A baby’s brain starts developing remarkably early, just three weeks after conception. At that point, a flat strip of cells called the neural plate forms along the embryo’s back, and by the end of week four, it folds and seals into the neural tube, the structure that becomes the entire brain and spinal cord. This tube is fully closed by day 28 after conception, often before many people even realize they’re pregnant.
From that tiny tube, the brain undergoes an extraordinary transformation over the remaining months of pregnancy, building billions of neurons, wiring them together, and folding into the wrinkled organ we recognize at birth.
Weeks 3 Through 4: The Neural Tube Forms
Around day 15 after conception, a structure called the primitive streak appears on the embryo. Over the following days, it triggers the formation of the neural plate, a thickened band of cells that develops raised edges (neural folds) with a groove running down the middle. By the end of week three, this plate is visible.
During week four, the edges of the neural folds rise up and fuse together, zipping shut to form the neural tube. Closure is complete by day 28 post-conception. If this tube doesn’t close properly, serious birth defects of the brain or spine can result, which is why folic acid intake matters so much before and during early pregnancy. The CDC recommends that all women who could become pregnant get 400 micrograms of folic acid daily, because the protective window closes before most pregnancies are confirmed.
Weeks 5 Through 8: The Brain Takes Shape
Once the neural tube is sealed, its front end begins to balloon outward into three distinct regions: the forebrain, midbrain, and hindbrain. These three vesicles, which differentiate between weeks five and eight, are the architectural blueprint for every major brain structure. The forebrain will eventually become the cerebral cortex (responsible for thinking, memory, and sensory processing), the midbrain will handle reflexes and relay signals, and the hindbrain will develop into the cerebellum and brainstem, controlling balance, breathing, and heart rate.
This is also when the basic body plan of the nervous system is being laid down. The embryo transitions from a tube into something with recognizable head and tail ends, and the brain regions begin specializing at a rapid pace.
Neuron Production Peaks Before Mid-Pregnancy
New brain cells, or neurons, start being produced around day 42 after conception, roughly the sixth week of pregnancy. Production ramps up quickly and is largely finished by mid-pregnancy. In the cortex specifically, neuron generation wraps up around day 108 post-conception, which falls near the end of the first trimester or early second trimester depending on how you count.
The sheer speed is staggering. During peak production, the fetal brain generates hundreds of thousands of neurons per minute. As neurons are produced, they migrate outward from deep zones near the center of the brain to their final positions in the cortex and other structures. This migration is carefully choreographed: deeper cortical layers form first, and newer neurons travel past them to build the outer layers.
Second Trimester: Wiring Begins
With most neurons in place by mid-pregnancy, the brain’s focus shifts to building connections. Neurons extend long fibers toward distant targets, forming the major pathways that will carry signals throughout the brain. These essential fiber pathways begin forming in the later part of the second trimester and are largely established by around 26 weeks.
One key example: neurons in the cortex’s fifth layer, which control voluntary movement, send fibers all the way down to the spinal cord. Those fibers reach the cervical spine by about 24 weeks of gestation. At the same time, fibers from the thalamus (the brain’s sensory relay station) grow toward the cortex. They arrive in a waiting zone beneath the cortex between weeks 13 and 15 but don’t actually penetrate into the cortex and form functional connections until after 24 weeks.
This means the second trimester is when the brain transitions from a collection of individual cells into an interconnected network. Synapses, the junctions where neurons communicate, begin forming in the cortical plate around week 23.
Third Trimester: Folding, Insulation, and Sensory Response
The third trimester is when the brain’s surface undergoes its most dramatic visual change. Around 20 weeks, the cortex begins folding into the ridges and grooves that give the brain its wrinkled appearance. This folding accelerates through the third trimester and continues after birth. The folds aren’t cosmetic: they pack a much larger surface area of cortex into the skull, which is directly tied to the brain’s processing power.
Starting around week 25, the brain begins insulating its nerve fibers with myelin, a fatty coating that speeds up electrical signals. Myelination starts in deeper brain structures involved in movement and sensory relay, then gradually extends to the cortex closer to 35 weeks. This process continues well into childhood, but its prenatal start is a turning point for how efficiently the brain can transmit information.
Sensory processing also comes online during this period. Research first established in the 1980s showed that babies can hear while still in the womb and distinguish voice patterns before birth. Studies have also found that third-trimester fetuses respond to visual patterns (red dots of light shone into the womb), confirming that the fetal brain is actively processing external stimuli months before delivery.
Why the Third Trimester Is a Vulnerable Window
The period from 24 weeks onward is sometimes called the “window of vulnerability” because the brain is undergoing so many simultaneous changes: forming synapses, myelinating, folding, and building cortical layers. Disruptions during this time can have outsized effects.
Maternal stress hormones, particularly cortisol, can cross the placenta and influence the developing brain. During the first half of pregnancy, cortisol promotes the uptake of serotonin, which transfers from the placenta to the fetal brain and can alter its chemistry. High cortisol exposure during the period when the hippocampus (a memory and stress-regulation center) is developing can cause changes in gene expression that affect how the child responds to stress later in life.
The timing of stress exposure matters. Stress during the first trimester has been linked to changes in cognitive development in offspring, while stress in the third trimester appears to affect social and emotional behavior more strongly. This doesn’t mean any amount of stress is harmful, as moderate, everyday stress is a normal part of pregnancy. But chronic or severe stress during these sensitive windows has measurable effects on fetal brain wiring.
What Supports Early Brain Development
Because brain development starts before most pregnancies are detected, the most impactful nutritional step happens before conception. Folic acid at 400 micrograms daily helps ensure the neural tube closes properly during that critical fourth week. Women who have previously had a pregnancy affected by a neural tube defect are typically advised to take a much higher dose of 4,000 micrograms daily.
Beyond folic acid, adequate nutrition throughout pregnancy supports each subsequent phase. Iron carries oxygen to rapidly dividing brain cells. Omega-3 fatty acids contribute to the cell membranes that neurons need to form connections. Iodine supports the thyroid hormones involved in myelination. These aren’t extras: they’re building materials for a brain that goes from a flat strip of cells to a 100-billion-neuron organ in roughly 37 weeks.