According to HowStuffWorks, babies have more bones than adults because some infant bones are separate pieces that gradually fuse after birth. In addition, newborns have cartilage that turns into bone over time. Adults typically have 206 bones, but infants have a combination of 300 bone and cartilage structures.
HowStuffWorks indicates that the skull is one example of a bone that grows together. At birth, the skull is in three fragments, so the head is able to fit through the birth canal. The sections of the skull eventually come together into one solid bone.
Some bones, such as kneecaps, begin as cartilage in babies, explains HowStuffWorks. The cartilage "ossifies," which means it slowly turns into bone. Cells called "osteoblasts" produce bone matter that covers the cartilage, and blood vessels form to spread nutrients throughout the new bone.