He’s not just getting comfy. Moving around in the womb is learning—here’s how.
Your developing baby’s brain grows at an incredible rate during your pregnancy. At four weeks in utero, his brain is not much bigger than a grain of salt, and at seven weeks, it barely measures a quarter inch. But fast-forward to the final trimester and your developing baby’s brain has increased in size by 260 percent! Most of this growth can be attributed to the rapid formation, growth, and proliferation of brain cells. The child is born with 100 billion neurons forming connections with one another and creating an elaborate messaging network that scientists consider the most complex biological system in the world. Here’s a deeper look at the brain development that you can’t see but can appreciate.
Your developing baby’s brain development technically begins during the third week of your pregnancy, when rapidly multiplying cells form what’s called the neural plate. This structure eventually folds in on itself to become the neural tube, which later gives rise to the forebrain, midbrain, hindbrain, and spinal cord. As early as five weeks after conception, neurons—the information-processing cells that pass signals throughout the central nervous system—begin to form, divide, and multiply in these regions of your developing baby’s brain.
The most active period of neuron proliferation takes place during the middle of the second trimester, when 250,000 neurons are created every minute. The neurons begin to migrate to different regions of your developing baby’s brain, where they take on specific roles, such as interpreting sounds and storing memories, and form connections with other neurons.
During this time of heightened activity, the cerebral cortex—the area associated with the brain’s higher functions, such as language and abstract thought—grows more rapidly than the brain’s other structures. By the seventh month of your pregnancy, it houses 70 percent of the neurons in your developing baby’s brain.
By the eighth, the auditory cortex, the visual cortex, and Broca’s area (a region of the brain associated with producing speech) begin to function, lending your developing baby a primitive ability to interpret sights and sounds and to distinguish language.
Your child is equipped for physical survival. Their brains and nerves are developed to the point that they can control basic reflexes and vital functions like breathing, swallowing, and sleeping. As you may have noticed, physical reflexes begin to develop during early pregnancy, when your developing baby becomes capable of reacting to stimuli by moving his arms and kicking his legs. In the seventh month, a process called myelination begins in which a dense, fat-based substance forms along the connections between neurons, allowing signals to pass between them with increasing speed. Among the first neurons to undergo myelination are those located in the brain regions responsible for motor development—the brain stem and spinal cord—and they’ll be the most fully developed by then.
Interestingly, even while your developing baby is in the womb, external stimuli can play a significant role in the development of his reflexes. By week 16, developing ears allow your unborn child to detect (though not interpret) sound. And by week 25, your unborn child is likely to respond to sounds outside the womb, such as music or your voice, by kicking or moving around. With each kick and movement, certain neurons are exercised, which encourages them to form even more connections. That, in turn, strengthens overall motor function and gradually allows for the more coordinated, complex, and deliberate movements you’ll see during your child's early years.