Your baby's brain is so complex that it needs the entire length of your pregnancy not only to grow, but also to mature. It is continually forming new connections and sensory pathways.
The nerves that make up the brain's gray matter started in the center of the brain on the outer surface of the lateral ventricles (there is one ventricle in each hemisphere of the brain). The lateral ventricles contain the part of the brain known as the choroid plexus, a loose, seaweedlike structure that produces the fluid that bathes your baby's brain and spinal cord. Constantly circulating around the brain, this fluid protects and buffers the brain from the harder structure of the bones of the surrounding skull.
The gradual, wavelike, outward movement of nerve cells in the gray matter, which started more than 12 weeks ago, is now complete. Coming to rest close to the brain's surface, these cells need to mature, branching out to make multiple connections, or "synapses," with other nerve cells.
The surface of your baby's brain is very smooth at this stage but as the cortex matures, beginning to form six clear layers, it takes on its familiar wrinkled appearance.