At birth, babies have around 305 bones, whereas an adult only has 206 bones. Why does the number of bones in the body decrease as we grow?

 

Newborn babies have more cartilage than bone, which means that the baby's bones consist mainly of cartilage and are therefore softer.
As they grow, some of the bones begin to fuse with others to form a bone. This process is called ossification. For example, in babies, the ilium, ischium and pubis are still separate. In adults, these bones have grown together to form a single bone (Os Coxae).

Strictly speaking, babies are born with the same number of skeletal limbs, which they retain throughout their lives. However, in adult humans, some of these skeletal limbs join together and therefore only count as one bone, whereas in babies they are still separate and are counted as several bones.