The anterior fontanelle is the diamond-shaped soft spot on the top of a baby’s head, located where the two frontal bones and two parietal bones of the skull meet but haven’t yet fused together. It’s the largest of six fontanelles on a newborn’s skull, and it plays a critical role in both birth and brain development. Most parents notice it within the first days of their baby’s life, and pediatricians check it at every well-baby visit during the first year and beyond.
Why Babies Have a Soft Spot
A newborn’s skull isn’t one solid piece of bone. It’s made up of five major bones (two frontal, two parietal, and one occipital) separated by connective tissue seams called sutures. The fontanelles are gaps where multiple sutures meet, filled with a tough but flexible membrane rather than bone.
This design serves two purposes. First, the flexible skull bones can overlap and shift during labor, allowing the baby’s head to mold to the shape of the birth canal. That cone-shaped head many newborns have after delivery? That’s the molding process at work, and it typically resolves within three to five days. Second, the open fontanelle gives the brain room to grow rapidly after birth. A baby’s brain roughly triples in size during the first two years, and fused skull bones would restrict that growth.
Size, Shape, and What It Feels Like
The anterior fontanelle sits toward the front-top of the skull and has a roughly diamond or kite shape. At birth, it typically measures about 2 to 3 centimeters across, though size varies from baby to baby. When you gently touch it, it should feel firm and very slightly curved inward. The membrane covering it is tough, so normal handling, bathing, and gentle touching won’t harm it.
One thing that catches many new parents off guard: you can often see or feel the soft spot pulsing. This is completely normal. The pulsations match your baby’s heartbeat and reflect blood flowing through vessels just beneath the membrane. It’s more visible when a baby is calm or lying down, and it’s not a sign of anything wrong.
When the Fontanelle Closes
The anterior fontanelle gradually shrinks as the skull bones grow toward each other. The median closure age falls between 12 and 18 months, but there’s a wide range of normal. Some babies’ fontanelles close a bit earlier, while others may remain slightly open past age two. In some cases, the fontanelle stays open until around 35 months without indicating a problem.
Pediatricians track fontanelle size alongside head circumference at routine checkups. A fontanelle that’s closing on a normal trajectory, paired with healthy head growth, is reassuring. What matters more than hitting a specific month is the overall pattern of change.
What a Normal Fontanelle Tells You
The anterior fontanelle acts like a window into what’s happening inside a baby’s skull. At routine visits, a pediatrician will gently feel the fontanelle to assess its tension and fullness. At home, parents can learn to recognize what their baby’s fontanelle normally looks and feels like, which makes it easier to spot changes.
A healthy fontanelle is slightly concave (curving gently inward) when the baby is upright and calm. It may look flatter or bulge slightly when a baby cries hard or strains during a bowel movement, then return to normal afterward. That temporary change with crying is expected and not concerning.
Sunken Fontanelle: A Sign of Dehydration
A fontanelle that curves noticeably inward, creating an obvious dip you can see without touching, is a warning sign that the baby doesn’t have enough fluid in their body. This is one of the more reliable visible indicators of dehydration in infants, alongside fewer wet diapers, dry mouth, and no tears when crying.
Dehydration in babies can happen quickly during illnesses that cause vomiting, diarrhea, or fever, or simply from not feeding well. If your baby’s soft spot looks distinctly sunken compared to its usual appearance, that’s a signal to seek medical attention promptly.
Bulging Fontanelle: A Sign of Pressure
A fontanelle that feels tense, swollen, or visibly raised even when the baby is calm and upright suggests increased pressure inside the skull. This is a more urgent finding than a sunken fontanelle. Conditions that can cause it include:
- Meningitis: an infection of the membranes surrounding the brain
- Hydrocephalus: a buildup of fluid inside the skull
- Encephalitis: swelling of the brain itself, usually from infection
A persistently bulging fontanelle, especially when accompanied by fever, vomiting, excessive sleepiness, or irritability, requires immediate medical evaluation.
Delayed Closure
When the fontanelle remains larger than expected or stays open well beyond the typical timeline, it can sometimes point to an underlying condition. The most common causes of a large or late-closing fontanelle include hypothyroidism (an underactive thyroid), rickets (a vitamin D deficiency that weakens bones), Down syndrome, achondroplasia (a form of dwarfism), and increased pressure inside the skull.
This doesn’t mean a fontanelle that’s still open at 18 months is automatically a problem. Many healthy babies close later. But if the fontanelle is unusually large or shows no signs of shrinking over time, your pediatrician may run blood tests or imaging to rule out these conditions.
Early Closure and Craniosynostosis
The opposite concern, a fontanelle that disappears too soon, can signal craniosynostosis. This is a condition where one or more skull sutures fuse prematurely, before the brain has finished growing. When a suture closes early, the skull stops expanding in that direction, which can cause the head to develop an abnormal shape as the brain pushes growth in other directions.
In more severe cases, especially when multiple sutures are involved, the brain may not have enough room to grow normally. This can lead to a dangerous buildup of pressure inside the skull, potentially causing vision problems, seizures, or developmental delays if untreated. One of the earliest signs parents or doctors may notice is the absence of a soft spot on the baby’s skull. Craniosynostosis is typically treated with surgery to reopen the fused sutures and allow the brain adequate space to develop.