None. As a newborn, none of the skull bones are fused; there are membranes/cartilage that fill spaces between them. Gradually this converts to bone (ossification), mostly during childhood, but process may not be complete till age 25! if they fuse prematurely, will cause intracranial pressure, head pain, visual problems, and unusual head shapes causing low self-esteem. Craniosynostosis requires surgery.
Craniosynostosis. The skull is made of many bone plates which are not fully fused at birth, allowing for adequate brain development. Fibrous tissue between the plates (cranial sutures) calcifies as brain growth slows during the first five or so years. Premature fusion of the plates can cause a mis-sharpen head or inhibit brain growth. Not all fused sutures are worrisome. The only treatment, if required, is surgery.
Growth plates closed. The cranium or brain part of the skull is comprised of 8 different bones, and although these joints don't move like those in the extremities, there is slight movement which allows an infant's skull slight movement to enable birth and growth occurs at the margins to allow head symmetry. The margins or sutures generally fuse by 22-39 months. At times they can be prematurely closed and no longer.
Surgery. This type of surgery is usually performed by a pediatric neurosurgeon, or a specialist called craniofacial surgeon. There are a variety of surgical approaches, which of course only the surgeon can answer based on each individual case.