Parents often worry about the soft spot on the top of their baby's head; it looks so unprotected and vulnerable. Babies actually have two major soft spots, medically termed fontanelles or fontanels. The one on the top of the head, called the anterior fontanelle, is the one normally called the soft spot. The one at the back of the head, the posterior fontanelle, is smaller and closes first. Fontanelles are spaces between cranial bones that allow your baby's skull to expand as he grows and make it easier for him to pass through the birth canal.
Your baby's skull consists of five separate bones joined together by tough, fibrous material called cranial sutures. The cranial suture is what you feel when you touch the baby's large soft spot on top of her head. It's the junction of four cranial bones; two parietal and two frontal bones. The posterior fontanelle is the junction of the two parietal bones and the occipital bone, which covers the back of the head. At birth and for some time after, these bones are loosely knit together, to give your baby's skull room to grow. Having some "give" between the bones also helps protect her brain from injury if she falls on her head. There are a total of six fontanelles at birth, but you will probably only be able to feel the two largest.
The anterior fontanelle, which has a rhomboid shape, varies considerably in size at birth. African-American babies have larger fontanelles than Caucasian babies, family practice physician Joseph Kiesler of the University of Cincinnati explains in the June 2003 issue of American Family Physician. This soft spot averages 2.1 centimeters but can range from 0.6 cm to 3.6 cm. The triangle-shaped posterior fontanelle ranges from 0.5 cm in Caucasian babies to 0.7 cm in African-American infants. Fontanelles often enlarge in the first few months of life, according to Dr. Kiesler. When your baby cries or when he's lying down, his soft spot might appear to bulge slightly, but it's not a problem as long as it becomes flat again when he stops crying and is upright. You can often see or feel his pulse at the soft spot.
The posterior fontanelle closes before the anterior fontanelle, usually closing by age 2 months. The soft spot on the top of the head closes earlier in boys than in girls; the soft spot closes by age 12 months in around 38 percent of babies and by 24 months, you can't fell the soft spot in 96 percent of babies. The average age for the soft spot to disappear is 13.8 months, according to Dr. Kiesler.
A soft spot that's depressed or one that suddenly starts bulging can indicate problems that need immediate emergency medical attention. If your baby becomes dehydrated, you might notice that the soft spot is sunken in slightly. A bulging fontanelle can indicate increased intracranial pressure from illnesses such as meningitis, from head injury or tumors, or from hydrocephalus, a condition where either too much spinal fluid is produced or not enough is absorbed. An unusually small anterior fontanelle can indicate microcephaly, a type of abnormal brain development that results in a smaller than normal head.