A fetus begins building its nervous system remarkably early. The foundations appear around week 3 after conception (about week 5 of pregnancy), when a flat strip of cells called the neural plate folds inward to form the neural tube. By week 6 of pregnancy, this tube is closing, and the brain and spinal cord are already developing from it. But there’s a big difference between having the structural beginning of a nervous system and having one that actually works. The full story unfolds across all three trimesters.
The Neural Tube: Weeks 3 to 6
The nervous system starts as a flat sheet of specialized cells running along the embryo’s back. During weeks 3 and 4 after conception, this sheet folds into a hollow tube. By around week 6 of pregnancy (four weeks after conception), the neural tube closes. Everything that will eventually become the brain, spinal cord, and peripheral nerves traces back to this structure.
Even at this early stage, the top end of the tube is already dividing into three bulges that will become distinct brain regions: the forebrain (which later becomes the cerebrum), the midbrain, and the hindbrain (which becomes the cerebellum and brainstem). By the seventh week after conception, these three sections have split further into five separate brain vesicles, setting the architectural blueprint for the entire brain.
First Movements: Weeks 7 to 12
The earliest signs that the nervous system is doing something functional show up surprisingly soon. Simple reflexive movements begin during the first trimester. Around weeks 7 to 8, the embryo starts making small, spontaneous movements, though they’re too faint for the mother to feel. These aren’t purposeful or coordinated. They’re more like test signals running through newly formed nerve-to-muscle connections.
By around week 12, the fetus can make more defined movements, including curling fingers and toes. Local reflexes are developing as motor neurons in the spinal cord begin connecting with muscle fibers. The movements are still jerky and uncoordinated, but they represent the nervous system’s first real output.
Synapses and Early Wiring: Weeks 12 to 22
Having neurons is one thing. Connecting them is another. Synapses, the junctions where one nerve cell communicates with the next, begin forming outside the main body of the cortex first. In the early second trimester, small numbers of synapses appear in layers above and below the cortical plate (the structure that becomes the outer brain). The cortical plate itself remains synapse-free during this period, essentially a dense layer of cells waiting to be wired.
By around 15 weeks after conception, the number of synapses increases, with most concentrated in a temporary structure called the subplate, which acts as a kind of staging area for brain connections. This subplate plays a critical scaffolding role: it holds connections temporarily until the cortex matures enough to take them over. Around 22 weeks, synapses finally begin appearing within the cortical plate itself, marking a turning point in brain circuit development.
Brain Electrical Activity: Midgestation to Week 24
The developing brain doesn’t sit quietly while it’s being built. Spontaneous bursts of electrical activity emerge around midgestation (roughly 20 weeks), though this early activity is disorganized. These bursts aren’t thoughts or sensations. They’re more like the nervous system testing its own wiring, and they play an important role in guiding how circuits mature.
A more significant milestone arrives around week 24. This is when connections between the thalamus (a deep brain relay station) and the cortex (the outer brain responsible for processing sensory information) begin maturing. With these pathways coming online, the brain can start producing more complex electrical signals, and for the first time, the cortex can generate measurable responses to stimulation from outside the body. This is also when myelination, the process of insulating nerve fibers to speed up signal transmission, begins in the brainstem. It starts in some brainstem pathways around week 20 and extends to other areas by weeks 23 to 24.
Sensory Systems Come Online
The senses don’t all develop at once. They come online in a roughly consistent order: touch and taste first, then hearing, and finally vision.
- Taste and smell: Flavors from the mother’s diet, including vanilla, carrot, garlic, and mint, pass into the amniotic fluid. The fetus swallows several ounces of this fluid daily. Studies have shown that babies exposed to carrot flavor in utero eat more carrot-flavored food after birth, suggesting the taste system is functional well before delivery.
- Hearing: Fetuses can hear and distinguish voice patterns while still in the womb. In a well-known study, mothers read The Cat in the Hat aloud during the final seven weeks of pregnancy. After birth, their babies consistently preferred a recording of that story over an unfamiliar one, indicating they had formed auditory memories before being born.
- Vision: This is the last sense to develop. In the third trimester, researchers have projected light patterns through the uterine wall and found that fetuses were about twice as likely to track dot patterns resembling a human face compared to random patterns, mirroring the same preference seen in newborns.
Pain Pathways: Weeks 24 to 28
The question of when a fetus can feel pain hinges on when the necessary wiring is complete. Pain signals from the body need to travel through the spinal cord, up to the thalamus, and then out to the cortex to be processed as a conscious experience. The thalamocortical connections that make this possible begin forming around week 24 and continue developing through the third trimester.
By around week 26, functional connections between the cortex and the spinal cord are likely present, meaning signals can travel in both directions between the brain and the body. This doesn’t settle every question about fetal pain experience, since conscious perception involves more than just having the right wires in place. But the physical hardware needed for pain processing is broadly established in the late second to early third trimester.
Autonomic Regulation: Late Second Trimester Onward
One of the nervous system’s most critical jobs is running basic body functions without conscious effort: heart rate, breathing rhythm, sleep-wake cycles. This autonomic nervous system undergoes rapid development in the second half of pregnancy at a pace that may never be matched again in a person’s lifetime.
Rest and activity cycles appear as early as 23 weeks, becoming more distinct between 26 and 32 weeks. The parasympathetic branch of the nervous system (the “rest and digest” side) shows a major wave of maturation at the end of the second trimester, driven partly by increasing insulation of the vagus nerve, which is the main nerve controlling heart rate. From 32 weeks onward, the fetus shows more sophisticated heart rate regulation, including responses to its own movements and early signs of the baroreflux (the system that keeps blood pressure stable).
Several key windows of autonomic development stand out: the transition from the late second into the early third trimester, around 30 to 32 weeks, and the near-term period beyond 35 weeks. By the time a baby is born at full term, the autonomic nervous system is functional enough to manage life outside the womb, though it continues refining for months after birth.
The Big Picture
There’s no single week when the fetal nervous system simply “switches on.” The structural foundation forms in weeks 3 to 6. Simple reflexes emerge by weeks 7 to 12. Synaptic wiring ramps up through the second trimester. Meaningful electrical activity and sensory processing begin around weeks 20 to 24. And the sophisticated regulation needed for independent life matures through the third trimester. Each milestone builds on the last, turning a hollow tube of cells into a functioning nervous system over the course of about 37 weeks.