In the infant and then in the baby, brain growth is not over, it needs space to grow over months. These cartilaginous spaces allow the brain to take on the full scale it needs.
The ossification of fontanelles varies from one child to another. Indeed, the volume of the skull reaches its adult size between 18 months and 3 years. It is at this age that the bones will gradually weld, which will lead to the disappearance of fontanelles.