Usually, A baby's head will take anywhere from 2 days to a few weeks to grow into its adorable, round shape after they are born. However, if babies spend too much time on their backs later in infancy, they can develop flat heads. So, it is important to keep an eye on your baby's head shape and growth during the early weeks and months of his or her life. When your baby's cranial sutures are fully closed, their head shape has fully developed. During childhood, the skeleton will continue to develop and remain flexible. At about four months, the soft spots in the head start to come together, and the fontanelle at the back of the head closes, and the frontal fontanelle closes between 9 and 18 months.