The Prenatal Period
The formation of neurons begins very early in the human embryo. By five weeks after conception, the cells in the developing brain begin dividing rapidly to form the 100 billion or so neurons that an infant’s brain has at birth. Once formed, neurons begin to migrate to the correct location in the brain, and some synapses begin forming.
Prenatal conditions — including temperature, pressure and fetal movements such as kicking— stimulate the development of synapses. Myelination of neurons also begins prenatally, starting with the neurons of the spinal cord and brain stem.