At birth, the cranial bones are attached by fibrous membranes called fontanels, which allow flexibility in the cranium, but bones fully fuse by the time the head is finished growing.
The bones of an adult cranium meet closely at sutures and are rigid, which protects the brain. The human cranium is relatively large, however, the cranium of an infant at birth could not pass through the birth canal and surrounding pelvic girdle if the cranium sutures were fixed. The bones of an infant's cranium are not yet fused at birth, and the bones adjust as the infant passes through the birth canal. This adjustment may temporarily deform a newborn's cranium after a vaginal delivery. Bones of the infant cranium are attached only by fontanels. A fontanel is any several fibrous membranes that connect an infant's developing cranial bones and cover spaces between the bones. Fontanels yield slightly to pressure and so are commonly known as soft spots. It is possible to feel the infant's pulsing blood under the fontanel. There are six primary fontanels located within particular skull sutures. The anterior fontanel, which is by far the largest fontanel, connects the frontal and parietal bones and is easy to find on an infant's head. The posterior fontanel separates the parietal bones and the occipital bone. Each of two sphenoid fontanels run horizontally on the side of the skull between the parietal bones and the temporal and sphenoid bones. Each of two mastoid fontanels lie between the rear of each temporal bone and the occipital bone.
Ossification of the fontanels—the replacement of the membranes as bone forms—occurs gradually in an infant. Most fontanels are completely ossified within the infant's first year of life. The posterior fontanel closes approximately two months after birth, the sphenoid fontanels close up at around three months, and the mastoid fontanels close slowly between 1 and 12 months. The anterior fontanel, however, takes the longest to ossify and can still be present in infants as old as 18–24 months. It generally is closed by the time a child is two years old.
Fontanels of the Cranium
In addition to providing flexibility, fontanels also permit rapid skull growth in infants. An infant's skull typically reaches half its adult size when the infant is 9 months old. A two-year-old has a skull that is three-quarters the size of an adult's. Even after fontanels have ossified, the skull of an infant or child continues to grow, adding bone along the edge of sutures. Most children's heads reach adult size at eight or nine years.