As humans grow and develop, an amazing number of changes take place. In fact, there are so many changes that a comparative analysis of the anatomy of an adult as opposed to a newborn infant makes it seem almost impossible that these are from the same species. Newborns are not simply much smaller than an adult; there are many structures that simply haven't formed yet or that are far from their adult form.
Infants have to undergo rigorous changes from conception until birth, many of which include changing shape and withstanding large amounts of pressure. As a result, the structures that make adult bodies rigid and strong cannot be in place in order for baby to be born healthy and alive. In other words, bones as they exist in the mature form cannot be in a newborn infant.
Adult versus infant skeletal structure
The adult body has approximately 206 bones, though these numbers can vary slightly by individual. In contrast, a newborn infant has over 270 bones. Babies born prematurely may even have over 300. Over time, some of the bones will fuse to create a complete adult bone.
The stages of bone formation
Bone starts out as cartilage that, with the addition of calcium, slowly becomes a mature bone. This is a process known as ossification. By the time of bone is fully developed, it has a tough structure that has no actual flexibility and has very little give when it encounters a direct force. The bone has spaces all throughout its matrix, giving it the appearance of a sponge. This allows it to withstand a wide range of direct trauma, but it will still break if it bends too far in a single direction.
The reason that babies have so many more bones than adults is because there are only sections of bone within their body, interspersed with the cartilage. Over time, these bits of cartilage also ossify to form complete mature bone all throughout a baby's body. Until then, each little bony section is counted as a separate bone.
A need for flexibility
During a normal delivery, a baby's head is compressed into shapes very different than the shape of a formed skull. If an adult’s head had to go through the same thing, it would simply shatter, sending fragments of bone into the brain beneath. For this reason, the skull has to have sections that are as pliable as rubber. The brain is also not developed to quite the same extent as it will be later in the child's life.
Long bones, such as arms and legs, and such critical elements as the spine, must also still be flexible at the time of birth. If you think of how an infant lays within its mother, curled up in what is so aptly called the fetal position, you will notice that it's more than joint flexibility that allows them to get into that position. Prenatal infants have curves to their arms and legs that will grow out later. This is why babies don't generally have broken bones when they're born and why their limbs and shoulders can undergo the rigors of natural birth.