The hippocampus begins forming remarkably early, with recognizable cell layers appearing by about nine weeks of gestation, and it does not reach structural maturity until early adulthood. That decades-long timeline makes the hippocampus one of the brain’s most drawn-out construction projects, and each phase of development carries distinct consequences for memory, spatial reasoning, and emotional regulation. The protracted schedule also means there is a long window during which things can go wrong, or go especially right.
The First Trimester Through Midpregnancy
At around nine gestational weeks, the hippocampal region already contains four distinct layers: a zone lining the brain’s fluid-filled cavities, an intermediate zone, a plate of immature neurons, and a wide outer margin.1Journal of Comparative Neurology. Human fetal hippocampal development: I. Cytoarchitecture, myeloarchitecture, and neuronal morphologic features At this stage the structure is a rough sketch, with all the neurons looking more or less alike. By weeks 15 through 19, individual subfields start to become distinguishable, and they do not all mature at the same pace. The subiculum, the hippocampus’s main output region, leads the way. The ammonic subfields (CA1 through CA3) trail behind, and the dentate gyrus brings up the rear.
During the early second trimester, imaging of fetal brains reveals that overall hippocampal volume grows in a linear fashion while its size relative to the rest of the brain follows a rise-and-fall curve, reflecting the uneven pace at which different brain regions expand.2PubMed Central. Development of the human fetal hippocampal formation during early second trimester Shape analysis shows prominent growth along both the inner and outer edges as the hippocampus rotates into its characteristic curved form. By 32 to 34 weeks of gestation, the CA2 and CA3 subfields have undergone rapid enlargement and morphological maturation, overtaking CA1, which still harbors immature neurons. The dentate gyrus only assumes a mature cellular architecture after 34 weeks.1Journal of Comparative Neurology. Human fetal hippocampal development: I. Cytoarchitecture, myeloarchitecture, and neuronal morphologic features
After Birth and the Dentate Gyrus Catch-Up
The dentate gyrus deserves special attention because so many of its principal neurons are generated after birth. In animal models, the cells that populate the dentate gyrus (called granule cells) are produced mainly postnatally: precursor cells migrate into the region, differentiate into neurons, and reach peak production rates during roughly the first postnatal week. That burst of neurogenesis tapers off by about the first postnatal month, though a reduced rate of new-neuron production continues throughout life.3PubMed Central. Electrophysiological characterization of granule cells in the dentate gyrus immediately after birth The dentate gyrus is the primary gateway for information entering the hippocampus, so its late maturation has an outsized impact on when the hippocampus becomes fully functional.
This delayed schedule helps explain a familiar phenomenon: we cannot remember much from our earliest years. Researchers have proposed that infantile amnesia reflects a developmental critical period in which the hippocampal learning system is still learning how to learn and remember, rather than simply being a matter of memories fading over time.4PubMed Central. Infantile Amnesia: A Critical Period of Learning to Learn and Remember In other words, the hardware is not fully online yet.
Spatial Navigation Comes Online Around Age Two
One of the hippocampus’s signature jobs is building mental maps. In children, the ability to navigate using a map-like sense of space (as opposed to just remembering a familiar route) emerges around 22 months of age. That timing aligns with when spatially tuned cells in the hippocampal circuit mature in rodents.5Learning & Memory. Construction and disruption of spatial memory networks during development
Research in rat pups has shown that some spatial firing patterns are present from the very first time an animal explores an environment, including adult-like directional signals. But the precision and stability of place cells, the neurons that fire when an animal is in a specific location, continue to develop throughout the juvenile period. Grid cells, which create a coordinate system for navigation, appear later but mature quickly once they emerge.6PubMed Central. Development of the hippocampal cognitive map in preweanling rats
A striking recent finding is that the mathematical structure underlying grid-cell networks can be detected in rat pups as early as postnatal day 10, before their eyes and ears have even opened and before they can walk on all four legs. These internally generated spatial maps appear to be preconfigured in the brain and only later get anchored to the external world through experience, producing stable, periodic firing fields by about three weeks of age.7bioRxiv. Toroidal topology of grid-cell activity precedes spatial navigation during development The implication is that the hippocampal navigation system is partly built-in, not entirely learned from scratch.
Childhood Through Adolescence
The hippocampus continues to change size and shape well into adolescence. In a study tracking people aged 4 to 22, most hippocampal subfields showed a nonlinear growth pattern, with volumes increasing until about age 13 to 15 and then leveling off. The right hippocampus was consistently larger than the left, and that asymmetry appeared to grow with age. Sex differences also showed up in several subfields, driven mainly by variation in children under 13.8PubMed Central. Development of hippocampal subfield volumes from 4 to 22 years
Not all subfields follow the same trajectory. Longitudinal data tracking individuals over time found that the subiculum, CA1, and the molecular layer showed nonlinear increases in volume during childhood before tapering, whereas some other subregions, including CA2/3 and the granule cell layer of the dentate gyrus, actually showed linear volume decreases over the same period.9PubMed Central. Longitudinal development of hippocampal subregions from childhood to adulthood These decreases likely reflect synaptic pruning and refinement rather than damage. The subfield that matters most for episodic memory, the ability to recall specific personal experiences, is the CA3/dentate gyrus complex, which shows age-related volume increases into early adolescence, particularly on the right side.10PubMed. Volume of hippocampal subfields and episodic memory in childhood and adolescence
The fact that hippocampal subfields are still gaining volume in adolescence means that the brain’s memory system is a moving target during the school-age years. A ten-year-old’s hippocampus is structurally different from a fifteen-year-old’s, and those differences track with measurable differences in how well they encode and retrieve memories.
Why the Slow Build Matters for Premature Infants
Because the hippocampus is still undergoing critical shaping during the third trimester, babies born very early miss out on some of that protected in-utero development. Very preterm infants with brain injury have been found to have smaller hippocampal volumes at the age when they would have been full-term, and the smallest hippocampi were seen in those who had severe bleeding events in the brain. Smaller hippocampal volumes in these infants were associated with worse motor performance across all groups and, specifically among those with brain injury, with lower cognitive scores at age two.11PubMed Central. Impaired hippocampal development and outcomes in very preterm infants with perinatal brain injury
Even in preterm neonates without major brain injury, the hippocampus shows signs of altered development. Premature infants have been found to have selectively smaller right hippocampal volumes, along with reduced thickness in the hippocampal head on both sides. The connections between the left hippocampus and the rest of the limbic system also appeared impaired compared to full-term peers.12Cerebral Cortex. Hippocampal asymmetry of regional development and structural covariance in preterm neonates These findings suggest that just being born early, even without obvious brain damage, can shift hippocampal development off its expected track.
Prenatal Alcohol Exposure and the Hippocampus
Alcohol during pregnancy is one of the clearest known threats to hippocampal development. In children with fetal alcohol spectrum disorders, several hippocampal subfields, including CA1, CA4, the subiculum, the presubiculum, and the hippocampal tail, have been found to be significantly smaller than in typically developing children.13PubMed Central. Hippocampal subfield abnormalities and memory functioning in children with fetal alcohol Spectrum disorders Animal studies fill in the mechanism: prenatal alcohol exposure reduces the total number of neurons in the hippocampus, lowers the density of dendritic spines (the small protrusions where neurons receive signals), and blunts the hippocampus’s ability to remodel itself in response to environmental enrichment.14PubMed. Effects of prenatal alcohol exposure on the hippocampus: spatial behavior, electrophysiology, and neuroanatomy
The damage is not limited to structure. Reviews of the literature identify several overlapping pathways through which alcohol disrupts the developing hippocampus: it interferes with the birth and differentiation of new neurons, triggers excessive cell death, generates oxidative stress, provokes neuroinflammation, and compromises the formation and function of synapses.15PubMed. Linking prenatal alcohol-induced transcriptomic alterations to hippocampal-associated structural and behavioral deficits Because the hippocampus is one of the last structures to mature in utero, it remains exposed to these insults for a longer window than many other brain regions.
Early Life Stress Leaves a Chemical Signature
The hippocampus is densely packed with receptors for stress hormones, which makes it unusually responsive to the biochemical environment a child grows up in. In animal models, chronic early-life adversity produces lasting deficits in hippocampus-dependent memory and makes the brain more vulnerable to the damaging effects of stress later in adulthood.16PubMed Central. Enduring memory consequences of early-life stress / adversity: Structural, synaptic, molecular and epigenetic mechanisms
Some of the molecular details are well documented. Rats that experienced maternal deprivation early in life were found in adulthood to have significantly reduced levels of BDNF (a protein crucial for neuron growth and survival) and of specific receptor subunits important for learning-related signaling, selectively in the hippocampus.17PubMed. Early maternal deprivation reduces the expression of BDNF and NMDA receptor subunits in rat hippocampus These receptor subunits normally undergo a developmental switch from one type to another as the brain matures, a process driven by neural activity and experience.18PubMed Central. mGluR5 and NMDA receptors drive the experience- and activity-dependent NMDA receptor NR2B to NR2A subunit switch When early adversity disrupts this molecular choreography, the hippocampal circuitry may never fully calibrate itself for efficient adult learning.
Seizures and the Immature Hippocampus
The developing hippocampus is especially seizure-prone. Temporal lobe epilepsy, the most common form of epilepsy in adults, and the associated scarring pattern known as hippocampal sclerosis commonly arise following prolonged seizures during early life, particularly febrile seizures that last an unusually long time.19PubMed Central. Origins of temporal lobe epilepsy: febrile seizures and febrile status epilepticus The exact causal chain between those early seizures and later epilepsy is still debated, but the vulnerability window overlaps with the period of rapid hippocampal neurogenesis and circuit formation, which makes the region more excitable and more susceptible to lasting damage from abnormal electrical activity.
Adolescent Pruning and Psychiatric Risk
During adolescence, the brain undergoes a large-scale pruning process in which excess synapses are eliminated to make remaining circuits more efficient. The hippocampus is not exempt. Genetic research has identified several genes involved in synaptic pruning that are both associated with schizophrenia risk and primarily expressed in the hippocampus. The hypothesis is that if pruning goes awry, particularly in the hippocampus, the resulting mis-wired connectivity could contribute to the kinds of perceptual and cognitive disturbances seen in schizophrenia, which typically emerges in late adolescence or early adulthood, right when pruning is most active.20PubMed. Hippocampal Pruning as a New Theory of Schizophrenia Etiopathogenesis
In autism spectrum disorder, the picture is different. Rather than a pruning deficit, the hippocampus in children with autism appears to be disproportionately large relative to overall brain size. When researchers adjusted for total hemisphere volume, both boys and girls with autism had relatively larger hippocampal volumes compared to typically developing children of similar brain size.21PubMed Central. Understanding Hippocampal Development in Young Children With Autism Spectrum Disorder Whether this represents overgrowth, underpruning, or something else entirely is an open question, but it underscores that atypical hippocampal developmental trajectories are a common thread across several neurodevelopmental conditions.
Does the Hippocampus Ever Stop Developing?
In a sense, no. The hippocampus is one of the very few brain regions where new neurons continue to be born in adulthood, a process called adult neurogenesis. This has been a contentious topic: two high-profile studies published within months of each other reached opposite conclusions about whether adult neurogenesis exists in humans. A careful review of the evidence argued that there is no good reason to abandon the idea that adult-generated neurons contribute to brain plasticity across the lifespan.22PubMed Central. Human Adult Neurogenesis: Evidence and Remaining Questions When brain tissue is handled properly after death, using short fixation times and appropriate processing, thousands of immature neurons can be found in the dentate gyrus even in people in their nineties.23Journal of Neuroscience. Evidences for Adult Hippocampal Neurogenesis in Humans
This ongoing neurogenesis is not just a biological curiosity. It appears to be functionally meaningful for learning and memory, and it responds to environmental input. In mice, voluntary wheel running boosts the proliferation of precursor cells in the dentate gyrus, while environmental enrichment (a more complex and stimulating living space) promotes the survival of those newborn cells. When the two are combined, running first followed by enrichment, the increase in new neurons is roughly 30 percent greater than from either stimulus alone.24PubMed Central. Additive effects of physical exercise and environmental enrichment on adult hippocampal neurogenesis in mice Physical activity, in effect, primes the hippocampus to benefit more from cognitive challenge.
How the Decline Compares Across Species
The rate at which hippocampal neurogenesis declines with age follows a strikingly consistent exponential curve across mammals. Whether you look at mice, rats, voles, marmosets, macaques, or foxes, the drop-off in new cell production relative to total neuron count is chronologically similar. Where species diverge is in the relationship between neurogenesis and major life milestones: long-lived animals like primates and foxes have significantly fewer proliferating cells at weaning, at first reproduction, and at average life expectancy compared to rodents at the same life stages.25PubMed. Comparing adult hippocampal neurogenesis in mammalian species and orders: influence of chronological age and life history stage Humans, presumably, sit at the low end of that curve, which may partly explain why age-related memory decline is more of a concern for us than for short-lived species that never live long enough to see it.
Sex Differences in Hippocampal Aging
Once the hippocampus has finished its long developmental climb, it does not hold steady forever. In males, hippocampal total volume declines significantly with age, while in females the change is not statistically significant.26PubMed Central. Sex-specific trajectories of hippocampal aging: structural changes and asymmetry across the lifespan This sex-specific pattern may reflect hormonal protection, differences in vascular risk, or some combination, and it adds another layer to the already complicated picture of hippocampal volume as a biomarker. When researchers measure someone’s hippocampal volume on a brain scan, they need to account for age, sex, and developmental history to know whether the number is genuinely abnormal or just part of that person’s normal trajectory.
Nutrition During the Critical Window
The hippocampus’s extended developmental schedule means its nutritional needs extend over a long period. Iron has emerged as a particularly important nutrient for hippocampal maturation. Research has identified two key mechanisms by which early nutrition shapes brain development long-term: the existence of critical periods during which certain nutrients are non-negotiable for normal growth, and nutrient-driven chemical modifications to DNA packaging that can permanently alter gene expression.27PubMed. Early life nutrition and brain development: breakthroughs, challenges and new horizons Iron deficiency during the prenatal or early postnatal period can impair hippocampal development in ways that supplementation later cannot fully reverse, precisely because the critical window has closed. For parents and pediatricians, this underscores that monitoring iron status is not just about preventing anemia. It is about protecting the brain structure most responsible for learning and memory during the narrow window when it is most susceptible to nutritional insult.