FactSentinel

Babies are born with roughly 270 to 300 bones, while adults have 206. The difference does not come from bones disappearing. It comes from fusion. Many of a newborn's bones are separate pieces of bone or cartilage that join together as the child grows, so the total number falls over time.

Several well-known examples illustrate the process. The skull of a newborn is made up of separate plates joined by flexible seams and soft spots called fontanelles, which allow the head to pass through the birth canal and make room for rapid brain growth. These plates gradually knit together during childhood. The sacrum at the base of the spine begins as five separate vertebrae that fuse, usually between late adolescence and the mid-twenties, and the tailbone also forms from several small segments. Long bones such as the femur have growth plates with separate ossification centers at the ends, which merge with the shaft once growth is complete. Much of an infant's skeleton is also cartilage that slowly hardens into bone, a process called ossification.

The exact figure varies by source because it depends on what is counted as a separate bone. Cartilage that has not yet begun to ossify is counted differently in different references, which is why estimates range from about 270 to 300. A common misconception is that babies have "extra" bones that are lost. In reality, the material is retained and simply merges into larger, stronger structures.