When you first stroke your newborn’s downy head, you may be surprised to discover that his skull has some soft spots, which are known as fontanelles. A newborn’s skull is not one solid bone but consists of several bony plates, which are connected by tough membranes called sutures. While the skull will eventually fuse and the fontanelles disappear, this flexible configuration allows bones to adjust during the birth process and the brain to expand during a child’s early years.
A baby’s bones form in the womb. During the first trimester of pregnancy, the developing skeleton is comprised of rubbery cartilage and connective tissue. Bones don’t start to ossify until the second trimester, at around 16 weeks, but parts of the baby’s skull remain soft and flexible even after birth.
Anthropologists have wondered why human newborns have fontanelles when other primates have skulls that become solid in utero. Some anthropologists theorize that it’s an evolutionary adaptation. Perhaps humans developed fontanelles because their brains need more room to grow during the early years and also because walking upright changed the shape of human birth canals, making it more difficult to give birth. The current configuration of fontanelles on infant skulls accommodates both needs.
The diamond-shaped fontanelle at the top of the skull is known as the anterior fontanelle, while the smaller one in the back of the middle of the head is known as the posterior fontanelle. The anterior fontanelle usually closes before the child reaches two years of age, while the posterior fontanelle can close as early as two months. While care should always be taken to protect a baby’s head, the membrane on these soft spots is tough. Parents need not be concerned about touching the soft spot.
Your doctor will examine your child’s fontanelles during well child visits. The fontanelles should feel flat and firm, not bulging, as a bulge might indicate increased pressure inside the skull. A depressed fontanelle can mean that a baby is dehydrated. The fontanelle closing too early or too late can also indicate a problem. If one or more of the sutures grows together too soon, it might force the skull to grow in an abnormal direction. Correcting this kind of deformity requires surgery.
Sometimes a child’s skull can seem temporarily lopsided, as a result of the skull’s malleability and the pressure of the birth canal. The shape almost always evens out. One reason that it’s important for babies to spend time on their stomach each day is so their head does not consistently rest on the same part of the skull. If a baby’s skull still seems lopsided at about six months after birth, the doctor may recommend that the baby wears a custom-molded helmet to apply pressure to the head and encourage symmetrical development. A flat spot that lasts on one side of the head or the back of the head is known as flat head syndrome.
Most infant skulls even out naturally without any intervention.
How big should a baby’s skull be at birth? The circumference of the average newborn’s head is between 13 and 14 inches, generally reaching 15 inches by the end of the first month. Because boys tend to be larger than girls, they tend to have slightly larger heads at birth, but the difference is usually less than an inch.