Fetal Brain Anatomy and Its Formation

The human brain begins forming in the third week after conception, when a flat sheet of embryonic cells folds into a hollow tube that will eventually give rise to the entire central nervous system. Over the next several months, this tube transforms through a staggering sequence of events: cells multiply by the billions, migrate to precise locations, wire themselves together, and begin folding into the wrinkled shape we recognize at birth. Each stage builds on the one before it, and the timing matters enormously. A disruption of even a few days during a critical window can reshape the brain’s architecture in lasting ways.

The Neural Tube and the Very First Steps

Everything starts with a structure called the neural tube. Around the third to fourth week of gestation, cells along the back of the embryo thicken into a plate, then curl upward and fuse into a closed tube. The front end of this tube will become the brain; the back end will become the spinal cord. Closure does not happen all at once. It begins in the middle and zips outward in both directions, with the last openings sealing around day 25 to 28. The molecular machinery driving this process is remarkably complex, involving signaling pathways and transcription factors that coordinate cell shape changes like apical constriction, convergent extension, and interkinetic nuclear migration.¹

When the neural tube fails to close completely, the result is a neural tube defect. These include spina bifida, where the spinal portion remains open, and anencephaly, a fatal condition in which much of the brain never forms.² This is why folic acid supplementation before and during early pregnancy is so heavily emphasized: much of the neural tube’s closure happens before many people even know they are pregnant.

How the Three-Vesicle Brain Takes Shape

Once the tube is sealed, its front end balloons outward into three swellings called primary brain vesicles. These appear by the end of the fourth week and represent the earliest anatomical blueprint. The front vesicle becomes the forebrain, which will eventually house the cerebral cortex and deep structures like the thalamus. The middle vesicle becomes the midbrain. The rear vesicle becomes the hindbrain, giving rise to the cerebellum and brainstem. By around five weeks, these three vesicles subdivide further into five distinct regions, each destined for different functional roles.

Early sulci, the grooves that begin to define brain topography, are already detectable by around 14 gestational weeks. A recent atlas project documented that the lateral fissure, calcarine sulcus, and cingulate sulcus are visible at this age, with the central and postcentral sulci appearing by 24 weeks.³ These landmarks are important for clinicians reading prenatal MRI scans, but they also show that the brain’s geography is being sketched out well before the third trimester.

Radial Glia and the Birth of Neurons

The walls of the early neural tube are lined with progenitor cells that serve as the brain’s factory workforce. Among the most important are radial glial cells, long scaffolding-like cells that stretch from the inner surface of the tube to the outer surface. For a long time, researchers thought these cells were just structural supports. It turns out they are the primary source of neurons throughout the brain. Fate-mapping studies have shown that the vast majority of neurons in all brain regions derive from radial glia, and that radial glial populations in different regions pass through their neuron-generating phase at distinct time points.⁴

Radial glia do not just produce neurons during fetal life and vanish. They also transform into a different kind of stem cell after birth. Recent work using live imaging and three-dimensional electron microscopy found that the birth process itself plays a role in this transformation: preterm birth disrupts the interactions between radial glia and blood vessel cells, impairing both the structure and the stem-cell properties of the adult neural stem cells that radial glia become.⁵ The timing of birth, in other words, is not just a logistical milestone. It is woven into the biology of brain cell development.

Neuronal Migration and the Routes Neurons Travel

Once neurons are born near the inner surface of the developing brain, they need to reach their final positions, which can be millimeters to centimeters away in a rapidly expanding organ. They travel along two main routes. Radial migration carries neurons outward along the fibers of radial glia, populating the cortical plate layer by layer. Tangential migration sends neurons sideways from a structure called the ganglionic eminence, delivering inhibitory neurons that weave into the developing cortex from a different direction. Diffusion MRI of human fetal brains has confirmed both pathways as distinct three-dimensional patterns of coherent movement.⁶

These two migration routes do not develop symmetrically. In fetuses 22 gestational weeks or younger, the volume of radial migration pathways is significantly larger in the left hemisphere than the right, and larger in posterior regions than anterior ones. Tangential pathways, by contrast, show no such asymmetry.⁷ This early left-right difference in radial migration could help explain why some brain regions end up with more excitatory neurons on one side, a structural asymmetry that eventually contributes to functional lateralization, like language dominance in the left hemisphere.

The Subplate and Cortical Construction

Beneath the developing cortical plate lies a temporary but crucial compartment called the subplate. For a window of fetal development, the subplate is actually thicker than the cortex itself. It is made up of scattered neurons, rich extracellular matrix, and a tangle of axons arriving from the thalamus and from distant cortical regions. The subplate serves as a kind of staging area: incoming connections “wait” there before finding their correct targets in the cortex above. It also generates early spontaneous activity, both synaptic and non-synaptic, and mediates the cortex’s first responses to thalamic input.⁸

Most of the subplate’s neurons eventually die off or are incorporated into the deepest cortical layers after birth. But during the second and early third trimesters, the subplate is indispensable. Damage to it during premature birth has been linked to later neurodevelopmental problems, because the subplate acts as the scaffolding on which cortical wiring is built.

When the Brain Starts to Fold

A human brain at 20 weeks looks almost smooth. By 28 weeks, the major grooves and ridges are taking shape. By birth, the cortex is deeply folded into the familiar pattern of gyri and sulci. This folding, called gyrification, is not random. It follows a reproducible sequence, and imaging studies that tracked 40 normal fetal brains between 20 and 28 gestational weeks found that specific regions fold at different rates. The cortical plate undergoing folding accelerates its growth relative to the deeper cerebral mantle, with the precentral and postcentral gyri, the right superior temporal gyrus, and the opercula showing especially increased local growth.⁹

The growth driving gyrification is not uniform in all directions, either. Tensor-based analysis of fetal brain scans reveals that folding produces directional growth patterns, and these patterns are asymmetric in regions like the area around the Sylvian fissure and the medial frontal lobe.¹⁰ The reason the cortex folds at all is still debated, but the leading idea is that the outer cortical layers grow faster than the inner ones, and this differential expansion buckles the surface. Folding is a way to pack an enormous surface area into a skull that can fit through the birth canal.

Building the Cerebellum

The cerebellum develops on its own timeline, lagging behind the cerebral cortex in some respects but ultimately becoming one of the brain’s most neuron-dense structures. Its main neuron type, the granule cell, is produced in a temporary outer layer called the external granular layer. Precursors in this layer are driven to multiply by sonic hedgehog signaling, the same pathway active in many other developing tissues.¹¹ The external granular layer thickens substantially between 16 and 28 gestational weeks, peaking at roughly 50 micrometers, then gradually thins during the late third trimester as its cells migrate inward to their permanent home in the internal granular layer.¹²

All cerebellar granule cells originate from a structure called the upper rhombic lip, which seeds the external granular layer as a secondary germinal zone that amplifies the precursor population before sending postmitotic granule cells on a radial migration to their final position.¹³ Because the cerebellum is still actively developing late in pregnancy and into the first year of life, it is particularly vulnerable to insults during the third trimester and in premature infants.

Early Wiring and Synaptic Activity

Long before birth, neurons begin extending axons to find their partners. One of the brain’s largest wiring projects is the corpus callosum, the thick band of fibers connecting the two hemispheres. In the developing human brain, midline glial structures guide callosal axons across the gap between hemispheres, along with pioneer axons from the cingulate cortex that blaze the trail. These same guidance mechanisms were first described in mice, but histological analysis of human fetal brains has confirmed that analogous structures are present during human development.¹⁴

Once axons reach their targets, synapses begin to form. The earliest synapses in the fetal brain are thought to play a role not in processing sensory information but in organizing the circuitry itself. These “pioneering” synapses generate spontaneous activity and resting-state patterns before any outside experience, essentially helping to wire the brain’s basic architecture through internal electrical conversations.¹⁵ Fetal sleep states further refine this process: the active and quiet stages of fetal sleep provide dual modes of activity-dependent synaptic organization.¹⁶

Blood Vessels and the Vulnerability of Prematurity

A growing brain needs a growing blood supply, and vascular development proceeds in tandem with neurogenesis. One region of particular clinical interest is the germinal matrix, a highly vascularized zone near the brain’s ventricles where neurons and glia are born. The vessels in this zone are immature and fragile, which is why germinal matrix hemorrhage is one of the most common and serious complications of premature birth. Studies of human fetuses and premature infants have found that the germinal matrix contains consistently higher levels of vascular endothelial growth factor and other angiogenic markers compared to the surrounding white matter and cortex.¹⁷

The problem is essentially one of timing: the rapid vessel remodeling that is normal during midgestation leaves vessels in a vulnerable state. Experimental overexpression of vascular growth factors near the ventricles produces a dense network of loosely connected endothelial cells that resembles the immature vasculature of the human fetal brain, and this fragile network is prone to hemorrhage around the time of birth.¹⁸ For babies born very early, the germinal matrix has not yet matured past this window, leaving them exposed.

Microglia Enter the Scene

The brain’s resident immune cells, microglia, are not born in the brain. They originate from the yolk sac and migrate into the developing brain around four and a half to five and a half gestational weeks, eventually making up roughly 10 to 15 percent of total brain cells. Once inside, microglia do far more than immune surveillance: they clear dying cells, prune excess synapses, and secrete signaling molecules that help maintain the developing brain’s equilibrium. Researchers have recently identified what appears to be a secondary formation center for microglia in the human fetal brain, a dense aggregate near the basal caudate that appears after 13 gestational weeks. Proliferating microglia in these aggregates accounted for about 23.5 percent of the total population, compared to about 6 percent in scattered microglia elsewhere.¹⁹ The timing coincides with the rapid expansion of the brain’s outer subventricular zone, suggesting the secondary center may help compensate for a growing brain’s increased demand for microglia.

Myelination Begins Before Birth

Myelin, the insulating sheath that allows nerve signals to travel quickly, begins its formation well before delivery. Oligodendrocyte precursors, the cells that produce myelin, are detectable in the human fetal brain as early as five gestational weeks and progress through a defined sequence of maturation stages through midgestation.²⁰ Actual myelin wrapping of axons starts in the brainstem and spinal cord during the third trimester and continues for years after birth, with some cortical regions not fully myelinated until early adulthood. Premature infants are at particular risk for white matter injury precisely because oligodendrocyte precursors at certain stages of development are highly sensitive to oxygen fluctuations and inflammation.

What Happens When Migration Goes Wrong

Because cortical construction depends on cells arriving at the right place at the right time, errors during migration produce a family of malformations with distinctive brain appearances. These disorders can affect proliferation, migration, or the final organization of neurons, and each stage of disruption leaves a recognizable signature.²¹

• Microcephaly or macrocephaly: Too few or too many cells produced during neurogenesis, resulting in a brain that is abnormally small or large.
• Periventricular heterotopia: Neurons that never leave their birthplace near the ventricles, forming clusters of misplaced gray matter.
• Lissencephaly: A smooth brain surface caused by widespread failure of neuronal migration, often linked to mutations in the LIS1 or DCX genes. LIS1 mutations tend to produce more severe malformations in the back of the brain, while DCX mutations affect the front more severely in males and cause a different pattern in females.²²
• Polymicrogyria: An excess of abnormally small folds, reflecting disrupted late-stage cortical organization.

Advances in exome sequencing and high-resolution MRI are rapidly expanding the list of genes tied to these conditions and refining how they are classified.²³ For families, this means genetic testing after a prenatal or postnatal diagnosis of a cortical malformation can sometimes identify a specific cause, inform recurrence risk, and guide follow-up care.

Maternal Health and the Fetal Brain

The fetal brain does not develop in isolation. Maternal conditions during pregnancy can alter its trajectory. Animal studies have shown that prenatal stress and maternal high-fat diet both increase markers of inflammation and oxidative stress in the placenta and fetal brain, with effects that differ by sex. In one mouse study, placental function was specifically disrupted in males, while female fetal brains showed increased expression of inflammation-related genes. An antioxidant (N-acetyl-cysteine) buffered these effects in both sexes.²⁴

Human research is beginning to link placental inflammation directly to fetal brain connectivity patterns. In one study, fetuses exposed to chronic placental inflammation showed altered resting-state functional connectivity across multiple brain networks, with the greatest differences in prefrontal, cerebellar, visual, and motor regions. Connectivity differences in the cerebellar-visual network were the strongest prenatal predictor of autism-related traits at age three.²⁵ These findings do not mean placental inflammation causes autism, but they suggest the placenta is a meaningful intermediary between maternal health and fetal brain wiring.

Fetal Movement and the Beginnings of Sensation

By the second trimester, the fetus is not a passive occupant. It moves, and those movements are not just reflexes. Observations and experiments on fetuses indicate that early motor activity allows the fetus to explore its body and the space around it, creating a form of primitive motor babbling. Over time, this activity becomes increasingly organized, showing characteristics of repetitive sensorimotor behavior, intentionality, and oriented reactions to sensory stimulation.²⁶ Touch sensitivity emerges before hearing and vision, and fetal responses to sound are detectable in the third trimester. This sensory experience does not just reflect brain development; it actively shapes it, reinforcing the circuits that mediate sensation and movement.

What Makes the Human Fetal Brain Different

Compared to other primates, the human brain grows disproportionately large and complex during fetal life. Comparative single-cell studies of primate brain development have found that while the basic cellular makeup of the cortex is broadly conserved across species, the progenitor cells that build it show significant evolutionary divergence. Human neural progenitors have undergone extensive rewiring in growth factor and extracellular matrix signaling pathways. One human-specific progenitor marker, ITGA2, when introduced into fetal mouse cortex, increased progenitor proliferation and the proportion of upper-layer neurons, the very cell types that expanded most dramatically in human evolution.²⁷ The implication is that much of what makes the human brain distinctive traces back not to novel cell types but to changes in how the same progenitor cells behave, how long they divide, how many neurons they produce, and which genes they turn on.

Imaging the Fetal Brain in Practice

Clinicians rely on ultrasound and fetal MRI to track brain development and detect anomalies. Ultrasound is the standard screening tool during routine prenatal care, and it performs well for confirming normal anatomy. When ultrasound raises a concern or cannot provide enough detail, intrauterine MRI offers higher-resolution images that can clarify findings. A systematic review and meta-analysis confirmed the strong diagnostic performance of both modalities, emphasizing that ultrasound remains an appropriate first-line screening technique and that MRI adds value when abnormalities are suspected.²⁸ Fetal MRI has become especially important for evaluating cortical malformations, posterior fossa abnormalities, and ventriculomegaly, conditions where the additional soft-tissue contrast of MRI can change clinical management.