The human brain at birth is roughly one-third the volume of an adult brain, and it undergoes a staggering growth spurt in the weeks and months that follow. In the first 90 days alone, overall brain volume increases by about 64%, driven by a combination of new connections forming between neurons, the insulation of nerve fibers, and the expansion of support cells that help wire everything together. But this growth is far from uniform: different regions develop at different speeds, shaped by genetics, sensory experience, caregiving, nutrition, and even gut bacteria.
Explosive Growth in the First Three Months
The speed of early brain growth is hard to overstate. An MRI study tracking healthy newborns found that whole-brain volume at birth averaged around 340,000 cubic millimeters, and the brain was growing at roughly 1% per day in those first weeks. That rate slowed to about 0.4% per day by the end of the third month, at which point the brain had already reached just over half the volume of a typical elderly adult brain.1JAMA Neurology. Structural Growth Trajectories and Rates of Change in the First 3 Months of Infant Brain Development Not every structure keeps the same pace. The cerebellum, which coordinates movement and balance, more than doubled in size over those 90 days. The hippocampus, involved in memory, grew at the slowest rate, increasing by about 47%.1JAMA Neurology. Structural Growth Trajectories and Rates of Change in the First 3 Months of Infant Brain Development
Regional differences persist beyond the first three months. Gray matter in sensory and motor areas, particularly the occipital lobe at the back of the brain, grows faster than the prefrontal cortex, the region behind the forehead that handles planning and decision-making.2PubMed Central. Regional gray matter growth, sexual dimorphism, and cerebral asymmetry in the neonatal brain This makes intuitive sense: a newborn needs to see, hear, and move well before it needs executive reasoning. Most brain regions continue gaining volume through early childhood, though some cortical areas in the frontal and parietal lobes actually start shrinking between ages seven and thirteen as the brain refines its circuitry.3Nature Communications. Tracking regional brain growth up to age 13 in children born term and very preterm
Sex differences emerge early. Male infants’ brains tend to grow slightly faster, widening the size gap with female infants over time, though longer gestation at birth correlates with larger brain volume regardless of sex. Infants born even one week earlier than average start out about 5% smaller and still lag behind at three months.1JAMA Neurology. Structural Growth Trajectories and Rates of Change in the First 3 Months of Infant Brain Development
Myelination Sweeps Through the Brain in a Predictable Wave
Much of what makes the postnatal brain faster and more efficient is myelination, the process by which nerve fibers get wrapped in a fatty sheath that speeds up electrical signals. Myelin is produced by a type of support cell called an oligodendrocyte, and the timing of its spread through the brain follows a remarkably consistent pattern. It begins in the cerebellum, the brainstem, and a deep white matter tract called the internal capsule. By three to four months, myelination reaches the back part of the corpus callosum (the bridge connecting the two hemispheres) and the visual pathways. Between four and six months, it moves through the occipital and parietal lobes. The frontal and temporal lobes come last, typically around six to eight months.4PubMed Central. Mapping infant brain myelination with magnetic resonance imaging
This back-to-front, bottom-to-top sequence mirrors the order in which different abilities come online. Vision sharpens before language. Motor control develops before impulse control. The frontal lobes, the last to myelinate in infancy, continue the process well into adolescence, which is part of why teenagers are famously impulsive. Understanding this timeline also helps explain why premature infants face particular risks: their oligodendrocyte precursors are still at a vulnerable, immature stage when they are exposed to the stresses of life outside the womb.
Building and Pruning Synapses
Growth in brain volume is only part of the story. What really defines postnatal development is what happens at the level of individual connections. After birth, neurons undergo massive arborization, branching out their dendrites to form synapses with neighboring cells. This synaptogenesis peaks at different times in different regions but is one of the dominant biological processes in the first two years of life.5PubMed Central. Genetic control of postnatal human brain growth At its peak, the infant brain is producing synapses at a rate far exceeding what it will ultimately keep.
That overproduction is by design. The brain builds more connections than it needs, then eliminates the ones that are not reinforced by experience. This pruning process is surprisingly active during sleep: REM sleep selectively prunes newly formed dendritic spines in the developing brain while also strengthening the synapses that remain.6PubMed. Rapid eye movement sleep and neuronal development Newborns spend a disproportionate amount of their time in REM sleep compared to adults, and this is not wasted downtime. It appears to be when the brain does much of its construction and quality-control work, sorting useful connections from redundant ones.
Glial Cells as Master Architects
For a long time, the non-neuronal cells of the brain, collectively called glia, were treated as passive scaffolding. That view has changed dramatically. Microglia and astrocytes turn out to be active orchestrators of postnatal circuit formation, regulating synapse creation and elimination, guiding axons to the right targets, modulating neuronal activity, and even promoting the survival of developing neurons.7PubMed Central. Microglia and Astrocytes in Postnatal Neural Circuit Formation
The interplay between these two glial types is more intricate than anyone expected. In experiments with postnatal mice, researchers found that microglia and astrocytes coordinate to remove synapses in an activity-dependent way. The process works something like this: when neural activity drops in a particular area, neurons signal to microglia through a chemical messenger. Microglia then release signaling molecules that instruct astrocytes to pull back from nearby synapses. This withdrawal creates a permissive environment for microglia to engulf and eliminate those synapses.8Cell. Microglia-astrocyte Wnt crosstalk coordinates activity-dependent synapse remodeling in the developing brain The takeaway is that synapse pruning is not a blunt process. It is carefully coordinated and depends on real-time feedback about which circuits are being used.
Neurons Still on the Move
One of the more surprising findings in recent neuroscience is that neuronal migration does not stop at birth. While the vast majority of neurons are generated prenatally, new neurons continue migrating into certain brain regions throughout infancy and even into adolescence. Multiple migratory pathways deliver inhibitory interneurons to destinations across the frontal and temporal lobes during infancy. Shorter-range migration of excitatory neurons also continues during adolescence, particularly near a structure called the amygdala paralaminar nucleus, which follows a notably delayed growth trajectory from infancy to adulthood.9PubMed Central. Which neurodevelopmental processes continue in humans after birth?
This extended migration is not just a curiosity. The frontal lobe, where many of these interneurons end up, is the seat of higher-order thinking and emotional regulation. Adding inhibitory neurons over a prolonged period could be one mechanism by which the brain fine-tunes its circuits for complex social behavior. And the amygdala, central to fear and emotional processing, receives new neurons well into the teen years, which may help explain why emotional regulation keeps improving throughout childhood and adolescence.
Sensory Systems Come Online Early
Even at two months of age, the infant brain processes sensory information in a surprisingly organized way. Brain imaging of awake infants shows that auditory stimulation activates localized areas in the primary and associative auditory cortex, and this response is not muddled by unrelated visual input.10PubMed. Selectivity and localization of cortical response to auditory and visual stimulation in awake infants aged 2 to 4 months This counters an older hypothesis that the immature cortex might process everything in a jumbled, cross-modal way. The sensory systems are more compartmentalized than expected, even in very young infants.
By six months, infants show something even more impressive: their brains generate expectation-based feedback. When six-month-olds learn that a particular sound predicts a visual event, their visual cortex begins responding to that sound in anticipation. If the sound is not predictive, the visual cortex stays quiet.11PubMed Central. Top-down modulation in the infant brain: Learning-induced expectations rapidly affect the sensory cortex at 6 months This means the infant brain is not just passively absorbing stimuli. It is building predictive models of the world, using prior learning to shape what it expects to see. That is a hallmark of sophisticated cortical processing, and it is already happening well before the first birthday.
Critical Periods and Why Timing Matters
Certain abilities can only develop properly if the right kind of input arrives during a specific window. These critical periods are most familiar in the context of vision, where early visual deprivation can cause permanent deficits, but they apply across sensory systems and even to aspects of emotional development. At the cellular level, the opening and closing of critical periods is governed in part by the maturation of a specific class of inhibitory neurons. These fast-spiking cells generate a particular type of brain oscillation associated with heightened plasticity. As they mature and become surrounded by a specialized protein structure called a perineuronal net, plasticity diminishes and the critical period closes.12PubMed Central. Critical period regulation across multiple timescales
Different brain regions reach their critical periods at different times, which reflects the staggered developmental timeline. Sensory cortices hit theirs early in life; higher-order association areas are thought to reach theirs later, perhaps not until early childhood or even later. This staggered schedule is why a cataract in a newborn’s eye is a medical urgency (the visual critical period is closing fast) while a child can begin learning a second language at age four and still sound like a native speaker.
How the Brain Fuels All This Growth
Building a brain at this rate is metabolically expensive. By some estimates, the infant brain consumes a larger fraction of the body’s total energy budget than at any other point in life. What it runs on is somewhat different from the adult brain. While adults rely almost entirely on glucose, neonates also lean heavily on ketone bodies, molecules produced from fat metabolism. In breastfed infants, ketone bodies can supply up to half of the brain’s energy needs, and they also serve as building blocks for the membrane lipids and cholesterol needed to construct new cells.13Current Opinion in Physiology. Ketone metabolism in pregnancy and neonates
This dual-fuel system is an elegant adaptation. Breast milk is rich in fat, and the newborn’s liver is primed to convert that fat into ketones. The arrangement means the developing brain has a steady supply of both energy and raw materials for building myelin sheaths and cell membranes, even when glucose availability fluctuates.
How Caregiving Shapes Brain Architecture
Postnatal brain development is not purely an internal biological program. It is deeply shaped by the quality of a child’s social environment. The brain structures most affected are the ones involved in emotion: the amygdala, the prefrontal cortex, and the hippocampus. Sensitive, responsive caregiving appears to regulate the development of these structures, programming how they will handle emotional challenges for years to come.14PubMed Central. Neurobiology of infant attachment: attachment despite adversity and parental programming of emotionality
During infancy, a caregiver acts as an external regulator for the baby’s stress response. When a parent soothes a distressed infant, they are not just calming the child in the moment; they are helping calibrate the circuitry that connects the amygdala (the brain’s alarm system) to the prefrontal cortex (which modulates that alarm). Disorganized or poor-quality caregiving, by contrast, can have lasting consequences on this frontoamygdala circuitry, and research on children adopted from institutional care has shown that the effects persist even after the child moves into a nurturing environment.15PubMed. Sensitive Periods of Emotion Regulation: Influences of Parental Care on Frontoamygdala Circuitry and Plasticity This does not mean these children cannot improve. But it does mean that early caregiving quality has biological consequences that go beyond psychology and into the physical wiring of the brain.
The Gut-Brain Connection in Infancy
One of the more unexpected discoveries of the past decade is that gut bacteria influence brain development. The gut microbiome communicates with the brain partly through microglia, the immune cells that also play a central role in synapse pruning. Preclinical evidence suggests that the composition of the gut microbial community regulates microglial maturation and function.16PubMed Central. Microbiome-microglia connections via the gut-brain axis
Animal studies have put some specifics on this link. In mice colonized with Bifidobacterium, a genus that dominates the breastfed infant gut, researchers found that early gut colonization influenced the expression of synapse-related genes and microglia markers in the cerebellum, cortex, and hippocampus. By three weeks of age, these colonized mice had normal synaptic density and neuronal activity, while germ-free mice (raised without gut bacteria) showed excess synaptic density and reduced neuronal firing rates.17PubMed Central. Bifidobacteria shape host neural circuits during postnatal development by promoting synapse formation and microglial function The implication is that normal gut colonization helps the brain reach the right balance of connections. Without those microbial signals, synapse pruning falls behind.
These are animal findings, and translating them directly to human infants requires caution. But they align with a growing body of evidence that breastfeeding, which shapes early microbial colonization, supports brain development through more pathways than just nutrition alone. Breast milk contains oligosaccharides, polyunsaturated fatty acids, and lactoferrin, all of which have been linked to neurodevelopment.18PubMed Central. A review on the relationship between breast milk nutrients and brain development in preterm infants Some of those oligosaccharides are not even digestible by the infant. They exist to feed Bifidobacterium, creating a feedback loop between breast milk, gut bacteria, and brain circuitry.
When Development Is Disrupted
Premature birth is one of the clearest natural experiments in how vulnerable the developing brain can be. The white matter of premature infants is especially susceptible to injury. Oligodendrocyte progenitors, the cells that will eventually produce myelin, are at an immature and vulnerable stage during the gestational ages when many preterm infants are born. These cells are particularly prone to damage from reduced blood flow and the resulting free radical-mediated injury.19PubMed. Maturation-dependent vulnerability of perinatal white matter in premature birth When oligodendrocyte progenitors are damaged, the downstream effect is disrupted myelination, which can translate to cognitive and motor difficulties that persist for years.
Environmental threats also reach the developing brain. Research on children growing up in heavily polluted urban environments has found evidence of neuroinflammation, disruption of protective barriers including the blood-brain barrier, and subtle cognitive effects.20PubMed Central. How air pollution alters brain development: the role of neuroinflammation The effects are described as subtle at the population level but widespread, appearing across multiple urban populations studied. The developing brain, with its rapid growth, high metabolic demands, and still-maturing blood-brain barrier, is likely more vulnerable to airborne pollutants than the adult brain.
Epigenetic Changes in the Postnatal Cortex
Not all developmental change involves building new structures. Some of it involves changing how existing genes are read. DNA methylation, one of the main ways cells switch genes on and off without altering the genetic code itself, shows distinct patterns in the postnatal cortex compared to prenatal development. A large-scale study of human cortical tissue found that the most dramatic methylation shifts happen during early and mid-gestation, but postnatal changes are different in character, not just a continuation of fetal programming.21Cell Genomics. Dynamic DNA methylation changes shape the developing human cortex
What this means in practical terms is that the postnatal brain is not simply executing a blueprint laid down before birth. Gene activity is being retuned after birth, responding to new environmental inputs that did not exist in the womb: light, sound, social contact, nutrition, and microbial colonization. These epigenetic modifications help explain why experience during infancy can have such lasting biological effects. The genome is the same, but the way it is read keeps changing, and the postnatal period is one of the most active phases of that remodeling.
Why Humans Develop So Slowly
Compared to other primates, the human brain takes an unusually long time to mature. This is not a design flaw. The extended period of postnatal brain development creates a wider window for environmental input to shape neural circuits, which enables the prolonged learning and acquisition of complex technical and social skills that characterize our species.22PubMed Central. Evolution, development, and plasticity of the human brain: from molecules to bones A chimpanzee infant is far more physically capable than a human newborn, but the tradeoff is a narrower window for experience-dependent sculpting of the brain.
This prolonged dependence is expensive. It requires years of caregiving, constant caloric investment, and a stable social environment. But it is also what allows human children to absorb language, cultural norms, tool use, and the kind of flexible problem-solving that no other species matches. The neonatal brain is not unfinished because something went wrong. It is unfinished because finishing slowly is the strategy that made us human.
How Scientists Watch All This Happen
Studying the living neonatal brain has historically been difficult. Traditional MRI requires the subject to lie still in a loud machine, which is not how infants prefer to spend their time. Researchers have increasingly turned to functional near-infrared spectroscopy, or fNIRS, a portable optical technique that shines harmless near-infrared light through the scalp and measures changes in blood oxygenation. It is non-invasive, tolerant of movement, and can be used at the bedside or even while an infant interacts with a caregiver.23PubMed Central. Monitoring neonatal brain function: recent advances in functional near-infrared spectroscopy (fNIRS) Many of the findings about early sensory processing described earlier in this article were made possible by fNIRS, because it can capture brain activity in awake, behaving infants in something close to real-world conditions. As the technology improves, our picture of what is happening inside the neonatal brain will only get sharper.