of them fade and which stay.

The newborn's reflexes.

HGD 0326 Newborn Reflexes

Newborn reflexes include rooting, turning toward a touched cheek; sucking; grasping; the Moro, or startle, reflex, flinging out the arms and legs at a loud noise; stepping; and the tonic neck reflex. Some aid survival, such as sucking; some are responses to stress, such as the Moro; some have no clear purpose.

Which reflexes fade, and which stay.

HGD 0327 Reflexes That Fade and Reflexes That Stay

Most newborn reflexes fade in the first months. Stepping fades by about two months, the Moro by two to four, rooting by three to four, and the palmar grasp by three to six, when deliberate grasping replaces it. The Babinski reflex, the toes fanning out when the sole is stroked, fades later, in the first year or two. Blinking, coughing, gagging and sneezing stay for life. Reflexes matter clinically because their timing is predictable: one absent when it should be present, or lingering long past its age, can signal a neurological problem. The order matters more than the exact month.

Look at what you can now do. You can read a growth chart: a drop across two percentile lines prompts a look for causes, while a steady low line may reflect small parents. You can state the safe-sleep guidance, on the back and alone in a crib, and the evidence: SIDS rates fell by forty-two to ninety-two percent after the back-sleeping campaigns. You can describe the brain's first two years: neurogenesis, synaptogenesis, pruning and myelination, with experience deciding what stays. You can explain plasticity, the capacity to change and compensate, and say that the same openness makes the infant brain more vulnerable. And you can say that most reflexes fade in the first months, that blinking and coughing stay, and that a reflex out of its time can signal a problem. Part two covers learning to walk.