In humans, the corpus callosum is made of millions of nerve fibres (over 190 million). In early pregnancy, many systems are involved in guiding corpus callosum fibres to connect the hemispheres. The first corpus callosum axons cross from one side to the other around 13-14 weeks post conception and the entire length of the callosum is complete by week 20.
Soon after birth the nerve fibres become covered with an insulating coat (made of myelin), which helps information travel faster along the axons. The myelin coat starts to form within the first year of life and makes the corpus callosum expand in size further until its shape resembles that of an adult. There is also some fine-tuning of the connections between nerve fibres, where new connections form and other connections are removed.
Our brains, and therefore our corpus callosum, continue to grow during childhood and adolescence as they become more effective and efficient. The eventual size and shape of a normal corpus callosum can vary between individuals.