The hippocampus begins forming around 13 to 14 weeks of gestation and continues developing well into adolescence. Unlike many brain structures that reach their adult form relatively early, the hippocampus has one of the longest developmental timelines in the human brain, with different subregions maturing at different rates over nearly two decades.
First Appearance in the Fetal Brain
By 13 to 14 weeks of gestation, the hippocampus is visible as an unfolded structure on the inner surface of the developing temporal lobe. At this stage, it surrounds a wide-open groove called the hippocampal sulcus. The structure is recognizable but far from functional. It looks nothing like the tightly folded, compact hippocampus of an adult brain.
Before that point, the cells destined to become the hippocampus are already dividing and organizing. The region that will become the dentate gyrus, a key area for forming new memories, starts as a small cluster of cells that separates from the neighboring tissue. Meanwhile, the pyramidal neurons that will form the main hippocampal layers are generated in a zone lining the brain’s ventricles and begin migrating outward toward their final positions. The cells that build the CA3 region settle into place about a day later than those in the CA1 region, at least in animal models, showing that even within the hippocampus, development is staggered.
How the Hippocampus Builds Its Layers
The hippocampus isn’t one uniform structure. It has distinct regions, and each one is assembled through a different migration pattern. Pyramidal neurons in the main body of the hippocampus (called Ammon’s horn) are born near the ventricle wall and migrate radially outward, like spokes on a wheel, to form organized layers. Cells destined for the dentate gyrus take a more complex route: they’re produced in a separate zone, travel in a stream, and then fan out radially to form the granule cell layer.
A signaling protein called Reelin plays a critical role in guiding these migrating neurons to the correct positions. When Reelin signaling is disrupted, neurons end up in the wrong layers, which can cause severe problems with hippocampal function. This cellular choreography is largely complete before birth, but the dentate gyrus remains a site of active neuron production long after.
Early Childhood: Rapid Growth, Then a Plateau
At birth, the hippocampus is far from its adult size, but it grows quickly. Longitudinal brain imaging studies suggest that total hippocampal volume stabilizes in children as early as age 4, meaning the bulk of its physical growth happens in the first few years of life. The nearby entorhinal cortex, which feeds information into the hippocampus, is stable by around age 8.
But “stable volume” doesn’t mean “fully mature.” The internal wiring, the connections between subregions, and the balance of different cell types all continue to change. This is why a 4-year-old has a hippocampus that looks roughly adult-sized on a brain scan but doesn’t yet support the kind of detailed, lasting episodic memories that older children and adults can form.
Why Young Children Forget: The Neurogenesis Trade-Off
One of the most striking features of the developing hippocampus is its extraordinarily high rate of new neuron production in infancy. The dentate gyrus churns out new neurons at a pace that drops off sharply with age. The density of young, immature neurons in this region falls from roughly 1,618 cells per square millimeter at birth to just 12 cells per square millimeter by age 7, and down to about 2 cells per square millimeter by age 13.
This rapid turnover likely explains infantile amnesia, the near-universal inability to remember events from the first two or three years of life. As new neurons integrate into existing hippocampal circuits, they appear to destabilize older memory traces, effectively overwriting them. It’s a trade-off: the young brain prioritizes building and expanding its circuits over preserving specific memories. Infants are excellent at extracting general knowledge from repeated experiences (learning that dogs have four legs, for instance) but poor at retaining the details of any single event.
Childhood Through Adolescence: From General to Specific
Even after infancy, the hippocampus takes years to support adult-like memory. Children between 1.5 and 4 years old show steady improvements in their ability to distinguish between similar locations in spatial memory tasks, a function tied directly to hippocampal maturation. But the shift from storing generalized impressions to encoding highly specific, detailed episodic memories continues through middle childhood and possibly into adolescence.
This protracted timeline may actually be an advantage. By defaulting to generalization early in life, children efficiently build a broad base of world knowledge, extracting patterns and rules from their environment. Detailed, specific memory encoding becomes more useful, and more affordable in terms of brain resources, only after that foundation is in place. The ability to remember the precise source of a piece of information (like who told you something or where you read it) depends more on frontal cortex maturation, while the ability to tell apart similar memories relies specifically on hippocampal development.
Structural Changes During Puberty
Brain imaging studies tracking children from early childhood through late adolescence reveal that hippocampal volume increases rapidly in early life, then plateaus during adolescence. But the shape continues to change. The posterior (back) portion of the hippocampus keeps expanding in thickness and surface area through late childhood and into the teenage years, consistent with the rapid improvements in memory performance seen during this period.
There are also notable sex differences. In females, the left hippocampal head and tail show enlargement by ages 9 to 10, with expansion of the left posterior body becoming more apparent around ages 14 to 15. Males show more modest left hippocampal growth, concentrated mainly in the head and tail. Both sexes show significant nonlinear growth patterns, with the right hippocampus exhibiting particularly dynamic shape changes during this period.
New Neurons in the Adult Hippocampus
The hippocampus is one of the very few brain regions where new neurons continue to be produced in adulthood, though the extent of this process in humans remains debated. One influential study using carbon dating estimated that roughly 700 new neurons are added to the human dentate gyrus each day. Other research has found thousands of immature neurons at various stages of development in neurologically healthy people up to their 90s.
However, some studies paint a more limited picture, finding almost no young neurons in people over 18. The current consensus sits somewhere in between: adult hippocampal neurogenesis persists but at a minimal level compared to childhood. Whether these new neurons play a meaningful role in adult memory or are mostly remnants of a developmental process that has largely wound down is still an open question.
What Can Disrupt Hippocampal Development
Because the hippocampus develops over such a long window, it’s vulnerable to disruption at many points. Prenatal maternal anxiety has been linked to slower hippocampal growth in the fetus, particularly during the late second and third trimesters. The left hippocampus appears especially sensitive to elevated stress hormones early in pregnancy, with measurable reductions in volume observed in children whose mothers experienced significant psychological distress.
Disrupted hippocampal development has also been linked to several neurodevelopmental conditions. In autism spectrum disorder, the relative volume of the hippocampus (adjusted for overall brain size) is reduced in individuals between ages 4 and 18 compared to matched peers. This reduction persists into adulthood, with bilateral hippocampal volume decreases observed in men with autism between ages 40 and 64. The right hippocampus, which is involved in encoding spatial relationships, shows particularly significant changes.
Fragile X syndrome, the most common single-gene mutation associated with autism, produces a different pattern: enlarged but structurally abnormal hippocampi, along with reduced hippocampal activation during visual memory tasks and deficits in episodic memory. In animal models, disrupting neurogenesis in the dentate gyrus during the equivalent of adolescence impairs social interaction, a core feature of autism, suggesting that the timing of new neuron production during development has direct behavioral consequences.