The fetal nervous system begins forming remarkably early, starting around day 15 after conception when the first structural precursor appears. By the end of the fourth week of pregnancy, the foundational structure of the entire nervous system, the neural tube, is already complete. From there, development unfolds in stages over the full course of pregnancy, with new neurons being produced, connections forming, and the system gradually becoming functional.
The Neural Tube Forms in Weeks 3 and 4
The very first step happens around day 15 after conception, when a structure called the primitive streak forms on the surface of the embryo. This streak helps reorganize the embryo’s cells into distinct layers, one of which will become the nervous system. By the end of week three, the edges of this layer begin folding upward to create the neural plate, a flat sheet of cells that is the direct precursor to the brain and spinal cord.
During week four, those raised edges (called neural folds) continue folding inward and fuse together at the midline, forming a hollow tube. This is the neural tube, and by the end of week four it is fully closed. The front end of the tube will become the brain. The rest becomes the spinal cord. If the tube fails to close properly between days 21 and 28 after conception, it results in neural tube defects like spina bifida or anencephaly. This is why folic acid intake before and during early pregnancy matters so much: the window of vulnerability is extremely narrow and often passes before a person even knows they’re pregnant.
The Brain Divides Into Regions by Week 5
Once the neural tube closes, the front portion quickly begins to balloon outward into three distinct sections called brain vesicles. These eventually become the forebrain, midbrain, and hindbrain. The forebrain later develops into the cerebral hemispheres and deep structures responsible for consciousness, sensory processing, and hormone regulation. The hindbrain gives rise to the cerebellum and the brainstem, which controls basic functions like breathing and heart rate. This initial regional organization is largely in place by the end of the fifth week.
By the end of the eighth week, all major organ systems, including the basic architecture of the brain, have formed. This is the end of the embryonic period and the beginning of the fetal period, when the focus shifts from building new structures to refining and growing the ones already in place.
Neurons Start Forming Around Day 42
Neuron production begins on approximately day 42 after conception, or about six weeks into pregnancy. At first, only a small number of precursor cells switch from simply multiplying to producing actual neurons. But the pace picks up dramatically over time, and the bulk of neuron production is complete by midgestation, around 18 to 20 weeks. For the outer layer of the brain (the cortex), neuron production wraps up by roughly day 108 after conception, or about 15 to 16 weeks.
As neurons are produced, they don’t stay where they were made. They migrate outward from deep within the brain to their final positions, a process that is essential for building the layered structure of the cortex. Disruptions during this migration period can lead to significant developmental problems, because neurons that end up in the wrong place can’t form the right connections.
The First Synapses and Brain Activity
Around five weeks after conception (approximately seven weeks of gestation), the fetus forms its first synapses in the spinal cord. Synapses are the junctions where one nerve cell communicates with another, and their appearance marks the very beginning of a functioning nervous system. This is the earliest point at which any neural signaling can be detected.
From this starting point, synapse formation accelerates. The connections spread from the spinal cord up into the brainstem and eventually into higher brain regions. This bottom-up pattern is a consistent theme in nervous system development: simpler, more primitive circuits come online first, and the complex circuitry of the cortex matures last.
Reflexes Emerge in the Second Trimester
As neural circuits become functional, the fetus begins to move. The grasp reflex, where the hand closes around an object, has been observed as early as 16 weeks of gestation. Fetuses have been seen grasping the umbilical cord during ultrasound exams around this stage. These early movements are reflexive rather than intentional, driven by spinal cord and brainstem circuits rather than conscious decision-making in the cortex.
Other reflexes and movements develop across the second trimester, including swallowing, sucking, and responses to touch. These are signs that the lower portions of the nervous system are wiring up and beginning to function, even though higher brain processing is still months away from maturity.
Sensory Pathways and Pain Perception
One of the most discussed milestones is when the fetus can perceive sensory input, particularly pain. This depends on connections forming between the thalamus (a deep brain relay station) and the cortex (where conscious experience is thought to occur). These connections begin developing after 23 to 24 weeks of gestation, when nerve fibers from the thalamus first reach the outer layers of the developing cortex.
The connections are generally considered functional between 24 and 28 weeks of gestation. Under the widely held view that cortical involvement is necessary for conscious pain perception, this window represents the earliest point at which a fetus could experience pain in any meaningful sense. Before this stage, the fetus can respond reflexively to stimulation, but those responses are processed at the spinal cord and brainstem level without cortical awareness.
Myelination Begins in the Fifth Month
Myelin is the insulating coating that wraps around nerve fibers and allows electrical signals to travel quickly and efficiently. Without it, nerve signaling is slow and unreliable. In human development, myelination follows a predictable sequence: peripheral nerves first, then the spinal cord, and finally the brain.
The motor nerve roots of the spinal cord begin to myelinate during the fifth fetal month, around 17 to 20 weeks of gestation. But the process is far from finished at birth. The brain is not almost completely myelinated until the end of the second year of life. This prolonged timeline explains why newborns and infants have limited coordination, slow reaction times, and gradually improving motor and cognitive abilities over their first two years. The nervous system at birth is structurally complete but functionally immature.
Why Timing Matters for Pregnancy
Because the nervous system develops across the entire pregnancy, there is no single “safe” or “dangerous” window. But some periods carry higher stakes than others. The neural tube closes between days 21 and 28, making the third and fourth weeks after conception the most critical period for structural defects. Exposure to certain medications during this window, including some anti-seizure drugs, has been linked to neural tube defects.
Later exposures carry different risks. The period of rapid neuron production (weeks 6 through 18) and the period of synapse formation and cortical wiring (second and third trimesters) are both sensitive to environmental factors including alcohol, certain medications, and infections. Research consistently shows that the gestational timing of an exposure is a significant factor in determining its impact on neurodevelopment, sometimes more important than the dose or duration.