A newborn baby has around 300 bones in their body, but during their life this number will decrease. At birth, many of a baby's bones are unfused--that is, they are a little flexible and many will later grow into a single, hard bone. A great example is found in the bones of the skull. Newborn babies have several plates of bone in the skull that shift during birth to allow the baby to pass through the mother's birth canal without injury to either person. For about the first year of infancy, babies will have a "soft spot" on top of the head where these bones haven't quite grown together yet. During early childhood, these bones in the skull will grow together and harden into a single cranial bone.
Many other parts of the body, like the elbow and knee joints, the long bones, and the ribs are either unfused or are entirely made of cartilage at birth but will later grow into hardened, solid adult bone. Different bones (and joints) fuse at different times. Have you ever heard of someone having a growth spurt? During puberty, people may experience several growth spurts as their bones finish ossifying and the growth plates fuse shut. In late adolescence or early adulthood, a person will have their final growth spurt and the last unfused bones of the body--the clavicles--will complete their growth!
This long process of bones growing together and hardening will reduce the skeletal count from around 300 to just 206 in adulthood.
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