The brain begins to develop in the third week of pregnancy. This is when the neural tube, which eventually becomes the brain and spinal cord, begins to form. The neural tube starts as a flat sheet of cells and then folds into a tube-like structure. The top part of the tube eventually becomes the brain, while the bottom part becomes the spinal cord. As the neural tube develops, the cells within it differentiate into various types of neurons and glial cells, which are the building blocks of the nervous system. The development of the brain is a complex process that continues throughout pregnancy and beyond, but the initial formation of the neural tube is a critical first step in this process.