An adult human has 206 bones. However, at birth, a newborn has more than 270 bones. Eventually, some of these bones fuse together when a child grows. For example, the bones of the skull also known as the cranium, as well as the bones of the sacrum and coccyx of the backbone, fuse together. The skeleton accounts for 12-20 percent of a person's body weight. There are also fused bones in the pelvic area. During pregnancy, the skeleton of a fetus is cartilaginous and this eventually becomes replaced by bone. Bones are places for muscles to attach and provide a framework and a support for the body.
When a baby is born he contains 300+ bones that are in form of cartilage, as to make it possible for a baby to be born without being hurt because cartilage can easily bend. When the baby grows his bones fuse together forming large strong bones. Therefore when the baby is an adult and the growth and development of bones stop, he has 206-207 bones in his body.