Quick answer
Babies have fontanelles—membrane-covered spaces between skull bones—because the skull must remain flexible. The bones can shift slightly and overlap during birth, helping the head pass through the birth canal. After birth, open sutures and fontanelles allow the skull to expand as the brain grows rapidly. The smaller posterior fontanelle usually closes earlier; the larger anterior fontanelle closes gradually with wide normal variation. A tense bulge, pronounced sinking, abnormal head shape or other concerning symptoms should be assessed by a healthcare professional.
On the top of a newborn's head, the skull can feel different from the hard dome an adult expects. There may even be a visible pulse beneath the skin. The area is called a fontanelle, and it exists because the infant skull is still being assembled around a rapidly growing brain.
Fontanelles are not holes with the brain exposed. They are spaces where several skull bones have not yet met, covered by strong fibrous membranes and normal layers of scalp. Their flexibility helps during birth and preserves room for growth afterward.
The newborn skull is a flexible construction
The upper skull is formed from several flat bones joined by fibrous seams called sutures. Where multiple bones meet, the membrane-filled spaces are wider and form fontanelles. The best known is the diamond-shaped anterior fontanelle between the frontal and parietal bones. A smaller posterior fontanelle sits farther back.
The fibrous covering protects the tissue beneath while remaining more flexible than mineralized bone. Normal handling, gentle hair washing and routine care do not expose the brain. The area should still be treated gently, as the rest of an infant's head should be.
A pulse may be visible because blood flow and pressure changes in vessels are transmitted through the flexible covering. A brief change with crying or lying down can differ from a persistent tense bulge. Clinical interpretation always depends on position, hydration, behavior and other symptoms.
Flexibility solves two problems at once
During vaginal birth, skull bones can move at the sutures and overlap slightly, a process called molding. That flexibility helps the head adapt to the birth canal. Temporary shape changes are common, although clinicians distinguish them from swelling, bleeding and abnormal suture fusion.
The second problem lasts much longer. The brain grows rapidly in infancy, and the skull must enlarge with it. Bone is added along the edges of open sutures while the plates expand. If one or more sutures fuse too early—a condition called craniosynostosis—growth becomes restricted across that seam and the head may develop a characteristic shape.
Fontanelles therefore do not close like a lid snapping shut. Bone growth gradually reduces the membrane-covered space while sutures continue to support skull expansion. The process is coordinated with overall head growth rather than triggered by one birthday.
Closure follows a range, not a deadline
The posterior fontanelle is usually small and often closes within the first few months. The anterior fontanelle remains longer and commonly closes during the second year, but healthy timing varies. Studies of newborns also find substantial variation in starting size across individuals and populations.
That variation is why one measurement cannot diagnose a child. Clinicians follow head circumference, shape, suture ridges, development and changes over time. A fontanelle that seems small, large, early or late is interpreted within that larger picture.
A persistently bulging fontanelle when a baby is calm and upright, marked sinking with signs of dehydration, fever with unusual sleepiness, vomiting, seizures or an abnormal head shape needs prompt professional assessment. Parents should not press the area to test it or use an online timeline as a substitute for an examination.
From separate bone plates to a growing skull
Fibrous sutures connect the skull bones while wider membrane-covered fontanelles occupy their junctions. During birth, the arrangement permits small shifts in position. During infancy, new bone forms at the edges while the brain and skull expand.
As growth progresses, the fontanelle narrows and becomes bone. Sutures remain biologically active beyond the moment the soft spot can no longer be felt, allowing the skull to continue changing through childhood.
Clinical reviews explain how normal skull growth differs from premature fusion of cranial sutures.
Studies of healthy term newborns demonstrate wide variation in anterior fontanelle size.
Try it yourself
Use routine care, not a home examination.
- Notice the baby's head shape during ordinary care without pressing or repeatedly touching the fontanelle.
- Bring questions about size, pulsing or closure to routine health visits, where head growth can be measured over time.
- Seek prompt medical advice when a change appears with fever, unusual behavior, vomiting, dehydration signs, injury or an abnormal head shape.
This article cannot assess an individual infant. A qualified healthcare professional should evaluate concerns about a fontanelle or head growth.
Why it matters
Fontanelles make visible a central rule of development: structures that seem unfinished can be precisely adapted to the next stage of life. Flexibility is not a flaw in the infant skull; it is part of how birth and growth are made possible.
They also give clinicians one piece of a larger developmental picture. Head shape, growth over time, hydration and neurological behavior matter together, which is why isolated comparisons with a single closure age can mislead.
The soft spot is flexible because the skull is still growing.
A fibrous membrane protects the junction while skull bones mold during birth and expand around the growing brain. Closure is gradual and normal timing varies.
Research behind this story
We link to the primary study or an authoritative indexed review wherever possible. Caveats in the text reflect the limits of that evidence.
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