When Does Spinal Cord and Brain Develop?

The spinal cord and brain begin developing around the third week of pregnancy when the neural plate forms, which folds into the neural tube by week 4. This neural tube closes around week 6-7, with the upper portion becoming the brain and the lower portion forming the spinal cord. Development continues throughout pregnancy and extends into early adulthood, with the brain reaching structural maturity around age 25.

The First Month: Foundation Building

The nervous system starts taking shape remarkably early. Around day 15-16 after conception, a thickened layer of cells called the neural plate appears on the embryo’s surface. This plate represents the very beginning of what will become the entire central nervous system.

Over the next several days, the edges of this plate rise and fold inward, creating a groove. By the end of week 4, these edges meet and fuse together, forming a hollow tube—the neural tube. This tube formation happens during the third and fourth weeks of gestation through a process called neurulation, where the neural plate bends and closes to create the neural tube that will become the brain and spinal cord.

The tube’s structure reveals its future: the cranial (head) end expands and will become the brain, while the caudal (tail) end remains narrower and will form the spinal cord. Week 3 marks when the neural plate forms, with the caudal end remaining narrow compared to the cranial end which rapidly expands.

What’s particularly interesting is the timing. The neural tube forms during week 5, along with the heart tube which will pulse at 110 times per minute by week’s end. This means the brain and spinal cord are among the first organs to begin development—even before most women realize they’re pregnant.

Weeks 5-7: Neural Tube Closure and First Signals

Week 5 through 7 marks a critical transition. The neural tube, initially open at both ends, must close completely. The neural tube closes around week 6 or 7, and at this point the cephalad portion (the rudimentary brain) separates into three distinct parts: front brain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon).

This closure isn’t just a structural milestone—it’s a functional one. Around five weeks after conception (approximately seven weeks of gestation), the fetus forms the first synapses in the spinal cord, signaling the beginning of brain activity detectable through advanced imaging. These first synaptic connections in the spinal cord enable the embryo’s earliest movements.

Just one week after these initial connections form, something remarkable happens: movement. Just one week after the first synapses form, these neural connections make possible the very first physical movements, with early arches and curls visible through ultrasound. The pregnant person can’t feel these movements yet, but they’re a sign that the nervous system has begun functioning.

When closure fails, the consequences are serious. Anencephaly results from failure of the neural tube to close at the cephalic end, leading to partial absence of the brain and skull, while spina bifida results from incomplete closure at the caudal end.

Weeks 8-12: Explosive Neuronal Growth

Once the neural tube closes, neuron production accelerates dramatically. The brain must grow at the rate of about 250,000 nerve cells per minute throughout pregnancy to arrive at the more than 100 billion neurons in a newborn baby. This isn’t a typo—that’s four thousand neurons created every single second.

From the time the neural tube closes around week 7, the brain grows at a rate of 250,000 neurons per minute for the next 21 weeks. This period of neurogenesis, or neuron creation, represents one of the most rapid growth phases in human development.

By week 8, the embryo officially becomes a fetus. At 8.5 weeks, the embryo resembles a human, with facial features continuing to develop and long bones beginning to form. The basic structure is now in place, though refinement will continue.

Almost all organs are completely formed by about 12 weeks of pregnancy, with the brain and spinal cord being exceptions—they continue to form and develop throughout pregnancy. This extended development window makes the brain uniquely vulnerable to environmental influences but also remarkably adaptable.

During this period, the fetus also develops more complex movements. By eight to ten weeks of gestation, the fetus starts to move its arms, legs, fingers, and toes, with movements becoming more complex and including stretching, yawning, hiccuping, swallowing, and even thumb-sucking.

Second Trimester: Brain Takes Command

The second trimester sees the brain begin directing bodily functions with increasing sophistication. The three initial brain divisions from week 7 subdivide into five secondary structures that will form all major brain regions.

Studies report that three primary structures form in the sixth gestational week—forebrain, midbrain, and hindbrain. Five secondary structures originate from these in the seventh gestational week: telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.

Around week 16, new abilities emerge. The brainstem begins controlling reflexes that support life after birth, with coordinated sucking and swallowing appearing between 12 and 16 weeks of gestation. These aren’t random movements anymore—they’re purposeful actions controlled by developing brain circuits.

The fetus also starts “practicing” breathing. During the second trimester, the brainstem begins to control rhythmic chest and diaphragm movements preparing the body for breathing air, though the lungs won’t function fully until after birth.

By 24-26 weeks, a critical threshold is reached. By 24 to 26 weeks of gestational age, the fetal brain has matured enough that a baby could potentially survive with medical support, marking a critical point where the nervous system begins taking over more complex regulation of vital functions like heart rate and temperature.

The brainstem develops particularly quickly during this period. The fetal brain stem is almost entirely developed by the end of the second trimester, located just above the spinal cord.

Third Trimester: Rapid Expansion and Refinement

The third trimester brings explosive growth in brain size and complexity. Brain development in the third trimester is marked by rapid neuron development and explosive growth, with the baby’s brain tripling in size during this time, growing from a little over 3 ounces to almost 11 ounces at birth.

The cerebellum, responsible for motor control, becomes the fastest-growing part of the brain. This explains why fetal movement increases so much during these final weeks—the baby is constantly kicking, stretching, and moving as motor circuits mature.

Surface features also develop. The appearance of cortical folds takes place during 24 and 32 weeks of gestation. These folds, called gyri and sulci, dramatically increase the brain’s surface area without requiring a larger skull.

The cerebral cortex—the brain’s outer layer responsible for voluntary movement, memory, and awareness—becomes increasingly active. During the third trimester, the cerebral cortex becomes more active, though babies born prematurely may already show some cortical activity, with full development continuing through the final weeks of gestation.

An interesting capability emerges late in pregnancy: learning. A key sign of advanced fetal brain development during the third trimester is habituation, where the fetus can recognize and then stop responding to repetitive, non-threatening stimuli like a repeated loud noise. This represents one of the earliest forms of learning.

Birth and Beyond: The Extended Timeline

At birth, the brain is far from finished. Although a full-term baby is born with a functioning brainstem and active reflexes, the cerebral cortex is still developing, which is why newborns rely so heavily on caregivers.

The numbers at birth are staggering. The average adult human brain contains approximately 100 billion neurons, of which 20 billion are located in the cerebral cortex, with each cortical neuron having on average 7000 synaptic connections to other neurons.

The first year sees dramatic growth. At birth, the brain is about one quarter of the size of an adult brain, and by age 2, the brain is about 80% of its adult size as neuron circuitry matures and protective glial cells are born.

At birth, the average baby’s brain is about a quarter the size of the average adult brain and doubles in size in the first year, growing to about 80% of adult size by age 3 and 90% by age 5. This rapid growth reflects not just size increases but the formation of connections.

In the first few years of life more than 1 million new neural connections form every second. These connections don’t just appear randomly—they form in response to experiences, relationships, and interactions with the environment.

The process of myelination—wrapping nerve fibers in a fatty insulation that speeds up signals—continues for years. Regions of the brain in sensory and motor areas are myelinated earlier in a process complete around the preschool period, while regions involved in higher cognitive abilities such as the prefrontal cortex are not complete until adolescence or early adulthood.

Childhood and Adolescence: Ongoing Refinement

Brain development doesn’t follow a simple “bigger is better” trajectory. Around early childhood, something counterintuitive happens: the brain begins eliminating connections.

The overproduction of synapses is followed by pruning back of unused and overabundant synapses, with the process of pruning largely experience-driven and dependent on the area of the brain. In visual and auditory regions, pruning completes between ages 4-6. In areas governing higher functions like inhibitory control and emotion regulation, it continues through adolescence.

Cortical grey matter development peaks at approximately 12 years of age in the frontal and parietal cortices, and 14-16 years in the temporal lobes. This doesn’t mean the brain is “shrinking”—it’s becoming more efficient by eliminating unnecessary connections and strengthening important ones.

White matter tells a different story. Cortical white matter increases from childhood (approximately 9 years) to adolescence (approximately 14 years), most notably in the frontal and parietal cortices.

The prefrontal cortex—responsible for decision-making, impulse control, and planning—matures last. Brain areas such as the prefrontal cortex, which subserves higher cognitive functions such as behavioral control, planning, and assessing the risk of decisions, mature later than cortical areas associated with sensory and motor tasks.

By age 14, the brain reaches its full size, but the circuitry continues to rewire until early adulthood, and by age 25, the brain is hardwired with its neural connections.

Critical Factors and Vulnerabilities

The extended development timeline creates both opportunities and vulnerabilities. During the first trimester, the baby is most at risk for damage from things that may cause birth defects, including certain medicines, illegal drug use, heavy alcohol use, and infections such as rubella.

Folic acid plays an outsized role in early development. Decreased folic acid uptake during pregnancy can lead to severe neural tube defects because folic acid is essential in neural tube closure and is a vital component in DNA methylation. Low levels of folic acid in a woman’s body before and during early pregnancy appear to play a part in neural tube defects.

The impact extends beyond the prenatal period. Much of brain development that occurs postnatally is experience-dependent and defined by gene-environment interactions. This means that while genetics provides the blueprint, experiences shape the final structure.

Brain growth during the first three years of life is faster than at any other time, with experiences during this period being crucial, as safe, stable, and responsive caregiving can support strong neural connections and help babies develop emotional and cognitive skills that last a lifetime.

The Spinal Cord’s Parallel Journey

While the brain gets most of the attention, the spinal cord follows its own developmental path. The spinal cord forms from the lower part of the neural tube, with the wall consisting of neuroepithelial cells that differentiate into neuroblasts, forming the mantle layer (gray matter) and marginal layer (white matter).

The spinal cord develops distinct functional regions early on. The ventral part of the mantle layer (the basal plates) forms the motor areas of the spinal cord, while the dorsal part (the alar plates) forms the sensory areas, with an intermediate layer containing neurons of the autonomic nervous system.

By mid-gestation, the spinal cord has established its basic organization, though myelination and refinement continue postnatally. The spinal cord’s relatively earlier maturation compared to higher brain regions reflects the evolutionary priority of basic sensory and motor functions.

Frequently Asked Questions

When is the most critical period for brain development?

The first trimester represents the highest-risk period for major structural defects because the neural tube is forming and closing. However, each stage has its own vulnerabilities. The period from conception through age 3 is considered critical for establishing foundational neural architecture, as the brain is most plastic and responsive to environmental influences during this time.

Can anything speed up or slow down brain development?

Proper nutrition, particularly adequate folic acid in early pregnancy, supports normal development rates. Conversely, alcohol exposure, malnutrition, chronic stress, and certain infections can slow or impair development. However, the brain follows a genetically programmed timeline that generally can’t be accelerated beyond its natural pace—attempting to force early development rarely produces lasting benefits.

Why does brain development take so much longer than other organs?

The brain’s extended development reflects its extraordinary complexity. With 86 billion neurons each forming thousands of connections, the brain requires prolonged experience-dependent refinement that other organs don’t need. This extended plasticity allows humans to adapt to diverse environments and learn complex skills, but it also means the brain remains vulnerable to influences—both positive and negative—for much longer than other body systems.

Does brain development really continue until age 25?

Yes, though what continues is refinement rather than growth. The brain reaches its full physical size around age 14, but myelination and synaptic pruning continue into the mid-20s, particularly in the prefrontal cortex. This explains why risk assessment, impulse control, and long-term planning continue improving through early adulthood.

The journey from a small neural plate to a fully functioning adult brain spans more than two decades. It begins with that first fold of cells in week 3 of pregnancy and doesn’t truly conclude until the mid-20s. This extended timeline reflects both the brain’s remarkable complexity and its dependence on experience to shape its final form. The spinal cord matures somewhat earlier, establishing basic sensory and motor functions, but even it continues refining connections long after birth.

Understanding this timeline helps explain why early experiences matter so profoundly and why different capabilities emerge at different ages. The 250,000 neurons forming every minute during pregnancy aren’t just numbers—they’re the foundation for every thought, movement, and emotion across a lifetime.

Data Sources

1. Cleveland Clinic – Fetal Development: Week-by-Week Stages of Pregnancy (https://my.clevelandclinic.org/health/articles/7247-fetal-development-stages-of-growth)
2. UNSW Embryology – Neural Spinal Cord Development (https://embryology.med.unsw.edu.au/embryology/index.php/Neural_-_Spinal_Cord_Development)
3. NCBI StatPearls – Embryology, Neural Tube (https://www.ncbi.nlm.nih.gov/books/NBK542285/)
4. Healthline – When Does a Fetus Develop a Brain? (https://www.healthline.com/health/when-does-a-fetus-develop-a-brain)
5. Mount Sinai – Fetal Development (https://www.mountsinai.org/health-library/special-topic/fetal-development)
6. Flo Health – Fetal Brain Development Stages (https://flo.health/pregnancy/pregnancy-health/fetal-development/fetal-brain-development)
7. Wikipedia – Development of the Nervous System in Humans (https://en.wikipedia.org/wiki/Development_of_the_nervous_system_in_humans)
8. ZERO TO THREE – When Does the Fetus’s Brain Begin to Work? (https://www.zerotothree.org/resource/when-does-the-fetuss-brain-begin-to-work/)
9. TeachMeAnatomy – Development of the Central Nervous System (https://teachmeanatomy.info/the-basics/embryology/central-nervous-system/)
10. PMC – Brain Development and the Role of Experience in the Early Years (https://pmc.ncbi.nlm.nih.gov/articles/PMC3722610/)
11. UQ Stories – Timeline of Brain Development (https://stories.uq.edu.au/the-brain/2022/timeline-of-brain-development/index.html)
12. NCBI – The Developing Brain: From Neurons to Neighborhoods (https://www.ncbi.nlm.nih.gov/books/NBK225562/)
13. First Things First – Brain Development (https://www.firstthingsfirst.org/early-childhood-matters/brain-development/)
14. PMC – Brain Development During Adolescence (https://pmc.ncbi.nlm.nih.gov/articles/PMC3705203/)
15. PMC – The Neurobiology of Childhood Structural Brain Development (https://pmc.ncbi.nlm.nih.gov/articles/PMC4114219/)
16. Wikipedia – Human Brain Development Timeline (https://en.wikipedia.org/wiki/Human_brain_development_timeline)
17. PMC – Imaging Structural and Functional Brain Development in Early Childhood (https://pmc.ncbi.nlm.nih.gov/articles/PMC4495345/)
18. NCBI – Physical Biology of Human Brain Development (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4495345/)