Dr. Greene's Answer
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 dwarfism.
- Apert syndrome. These babies have webbed digits, broad thumbs and big toes, and characteristic faces.
- Cleidocranial dysostosis. These babies have tiny to absent collarbones, tiny chests, and abnormally shaped skulls. This is 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.
- 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.
- 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.
- 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.