A fontanel is a gap between the bones of an infant’s skull, covered by a tough but flexible membrane, commonly called a “soft spot.” Every baby is born with six of them, and they serve a dual purpose: allowing the head to compress slightly during birth and giving the brain room to grow rapidly in the months that follow.1PubMed. A comprehensive review of the anterior fontanelle: embryology, anatomy, and clinical considerations Most fontanels close on their own during the first year or two of life, but the timing, size, and feel of these openings give pediatricians a surprising amount of diagnostic information about what is happening inside a baby’s skull.
Why Babies Have Fontanels in the First Place
An adult skull is a fused shell of bone, but a newborn’s skull is more like a jigsaw puzzle with flexible seams. The flat bones of the skull grow from separate centers of ossification, and where the edges of those bones have not yet met, fibrous membrane fills the gap. The largest of these gaps sits right on top of the head, between the two frontal bones and the two parietal bones. That is the anterior fontanel, the one parents typically notice and worry about. Four smaller fontanels sit at the back and sides of the skull.
This design is not a defect. It solves two problems at once. First, during vaginal delivery, the skull bones can overlap and shift, letting the head change shape to fit through the birth canal. This process, called head molding, is a normal part of labor.2PubMed. A numerical study on fetal head molding during labor Second, after birth, a human baby’s brain roughly triples in size during the first two years. If the skull were a rigid box from the start, there would be no room for that growth. The fontanels and the fibrous seams between the bones, called sutures, give the skull the flexibility it needs to expand outward as the brain pushes from within.
Interestingly, this is not unique to humans. Research on nonhuman primates has found that fontanels and widely open sutures create flexibility at exactly the widest part of the skull, suggesting that head molding during birth is more common across primate species than previously thought.3PubMed. Some nonhuman primates have the capacity for head molding at birth
How Birth Itself Affects Fontanel Size
Because fontanels are flexible, the physical stress of labor can change their dimensions. A study comparing newborns delivered vaginally with those delivered by cesarean section found that babies born by C-section tended to have larger anterior fontanels, since their heads had not been compressed through the birth canal.4Yemeni Journal for Medical Sciences. Impact of Labor Variables on The Anterior and Posterior Fontanels Morphometry of Newborn The duration of labor also played a role: longer labors were associated with differences in anterior fontanel size, though the posterior fontanel was not significantly affected. The baby’s presentation, whether head-first or otherwise, mattered too.
This means the size of the fontanel you see in a newborn is not just genetically predetermined. It is partly the product of what happened during delivery. Pediatricians factor this in when they assess a baby for the first time, rather than relying on a single “normal” number.
Normal Closure Timeline
Parents often want to know exactly when the soft spot should close, and the honest answer is that the range is wide. One study tracking fontanel size from birth to 24 months found that the anterior fontanel averaged about 2.55 cm at birth, grew slightly to a peak of roughly 3.37 cm around three months of age, and was fully closed in all children by 15 to 18 months. The posterior fontanel, which is much smaller (about 0.8 cm at birth), closed in all infants by two months.5PubMed Central. Fontanel Size from Birth to 24 Months of Age in Iranian Children
That counterintuitive detail, that the anterior fontanel actually gets a bit bigger before it starts shrinking, catches many parents off guard. The brain is growing rapidly in the first few months, so the fontanel stretches to accommodate it. Only after brain growth slows relative to bone growth does the fontanel begin to narrow and eventually close.
A separate study looking at both full-term and preterm infants found enormous variability in fontanel size and closure age from baby to baby, but no significant differences between preterm and full-term infants or between boys and girls.6Pediatrics. Anterior Fontanel: Size and Closure in Term and Preterm Infants So while the average closure window is somewhere around 12 to 18 months, some perfectly healthy babies close earlier and some close later. A pediatrician who sees a fontanel still open at 18 months is not automatically alarmed; they are looking at the full picture.
What Doctors Can Learn from Touching the Soft Spot
For such a small area, the anterior fontanel gives pediatricians a remarkable amount of information. When a doctor presses gently on the soft spot, they are checking several things at once. A fontanel that is level with the surrounding bone and feels soft and slightly pulsatile is normal. One that is bulging outward and feels tense can signal increased pressure inside the skull, which might be caused by conditions ranging from infection to fluid buildup. A sunken fontanel, one that dips inward, often points to dehydration.
The fontanel also serves as a window for imaging. Before the bones close, doctors can place an ultrasound probe directly on the anterior fontanel and get a clear, real-time image of the brain and its fluid-filled ventricles. This technique avoids radiation, requires no sedation, and can be done right at the bedside. It was a major advance in neonatal medicine, making it possible to quickly check for bleeding, swelling, or structural problems in the brain.7PubMed. Real-time sonography of the brain through the anterior fontanelle Once the fontanel closes, that acoustic window disappears, and imaging the brain requires CT or MRI.
Normative growth charts for fontanel and suture dimensions are being developed to make these assessments more precise. Researchers hope these reference ranges will improve the ability to spot conditions like craniosynostosis earlier and may even support forensic investigations in cases of suspected abusive head trauma.8medRxiv. Growth Charts of Sutures and Fontanelles for The First Year of Infancy
When the Fontanel Closes Too Early
Craniosynostosis is the condition where one or more of the skull’s sutures fuse prematurely, and it can involve early closure of the fontanel as well. When sutures fuse too soon, the skull cannot expand normally in the direction perpendicular to the fused seam. The brain keeps growing regardless, so the skull compensates by bulging in other directions, creating an abnormal head shape.
Research comparing fontanel closure in children with craniosynostosis to healthy controls has found that the accelerated closure likely involves both genetic and biomechanical factors. On the genetic side, overactivity of certain growth-signaling pathways, particularly the fibroblast growth factor (FGF) pathway, drives bone to form faster than it should.9PubMed Central. Difference in anterior fontanelle closure between non-syndromic craniosynostosis and normal controls: a retrospective cross-sectional study Mutations in specific genes, including FGFR1, FGFR2, FGFR3, TWIST, and MSX2, have been identified in various forms of the condition.10PubMed Central. Genetics of craniosynostosis: review of the literature These mutations disrupt the normal balance between bone growth and the signals that are supposed to keep sutures open until the brain is done expanding.11Human Molecular Genetics. Craniosynostosis: Genes and Mechanisms
Craniosynostosis typically requires surgical correction, both to normalize skull shape and to prevent increased pressure on the brain. The type of surgery depends on which sutures are affected and how severe the fusion is. Some cases involve removing and reshaping pieces of the skull vault, sometimes combined with procedures to correct associated problems like widely spaced eye sockets.12PubMed. Cranium Bifidum Occultum Associated with Hypertelorism Treated with Posterior Vault Reconstruction and Orbital Box Osteotomies
When the Fontanel Stays Open Too Long
A fontanel that remains large or open well past the expected window can also signal a problem, though the list of possible causes is different. The most common conditions associated with delayed fontanel closure include hypothyroidism, Down syndrome, rickets, achondroplasia (a form of dwarfism), and increased intracranial pressure.13PubMed. The abnormal fontanel In each of these, the mechanism is different. Hypothyroidism slows bone development broadly. Rickets, caused by vitamin D deficiency, weakens the mineralization process that turns soft tissue into bone. Down syndrome and achondroplasia each involve genetic differences that alter the pace at which skull bones grow and fuse.
One particularly striking example is cleidocranial dysplasia, a genetic skeletal condition in which the fontanels can remain wide open not just through childhood but potentially for life. People with this condition typically also have underdeveloped or absent collarbones and extra teeth.14PubMed. Cleidocranial Dysplasia Spectrum Disorder The condition is inherited in an autosomal dominant pattern and represents a spectrum, from the full triad of features to milder forms with only dental anomalies.15Journal of Medical Genetics. Cleidocranial dysplasia: clinical and molecular genetics
A persistently open fontanel in isolation, without other signs, is not necessarily cause for alarm. But it should prompt a pediatrician to check for underlying conditions, especially if the child is also showing slow growth, developmental delay, or unusual body proportions.
Head Shape Worries and How to Tell What Is Going On
One of the most common reasons parents bring a baby to the doctor about the skull is an asymmetric or unusual head shape. The tricky part is that the two most common causes, craniosynostosis and positional plagiocephaly, can look similar but require very different responses.
Positional plagiocephaly is flattening that happens because an infant spends too much time with pressure on one part of the skull. It became much more common after pediatricians began recommending that babies sleep on their backs to reduce the risk of sudden infant death.16PubMed Central. Distinguishing Between Lambdoid Craniosynostosis and Deformational Plagiocephaly The flattening is cosmetic and typically responds to simple measures like repositioning the baby and supervised tummy time. No surgery is needed.
Craniosynostosis, by contrast, involves premature fusion of a suture and usually does require surgery. True lambdoid suture fusion, which produces flattening at the back of the skull and can look like positional plagiocephaly, is rare.17PubMed. The differential diagnosis of abnormal head shapes: separating craniosynostosis from positional deformities and normal variants Most cases of posterior skull flattening turn out to be positional. Still, distinguishing between the two is important because a misdiagnosis in either direction means either an unnecessary surgery or a missed one. Pediatricians and craniofacial surgeons use physical examination, the pattern of the deformity, and sometimes imaging to make the distinction.18PubMed. Positional Plagiocephaly and Craniosynostosis
For parents, the practical takeaway is that a slightly asymmetric head in a young baby, especially if the baby has a preferred sleeping position, is usually positional and not dangerous. But any head shape that looks unusual or worsens over time is worth having evaluated, because the small number of cases that are craniosynostosis are best treated early.
What Fontanels Reveal About Human Evolution
The fact that human babies are born with such large, flexible gaps in their skulls is itself a clue to our evolutionary history. Humans have an unusual combination of traits: very large brains relative to body size and a pelvis reshaped for upright walking. These two features create a tight fit during birth, sometimes called the obstetric dilemma. Fontanels and open sutures are part of the anatomical compromise that makes human birth possible at all.
Fossil evidence supports this. Researchers studying the skull of the Taung child, a famous specimen of Australopithecus africanus that lived roughly two to three million years ago, found that certain suture-fusion patterns in early hominins resemble those of modern humans more than those of apes. The data suggest that delayed fusion of frontal skull sutures became advantageous early in human evolution, driven by the need to fit a bigger-brained baby through a pelvis adapted for bipedalism, combined with the rapid brain growth that continues after birth.19PubMed Central. Metopic suture of Taung (Australopithecus africanus) and its implications for hominin brain evolution In other words, our fontanels are not just a developmental convenience. They are an evolved feature, shaped by millions of years of competing pressures between brain size, birth-canal geometry, and postnatal brain growth.
A Long History of Shaping Infant Skulls
The flexibility of the infant skull has not gone unnoticed throughout human history. Intentional head shaping, using boards, bindings, or repeated hand massage during infancy, has been practiced on every inhabited continent and in virtually every era of recorded history. Motivations varied widely: some cultures associated elongated skulls with beauty, others with noble birth or ethnic identity, and some believed specific head shapes conferred health benefits.20PubMed. In God’s image? The tradition of infant head shaping
These practices worked precisely because fontanels and open sutures make the infant skull pliable. Sustained pressure during the period when the bones are still growing and the fontanels are still open can permanently alter skull shape. The practice has mostly fallen out of use, but its global ubiquity underscores just how malleable the infant skull really is during the window when fontanels are open. It also explains, in part, why modern pediatricians take positional plagiocephaly seriously: if even intentional reshaping can permanently change skull form, unintentional pressure from a preferred sleeping position can too, although the resulting changes are typically mild.
Devices like cradleboards, once widespread among indigenous cultures in the Americas, applied gentle, sustained force to the back of the skull while the baby was swaddled. The skulls of adults who underwent this process in infancy show dramatic reshaping, with no evidence of neurological harm, at least not from the shape change itself. Anthropologists and forensic specialists encounter these modified skulls in the archaeological record and must distinguish them from pathological deformities, a task that requires understanding the normal range of fontanel-related skull flexibility.