The fetal brain develops across all three trimesters, not just one. But the work happening in each phase is dramatically different. The first trimester lays the basic structure, the second trimester builds and wires billions of neurons, and the third trimester is when the brain quadruples in size and begins processing sensory input from the outside world.
First Trimester: The Foundation Forms
Brain development begins earlier than most people realize. During weeks three and four of pregnancy, a flat strip of cells called the neural tube folds and closes. The upper portion of this tube becomes the brain and skull, while the lower portion becomes the spinal cord. This is why folic acid before and during early pregnancy is so critical: the neural tube closes before many women even know they’re pregnant, and defects at this stage can cause serious conditions like spina bifida.
By roughly the fifth week after conception (about seven weeks of gestation), the first synapses form in the spinal cord. These are the earliest connections between nerve cells, and they mark the true beginning of neural activity in the embryo. Over the remaining weeks of the first trimester, the brain divides into three primary regions that will eventually become the forebrain, midbrain, and hindbrain. At this stage the brain looks nothing like the wrinkled organ you’d recognize. It’s smooth, tiny, and just getting its basic architecture in place.
Second Trimester: Neurons Move Into Position
The second trimester is when the brain transforms from a simple tube into a complex, layered organ. Between weeks 12 and 20, billions of newly formed neurons migrate from deep within the brain toward the outer surface, where they’ll eventually form the cortex. Think of it like workers moving from a central staging area to their assigned positions throughout a building. Each neuron has to travel to a precise location, and errors during this migration can lead to developmental disorders.
By around week 22 to 24, once most neurons have reached their destinations, the next phase begins: organization. Neurons start sprouting branches (axons and dendrites) and forming the earliest connections with neighboring cells. This wiring process, called synaptogenesis, continues well into infancy, but its origins trace back to the middle of pregnancy.
The cerebellum, the brain region responsible for coordination and balance, also hits a growth spurt during this period. Starting around week 17, the cerebellum begins growing faster than the rest of the brain. Between weeks 15 and 22 alone, its volume increases more than fivefold. This accelerated growth continues through the third trimester and into the first years of life.
Second Trimester: The Brain Starts to Fold
Between the fifth and sixth months of pregnancy, the brain’s surface begins forming the distinctive folds and grooves that give it its wrinkled appearance. These folds aren’t just cosmetic. They dramatically increase the brain’s surface area, allowing far more neurons to pack into the skull than a smooth brain ever could.
Around weeks 24 to 25, several major landmarks emerge almost simultaneously: the central sulcus (which separates the motor and sensory areas), grooves in the frontal lobe, and the superior temporal sulcus (important for language processing later in life). Between weeks 20 and 28, the brain’s cortical surface area roughly triples, expanding at a rate of about 11 square centimeters per week. Researchers using fetal MRI have also detected subtle left-right asymmetries in folding patterns during this window, hinting that the brain’s lateralization, its tendency to specialize each hemisphere for different tasks, begins before birth.
Third Trimester: Rapid Growth and Early Function
The third trimester is the most explosive period of brain growth. The brain increases roughly fourfold in size between weeks 28 and 40, accompanied by a surge in surface folding as smaller, more detailed grooves appear across the cortex. Multiple processes are running in parallel during these final months: neurons are forming synapses at an accelerating rate, the cortex is organizing into its mature six-layered structure, and the protective insulation around nerve fibers (myelin) is starting to form.
Myelination, the process of coating nerve fibers to speed up electrical signals, begins in the brainstem around week 20 but picks up significantly in the third trimester. By weeks 34 to 36, myelination of the spinal nerve roots is well advanced. This insulation is essential for coordinated movement and fast signal transmission, and it continues developing for years after birth.
Around week 26, the motor cortex in the frontal lobe likely begins sending direct signals to muscles in the limbs and head. This is when fetal movements start becoming more coordinated and purposeful rather than the reflexive twitches of earlier months. By weeks 26 to 28, the brain is also developed enough for the fetus to respond to external sounds. This is why some parents notice their baby startling at loud noises or seeming to calm in response to familiar voices during the final trimester.
What Keeps Developing After Birth
Birth doesn’t mark the finish line for brain development. The cortical organization process that begins around week 22 continues through infancy, with synapse formation peaking in the first two years of life. Myelination continues into a person’s mid-twenties, particularly in the prefrontal cortex, the region responsible for planning, impulse control, and decision-making. The brain at birth is roughly a quarter of its adult size, and most of its growth in the first year comes from myelination and synapse formation rather than the creation of new neurons.
Nutrients That Support Fetal Brain Growth
Because the brain is developing throughout pregnancy, nutritional support matters in every trimester. Two nutrients stand out for their direct role in fetal brain development.
Iodine is essential for thyroid hormone production, which in turn regulates brain maturation. The World Health Organization recommends 250 micrograms of iodine daily during pregnancy. In regions with low iodine in the diet, supplementation of 150 to 200 micrograms per day is recommended starting before conception and continuing through breastfeeding. Even mild iodine deficiency during pregnancy has been linked to lower cognitive scores in children.
DHA, an omega-3 fatty acid found in fatty fish and algae-based supplements, is a major structural component of brain cell membranes. It accumulates most rapidly in the fetal brain during the third trimester, when growth is fastest. Folic acid remains critical in the first trimester for neural tube closure, while iron supports oxygen delivery to the rapidly growing brain throughout pregnancy. No single nutrient matters only in one trimester, because the brain never stops building during the full 40 weeks.