By the time of birth, the infant's skeleton has the same basic framework as an adult. All the bones are in place---including many extras---and the structure allows for rapid growth and continued development. Much of the critical development takes place in the first few years of life, but bone growth continues until after puberty.
To form bones, the bone cells of the baby lay down calcium over a framework of collagen, a process that takes time and a good supply of calcium from the mother. A baby's bones begin to form during about the 12th week of pregnancy, when centers of bone formation or ossification develop in most bones, says "Your Pregnancy Week by Week." By the 15th week, bones are rapidly hardening and gathering calcium and the skeleton would be visible with X-ray.
Although babies are born with all the bones they will ever have, many parts of the skeleton still consist of cartilage that will ossify over time. Babies have far more bones than adults, with about 270 to our 206, because many of the bones will grow and fuse together over the course of development, according to European Medical Tourist. The "tailbones," for example, start as five separate bones that fuse into one solid structure.
The soft spots or fontanelles of a baby's skull provide the most obvious example of skeletal growth. Soft spots occur where bones of the skull do not yet meet; their unfinished nature allows for rapid brain growth after birth. The posterior or rear fontanelle knits together a few months after birth, while the anterior or top fontanelle closes after about a year, states Pregnancy for Dummies.
Bone growth occurs at particular sites called growth plates. As new cartilage cells form toward the edges of the bone, the old ones are pushed back and die. They provide a platform for calcium deposit that gradually hardens the bone. When growth plates deactivate, bone growth stops. For long bones like the thigh bone, growth plates remain active until after puberty, when the body reaches its final height.
Babies from 3 to 36 months old have the greatest risk for the bone disease rickets because of the rapid skeletal growth occurring during this critical window. Due to a prolonged and severe lack of vitamin D, which helps bones absorb needed minerals, rickets weakens bones. Bones are deformed and fragile and growth can be stunted. Usually, vitamin D supplements can restore the bones to normal strength, according to the Mayo Clinic.