Anterior Fontanel (Soft Spot)

Dr. Greene's Answer

How crazy-making! The message you are left with is, “Don’t worry, but your son might be developmentally delayed — and it’s your responsibility to pick up on the clues.” No wonder you feel the way you do. Before I answer your question, Alex, I want to tell readers who don’t have your training a little about the anterior fontanel.

The anterior fontanel, or soft spot on a baby’s skull, is a cause of concern for many parents. Elsewhere, the baby’s brain is protected by a wall of bone; here only soft, squishy tissue separates the brain from the traumas of the outside world. The soft spot seems so vulnerable. I spoke with a mother today who had never touched her son’s soft spot — she was afraid she would put her finger through it.

In truth, the soft spot is another example of the amazing design of the human body. At birth there are six soft spots, but only two are noticeable (the largest, up on top, is the anterior fontanel). The loose connections of the skull bones that intersect in the soft spots make labor and delivery possible. Without this flexible anatomy, either human babies would have to have smaller brains or human mothers would have to have wider hips if any babies were to be born.

The value of the soft spot isn’t gone when you first hold your baby in your arms. Far from making the baby more vulnerable, the soft spot protects a baby from injury. True, it makes some uncommon accidents more dangerous (landing head first on top of a car’s radio antenna), but for the common falls experienced by all babies, the soft spot cushions and protects — making the skull function rather like a football helmet.

Every week, frantic parents rush into my office after their babies have fallen off a bed or table or highchair. It happens so quickly, babies can fall even with careful and attentive parents — it’s even happened to me, but don’t tell :^). When babies fall, they usually land head first, since their centers of gravity are in their heads (adults’ centers of gravity are in our bottoms). The head hits the floor with a terrible, ripe-melon-like “thwunk.” Thanks to the cushioning of the soft spot, most of these head injuries are quite minor. And by the way, although the spot is soft, it actually consists of a surprisingly tough fibrous membrane.

At birth, babies’ soft spots come in a very wide range of sizes. If the soft spot is small, it will usually enlarge over the first several months. Conversely, large ones tend to get smaller. By the time a baby is 2 months old, the anterior fontanel is usually about 1 and 1/4 inch across (actually one by one and a half inches), +/- 3/4 inch (Journal of Pediatrics, 80:749, 1972). A persistent, larger anterior fontanel is usually completely fine, but is sometimes associated with a variety of uncommon disorders (list adapted from Nelson Textbook of Pediatrics, Saunders, 2007):

• Achondroplasia, or true dwarfism. You would already know, by size and appearance, if your son were a dwarf.
• Apert syndrome. These babies have webbed digits, broad thumbs and big toes, and characteristic faces. Again, you would already know.
• Cleidocranial dysostosis. These babies have tiny to absent collarbones, tiny chests, and abnormally shaped skulls. Again, evident before 6 months of age.
• Congenital hypothyroidism. This condition might not be picked up, if it weren’t for the newborn screening blood test. Before newborn screening, large fontanels were a common way to diagnose hypothyroidism. If your son’s screening test was normal, no worry here.
• Congenital rubella. This virus is mild in children, but devastating to unborn babies. Those who survive generally are blind, deaf, and with significant heart murmurs. These are not healthy appearing babies.
• Hallerman-Streiff syndrome. Characterized by big foreheads, tiny eyes (with cataracts), and very tiny, underdeveloped noses, these babies are usually blind from birth and remain blind despite surgery. The diagnosis is usually made shortly after a baby is born.
• Kenny-Caffey syndrome. An extremely rare inherited skeletal disorder associated with low blood calcium levels and short stature.
• Hydrocephalus, or water-on-the-brain. In hydrocephalic babies, the fontanel is kept open by the pressure and by the rapidly increasing head size. I’m glad your son’s head is growing fine.
• Hypophosphatasia. This inborn error of metabolism often is severe and lethal in newborns. There is a very mild form of the disease which can be diagnosed by x-ray (moth-eaten appearance of the bones) or blood or urine tests. These kids look like kids with rickets, and are not developmentally delayed.
• Intrauterine growth retardation. Kids who don’t grow well in the uterus (for any reason) will often have persistent large fontanels. Once again, you would already know.
• Osteogenesis imperfecta. This condition is characterized by fragile bones, hyperextensible joints, deep blue sclerae (the white part of the eyes), and unusual, translucent teeth. The diagnosis is sometimes missed in the mild forms of this disease, but it can be identified on x-ray. These babies all have multiple bone fractures, which lead to the x-rays.
• Prematurity. The soft spots in premature babies are larger than average, but there is no known problem from this.
• Pyknodysostosis. These children are short, with unusual jaws, underdeveloped or absent collarbones, and very wrinkled fingers. The diagnosis may be clinched by x-ray. Pyknodysostosis is most common in children whose parents are close relatives of each other. It has been suggested that the artist Toulouse-Lautrec had pyknodysostosis, but I don’t know anything about his parents.
• Russell-Silver syndrome. These kids have congenital short stature, asymmetric limbs, and small incurved fifth fingers. This is usually identified at birth, or shortly thereafter.
• Trisomy 13, 18, or 21. These unfortunate chromosomal problems are readily detectable at birth. I went to the grave of my niece today. It still hurts.
• Vitamin D deficient rickets. This is rare in the United States, but could happen in exclusively breast-fed babies whose mothers do not take vitamins or go outside in the sunlight, at least briefly, each week. It is more common in dark-skinned individuals. Early on, rickets can be diagnosed by x-ray. Soon it is evident in curved limbs, potbellies, and a horizontal groove between the chest and the abdomen. In many kids, if you press on the skull, you feel a ping-pong-ball-like sensation.

Alex, when your pediatrician said everything was probably fine, he was right. He is also right to be vaguely concerned that large anterior fontanels are sometimes associated with certain syndromes, some of which (like the trisomies) involve developmental delay. Now you know what these syndromes are. The serious ones have already been ruled out. Even the mild ones are very unlikely in your thriving son. I would relax and enjoy your son, who is just starting to sit. Soon he’ll be moving across the floor. For most kids, the anterior fontanelle closes not long after they get steady on their feet (at 9 to 18 months). It stays open just long enough to protect them as they stumble their way toward walking on their own. How quickly it all goes!