When does the fetus’s brain begin to work?

Around the fifth week after conception, which is approximately seven weeks of gestation, the very first synapses begin forming in the spinal cord. This marks the start of detectable brain activity in the fetus. Just one week later, these early connections allow for the first visible movements, such as gentle arches and curls. Using ultrasound technology, scientists can observe these spontaneous motions, even though they’re too subtle for the pregnant person to feel.

As development continues, the range of movements expands. By about eight to ten weeks of gestation, a fetus begins to move its limbs and fingers. Soon after, you can observe more coordinated actions like stretching, yawning, hiccuping, swallowing, and even thumb-sucking. Although this rich movement repertoire is already developing in the first trimester, most parents won’t feel fetal movements until around the 18th week of pregnancy.

Second Trimester: The Role of the Brainstem in Vital Reflexes

In the second trimester, the brain continues to mature and begins to support essential reflexes that babies will need to survive outside the womb. These include breathing-like movements, rhythmic contractions of the diaphragm and chest muscles—even though the lungs won’t fully function until after birth. Reflexes like coordinated sucking and swallowing also begin to appear.

These actions are controlled by the brainstem, a region located just above the spinal cord and below the cerebral cortex. The brainstem is vital for basic life functions, including heartbeat regulation, breathing, and maintaining blood pressure. By 24 to 26 weeks of gestation, the fetal brain has developed enough that, with medical support, a baby born at this stage can often survive. This milestone is a major turning point in prenatal growth and development.

Third Trimester: Development of the Cerebral Cortex

The cerebral cortex, the part of the brain responsible for conscious thought, memory, and voluntary movements, is the last major structure to develop. During the third trimester, electrical activity begins to appear in the sensory and motor regions of the cortex, especially in babies born prematurely. But true cortical maturation continues in the womb throughout the final weeks of gestation.

As the fetus nears full term, its brain activity becomes increasingly sophisticated. One key development is habituation, or the ability to tune out repetitive stimuli. For example, if a loud noise is made repeatedly near the abdomen, the fetus eventually stops reacting to it. This is an early form of learning. In addition, fetuses start recognizing familiar sounds and smells from the womb environment, such as the mother’s voice or the scent of amniotic fluid.

Gradual Maturation of the Brain After Birth

Despite these advances in fetal brain activity, newborns still have an immature cerebral cortex at birth. The gradual development of this part of the brain explains much of a child’s emotional and cognitive growth in the first few years of life.