The formation of your baby's brain is a complicated process that occurs throughout your pregnancy. The embryo's brain and nervous system begin to develop at just six weeks, yet the sophisticated components of the brain continue to grow and develop until the conclusion of pregnancy, when development ends around the age of 25. The major milestones in fetal brain development are as follows: At 6 weeks, the brain consists only of a fluid-filled cavity with some nerve cells and blood vessels beginning to form connections with each other. By 8 weeks, the brain has grown in size and more neurons and brain cells are being produced every day. By 10 weeks, the fetus' brain contains many large neurons connected by thick bundles of fibers. By 12 weeks, the cerebral cortex (the part of the brain that controls thoughts and feelings) is growing in thickness. Around 14 weeks, the hippocampus (a part of the brain that plays a role in learning and memory) begins to form. By 16 weeks, the corpus callosum (a bundle of nerves that connects the left and right sides of the brain) can be seen on an ultrasound image. Between 17 and 20 weeks, the cerebellum (a small bowl-shaped structure that helps control movement) starts to form. At about 24 weeks, you can see the first signs of brain activity. Fetuses this age will relax between 28 and 32 weeks completely relaxed and without movements associated with sleep.