Fontanelles are the soft spots on a baby’s skull where the bone plates haven’t yet fused together. Every infant is born with six of them. They exist for two critical reasons: to let the skull compress and flex during birth, and to give the brain room to grow rapidly in the first years of life.
Why Babies Have Soft Spots
A baby’s skull isn’t one solid piece of bone. It’s made up of several flat bones connected by flexible, fibrous seams called sutures. Where multiple seams meet, the gaps are wider, forming fontanelles. These soft, membrane-covered openings are what you can feel (and sometimes see) on your baby’s head.
During delivery, the skull bones need to overlap and shift so the head can pass through the birth canal. That’s why many newborns have a slightly elongated or uneven head shape right after birth. The fontanelles and sutures make this molding possible. After birth, these gaps serve a second purpose: they allow the skull to expand as the brain grows. A baby’s brain roughly triples in size during the first two years, and it needs room.
The Six Fontanelles and Where They Are
The two fontanelles most people know about are the anterior and posterior ones, both along the midline of the skull. The anterior fontanelle sits at the top of the head, toward the front. It’s the largest and most noticeable soft spot, typically diamond-shaped. The posterior fontanelle is at the back of the head, much smaller and often triangle-shaped.
The other four are arranged in pairs on each side of the skull. Two sphenoidal fontanelles sit at the temples, and two mastoid fontanelles are located behind the ears. These are smaller and harder to feel, so they rarely get attention from parents.
When Each Fontanelle Closes
The posterior fontanelle is the first to close, typically by about 6 to 8 weeks of age, though some sources place full closure closer to 3 months. The sphenoidal and mastoid fontanelles on the sides of the skull close within the first several months as well.
The anterior fontanelle, the large one on top, stays open much longer. It generally closes between 12 and 18 months of age, though it can remain open until around 24 months and still be considered normal. This extended timeline reflects how much brain growth happens in the first two years. Pediatricians check the anterior fontanelle at routine visits precisely because it’s open long enough to provide useful clinical information.
Normal Size and Appearance
At birth, the anterior fontanelle typically measures around 2.1 centimeters across, with a normal range of roughly 0.6 to 3.6 centimeters. Size varies by ethnicity and individual genetics. Studies show that Black infants tend to have larger fontanelles, ranging from about 1.4 to 4.7 centimeters, which is a normal variation rather than a concern.
The soft spot should feel flat and firm when your baby is upright and calm. It may look slightly concave when the baby is sitting up, and it can appear slightly fuller when the baby is lying down or crying. Both are normal. If you look closely, you might notice a gentle pulsing at the top of your baby’s head. This is simply the pulse of blood flowing through vessels near the surface, and it’s completely expected.
What a Sunken Fontanelle Means
A fontanelle that looks noticeably sunken or depressed is a sign of dehydration. When a baby doesn’t have enough fluid in their body, the soft tissue covering the fontanelle sinks inward. This is especially important to watch for during illnesses that cause vomiting, diarrhea, or fever, since babies lose fluids quickly.
Other signs of dehydration in infants include fewer wet diapers, dry mouth, no tears when crying, and unusual sleepiness. A sunken fontanelle alongside any of these symptoms means the baby needs fluids and medical attention promptly. In more severe cases, rehydration through an IV may be needed.
What a Bulging Fontanelle Means
A fontanelle that bulges outward when the baby is upright and calm can signal increased pressure inside the skull. This is different from the temporary fullness that happens during crying or straining, which is harmless. A persistently bulging fontanelle can be associated with serious conditions, including meningitis (infection of the membranes surrounding the brain), encephalitis (brain inflammation), or hydrocephalus (a buildup of fluid inside the skull).
A bulging soft spot, especially combined with fever, vomiting, unusual irritability, or excessive sleepiness, requires immediate medical evaluation. Pressure inside the skull needs to be addressed quickly in infants.
When Skull Bones Fuse Too Early
In some babies, one or more of the seams between skull bones close prematurely, a condition called craniosynostosis. This is present at birth and prevents normal skull expansion in the affected area. The brain continues growing, but because one pathway is blocked, growth gets redirected through the remaining open seams. The result is an asymmetric or unusually shaped head.
The specific shape depends on which seam closes early. When the seam running along the top of the skull fuses too soon, the head grows long and narrow. When a seam on one side of the forehead closes early, the forehead flattens on that side and bulges on the other, and the nose may appear slightly turned. When both forehead seams close prematurely, the head looks short and wide with a forehead that tilts forward. Treatment for craniosynostosis is surgical, and outcomes are generally good when it’s identified early.
Craniosynostosis is one reason pediatricians pay attention to both the fontanelle and the overall shape of the skull at well-child visits. A fontanelle that closes unusually early, combined with an atypical head shape, can be an early indicator.