The human brain develops through five major stages that begin in the third week of pregnancy and aren’t fully complete until your mid-twenties. These stages are: neural tube formation, cell proliferation, neuronal migration, synaptogenesis (the wiring of connections), and myelination with synaptic pruning. Each stage builds on the one before it, and disruptions at any point can ripple forward through later development.
Stage 1: Neural Tube Formation
Brain development starts during the third and fourth week of pregnancy with a process called neurulation. A flat sheet of cells on the embryo’s surface begins folding inward, curling its edges upward until they meet and fuse into a hollow tube. This neural tube is the earliest structure of the central nervous system, and it will eventually become both the brain and spinal cord. By the end of week four, the tube has fully closed and separated from the surrounding tissue.
Almost immediately, the front end of the tube starts ballooning outward into three pouches. These pouches are the embryonic precursors of the forebrain, midbrain, and hindbrain. By the end of the embryonic period (around week eight), the three pouches subdivide into five distinct regions that establish the primary layout of the entire central nervous system. The forebrain splits into two sections: one that will become the cerebral cortex and one that will become structures like the thalamus. The hindbrain also splits into two, while the midbrain stays as a single unit.
This is the stage where folate plays its most critical role. Both folate and choline support DNA synthesis and cell division during this window, and deficiencies in either nutrient are linked to neural tube defects. Folic acid deficiency during this period decreases the number of progenitor cells and increases cell death in the developing brain.
Stage 2: Cell Proliferation
Once the basic tube structure is in place, the brain enters an explosive period of cell production. Progenitor cells lining the inside of the neural tube divide rapidly, generating the neurons and support cells that will eventually populate every region of the brain. The scale is staggering: during peak production, the developing brain generates thousands of new cells every minute.
This proliferation doesn’t happen all at once across the entire brain. Different regions ramp up production at different times, and progenitor cells in different zones cycle at very different speeds. Some populations divide in as little as six hours, while others take days to complete a single division cycle. The timing and rate of proliferation in each zone helps determine both the size and the cell composition of the brain region it feeds into. Most neuronal proliferation happens during the first and second trimesters, though some cell types, particularly the support cells called glia, continue to be produced well after birth.
Stage 3: Neuronal Migration
Newly born neurons face a problem: they’re created deep inside the brain, but most of them need to end up somewhere else entirely. During migration, billions of young neurons travel from their birthplace near the center of the neural tube outward to their final positions, sometimes crossing considerable distances relative to their size.
In layered structures like the cerebral cortex, cerebellum, and hippocampus, neurons get to their destinations by crawling along specialized guide cells called radial glia. These glial cells stretch like cables from the inner surface of the brain to the outer edge, and migrating neurons grip onto them and pull themselves along. When scientists isolate radial glia and immature neurons together in a dish, the neurons spontaneously attach to the glial fibers, assume the elongated shape of migrating cells, and start moving. It’s a built-in behavior.
The order in which neurons arrive determines the layered architecture of the cortex. Earlier-arriving neurons settle into the deeper layers, while later arrivals climb past them to form the outer layers. This inside-out construction pattern is essential. If neurons fail to migrate properly or end up in the wrong layer, the consequences can include epilepsy, intellectual disabilities, or other neurological conditions.
Stage 4: Synaptogenesis
Once neurons reach their destinations, they begin extending branches and forming synapses, the tiny junctions where one neuron communicates with another. This stage, synaptogenesis, is where the brain’s circuitry actually gets wired. It starts before birth and continues intensely through early childhood.
The numbers involved are extraordinary. Beginning around gestational week 34, the brain forms roughly 40,000 new synapses every second. This pace continues well into postnatal life, eventually pushing synaptic density to 140 to 150 percent of adult levels in some regions. The brain deliberately overproduces connections, building far more wiring than it will ultimately keep.
Different brain areas hit their peak connection density on different timelines. The primary visual cortex surges between 3 and 12 months of age. The prefrontal cortex, which handles planning, decision-making, and impulse control, starts building synapses around the same time but doesn’t reach peak density until 8 months and continues adding connections through the second year of life. This staggered timing follows a consistent pattern: sensory areas wire up first, then association areas, then the prefrontal cortex.
This sequence is why the brain has “sensitive periods” for different skills. Vision and hearing have their windows of peak plasticity earliest in life. Language development follows, with receptive language peaking before speech production. Higher cognitive functions have the latest and longest sensitive windows. During these periods, the brain is especially responsive to environmental input, and the right experiences at the right time shape circuits that become much harder to rewire later.
Stage 5: Myelination and Synaptic Pruning
The final stage is really two parallel processes that refine and optimize the connections built during synaptogenesis. Myelination wraps a fatty insulating layer around nerve fibers, dramatically increasing the speed of electrical signals. Synaptic pruning eliminates the connections the brain doesn’t need, streamlining circuits for efficiency.
Myelination begins before birth but continues for decades. It follows a predictable sequence, starting with basic sensory and motor pathways and progressing toward the higher-order association areas. The prefrontal cortex is among the last regions to be fully myelinated, which is a major reason this area doesn’t reach full maturity until the mid-twenties. Before myelination is complete, signals in these circuits travel slower and less reliably, which contributes to the impulsive decision-making and risk-taking typical of adolescence.
Pruning is equally important. The brain eliminates unused or weak synapses while strengthening the ones that get regular use. In the prefrontal cortex, the number of excitatory synapses peaks between ages 5 and 10, then decreases steadily. Estimates suggest that up to 40 percent of excitatory synapses in the prefrontal cortex are pruned between ages 10 and 30. This isn’t damage. It’s optimization. A brain with fewer, stronger connections processes information more efficiently than one cluttered with weak, redundant wiring.
Pruning is heavily shaped by experience. Synapses that fire frequently get reinforced, while those that rarely activate are tagged for removal. This is the biological basis of the “use it or lose it” principle in brain development, and it’s why enriched environments, consistent learning, and varied experiences during childhood and adolescence have lasting effects on brain structure.
The Brain Keeps Changing in Adulthood
Although the five major stages are largely complete by early adulthood, the brain doesn’t become static. The hippocampus, a region central to memory, retains populations of neural stem cells that continue producing new neurons throughout life. Recent multiomic sequencing studies of human hippocampal tissue have confirmed the presence of neural stem cells, immature neurons, and active developmental pathways in adults of all ages, including older adults with exceptional memory capacity. The rate of new neuron production declines with age, and it drops further in Alzheimer’s disease, but the basic machinery for generating new brain cells persists.
Synaptic plasticity also continues throughout life, allowing existing circuits to be remodeled by learning and experience. The adult brain can’t rewire itself with the same ease it had during childhood sensitive periods, but it retains meaningful flexibility. Physical exercise, learning new skills, and social engagement all promote ongoing plasticity and support the maintenance of healthy brain circuits well into old age.