A baby’s skull never fuses all at once. The familiar soft spots, called fontanelles, typically close by about 18 months of age, but the bony seams between the skull plates remain at least partially open for decades. The full answer depends on which part of the skull you mean, and the distinction matters more than most parents realize.
What the Soft Spots Actually Are
A newborn’s skull is not a single solid dome. It is made up of several flat bones separated by fibrous gaps called sutures, with wider membrane-covered spaces at certain intersections. Those wider spaces are the fontanelles. The largest and most familiar is the anterior fontanelle, the diamond-shaped soft spot near the front-top of the head. In newborns it measures roughly 4 cm front to back and 2.5 cm across. There is also a smaller posterior fontanelle near the back of the skull, along with several even tinier ones at the sides that most parents never notice.
The posterior fontanelle closes first. In a study of Iranian children, it was closed in all infants by two months of age.1PubMed Central. Fontanel Size from Birth to 24 Months of Age in Iranian Children The anterior fontanelle has a longer timeline: it tends to reach its largest size around three months, then gradually shrinks and closes somewhere between 15 and 18 months. A large Italian study found that an age of about 17.5 months was the best statistical cutoff for distinguishing open from closed anterior fontanelles in healthy children.2PubMed Central. Difference in anterior fontanelle closure between non-syndromic craniosynostosis and normal controls: a retrospective cross-sectional study That said, some perfectly healthy babies have a closed soft spot by 12 months, while others still have a palpable opening past their second birthday.
Why Babies Are Built This Way
Open fontanelles and flexible sutures serve two critical purposes. The first is delivery itself. During vaginal birth, the fetal head must squeeze through the birth canal, and the skull bones actually overlap at the sutures to make that possible. MRI imaging during labor has shown that all fetuses studied exhibited this head molding, with the parietal bones sliding beneath the frontal bone at the coronal suture and shifting relative to each other and the occipital bone at the lambdoid suture. The result is the temporarily elongated “sugarloaf” head shape many parents notice at birth.3PLOS ONE. Three-dimensional magnetic resonance imaging of fetal head molding and brain shape changes during the second stage of labor If the skull bones were fused solid, this compression could not happen safely. Excessive molding from prolonged labor or very strong contractions can cause problems, which is one reason clinicians monitor labor progress closely.4PubMed. A numerical study on fetal head molding during labor
The second purpose is brain growth. A human brain roughly triples in size during the first two years of life. The open sutures act as growth zones, allowing new bone to form at their edges while the expanding brain pushes outward. If the sutures locked up too early, the brain would run out of room. The fontanelles serve as pressure relief valves of sorts, and doctors use them diagnostically: a bulging fontanelle can signal increased pressure inside the skull, while a sunken one can indicate dehydration.
Variation by Ethnicity and Individual
Fontanelle closure timing is not uniform across all populations. A CT-based study in New Zealand found meaningful differences between ethnic groups. Among Māori and Pasifika children, about a quarter had a clinically closed anterior fontanelle by four to six months of age, rising to roughly half by 10 to 12 months and about 80 percent by 13 to 18 months. The posterior fontanelle also showed differences: it was still open in about 17 percent of Māori/Pasifika infants under one month and 7 percent of those one to three months old, whereas no cases of posterior fontanelle non-fusion were identified in the New Zealand European group at those ages.5PubMed. Closure of the anterior and posterior fontanelle in the New Zealand population: A computed tomography study
This kind of population-level variation means that using a single “normal” closure window for all babies can lead to unnecessary concern. A fontanelle that is still open at 20 months might be perfectly normal for one child while warranting a closer look in another, depending on growth trajectory, head circumference, and family background. Pediatricians consider the whole picture rather than a single number.
When Fusion Happens Too Early
Craniosynostosis is the condition where one or more skull sutures fuse prematurely, before the brain has finished its rapid growth phase. Depending on which suture closes early, the skull grows into an abnormal shape because the brain expands preferentially in the directions where sutures remain open. Sagittal craniosynostosis is the most common type, resulting in a long, narrow head.6PubMed Central. Cranial growth in isolated sagittal craniosynostosis compared with normal growth in the first 6 months of age Premature closure of the coronal suture can happen on one side or both. When it is one-sided, the skull becomes noticeably asymmetric, a condition called anterior plagiocephaly. When both coronal sutures fuse early, the skull is symmetric but abnormally wide and short.7PubMed Central. Unilateral and bilateral expression of a quantitative trait: asymmetry and symmetry in coronal craniosynostosis
Researchers have identified mutations in several genes that are responsible for many syndromic forms of craniosynostosis, particularly in fibroblast growth factor receptors (FGFR-1, FGFR-2, and FGFR-3) and genes called TWIST and MSX2.8PubMed Central. Genetics of craniosynostosis: review of the literature These discoveries have reshaped how the condition is classified and understood. But many cases of single-suture craniosynostosis have no identified genetic cause and are considered non-syndromic, meaning they occur in isolation without other developmental abnormalities.9PubMed Central. Growth of the normal skull vault and its alteration in craniosynostosis: insights from human genetics and experimental studies
Telling Craniosynostosis Apart from Positional Flattening
Not every misshapen baby head is craniosynostosis. Positional plagiocephaly, the flat spot that develops when a baby spends too much time lying on one side, is far more common and does not involve suture fusion at all. Telling the two apart matters because the treatment paths are completely different. Clinical features often distinguish them, and when there is uncertainty, ultrasound of the cranial sutures can confirm whether a suture is open or closed without exposing the infant to radiation.10PubMed. Occipital plagiocephaly: unilateral lambdoid synostosis versus positional plagiocephaly Positional skull deformities can be diagnosed and classified based on clinical features, with ultrasound reserved for unclear cases.11PubMed Central. Positional Skull Deformities
Imaging technology has evolved considerably. Traditional CT scans remain the gold standard for visualizing suture status, but they deliver ionizing radiation and often require sedation in young infants. Newer approaches including high-resolution ultrasound, 3D stereophotogrammetry, and advanced MRI offer radiation-free alternatives that can provide comprehensive assessment of the skull.12PubMed Central. Imaging Modalities in Craniosynostosis: A Systematic Review and Proposal of the ARCANA Protocol for Multimodal Radiation-Free Assessment For a condition that overwhelmingly affects very young infants, reducing radiation exposure is a genuine clinical priority.
How Craniosynostosis Is Treated
Surgery is the standard treatment. The earliest attempts date back to 1890 with strip craniectomy, where the fused suture was simply removed. Results were poor for decades, but successful strip craniectomies became routine in the 1920s. A major shift came in the late 1960s with the introduction of cranial vault remodeling, a more extensive procedure that reshapes the skull bones. By the 1980s, helmet therapy was being used as an adjunct, and in the 1990s, endoscopic techniques dramatically reduced the invasiveness of the surgery itself.13PubMed Central. Surgery of craniosynostosis: a historical review
Today, endoscope-assisted strip craniectomy combined with postoperative molding helmet therapy has become a well-established minimally invasive option for carefully selected infants. This approach takes advantage of the rapid brain growth in early infancy: once the fused suture is surgically released, the brain’s own expansion pushes the skull into a more normal shape while the helmet guides the reshaping process.14Seminars in Plastic Surgery. Endoscopic Suturectomy and Molding Helmet Therapy for the Treatment of Common Types of Craniosynostosis Compared to traditional open vault remodeling, this approach involves shorter surgery, less blood loss, and a shorter hospital stay. The trade-off is that the infant typically wears a custom molding helmet for several months afterward.
Timing matters. The endoscopic approach works best when done early, usually before about four months of age, because it relies on the period of fastest brain growth to do the reshaping work. Open vault remodeling can be performed later but is a bigger operation. Either way, the goal is to give the brain room to grow normally and restore a more typical head shape.
Does Early Fusion Affect the Brain?
Beyond cosmetic concerns, parents understandably worry about whether premature suture fusion affects brain development. The evidence suggests it can, though the effects are generally modest. A systematic review of cognitive outcomes in single-suture craniosynostosis found small to medium but persistent effects on both general and some specific cognitive functions across different age groups, even in higher-quality studies.15PubMed. Cognitive Development in Single-Suture Craniosynostosis – A Systematic Review The review also found limited evidence that surgical correction alone eliminated these effects, suggesting the relationship between skull shape and brain development is more complicated than simple mechanical restriction.
Brain imaging studies have added nuance. Different types of craniosynostosis appear to affect different brain networks. Sagittal synostosis has been associated with decreased connectivity in brain regions involved in visual-motor coordination and attention, while coronal synostosis has been linked to changes in circuits important for executive function and broader cognition. The severity of metopic synostosis may also influence the degree of neurological differences.16PubMed Central. An Investigation of Brain Functional Connectivity by Form of Craniosynostosis These findings reinforce that craniosynostosis is not just a cosmetic issue, and that the specific suture involved shapes the nature of any downstream effects.
When the Soft Spot Stays Open Too Long
While premature closure gets most of the attention, delayed closure of the anterior fontanelle also warrants evaluation. When the soft spot remains open well past two years, it can be a normal variant, but it can also signal conditions that a pediatrician should investigate. Increased intracranial pressure, thyroid problems, and certain skeletal disorders are among the recognized causes of persistently open fontanelles.17PubMed. Persistent open anterior fontanelle in a healthy 32-month-old boy In many cases, though, the fontanelle is simply on the tail end of the normal bell curve, and the child is developing fine. Context is everything: a delayed fontanelle in a child who is growing normally and hitting milestones is very different from one in a child with other concerning signs.
The Sutures Keep Fusing Well into Adulthood
Here is the part that surprises most people. Even after the fontanelles close in infancy, the sutures between the skull bones remain partially open for much of adult life. This process is so gradual that forensic scientists have long tried to use suture closure as a way to estimate age at death, with mixed success.
The general pattern in adults follows a predictable sequence. A study of the Gujarati population found that the sagittal suture (running front to back along the top of the skull) tends to close first, typically in the 30s. The coronal suture (running side to side behind the forehead) follows in the 40s. The lambdoid suture (at the back of the skull) often does not close until the 50s or later, with near-complete fusion across all sutures by about age 60.18Journal of Indian Academy of Forensic Medicine. Chronological Closure Patterns of Cranial Sutures in a Gujarati Population: A Forensic Age Estimation Study That means a 30-year-old still has partially open skull sutures, and it would be technically accurate to say the skull is not “completely fused.”
As a forensic tool, though, suture closure has real limitations. An analysis of Polish men using postmortem CT imaging found that the method was useful for broad categories, distinguishing people who were roughly 30 to 35 from those over 50, but could not be used independently for precise age estimation.19PubMed Central. Cranial sutures as an age indicator: verification of the method using postmortem CT acquisition material Individual variation is simply too large. Two people of the same age can have very different degrees of suture closure, and there is no reliable way to know where any given person falls without examining them directly.
How Open Sutures Protect the Infant Skull
The mechanical properties of an infant skull are strikingly different from an adult’s, and the open sutures are a big reason why. Finite element modeling of infant skulls has shown that unossified (open) sutures absorb and distribute impact forces differently than fused ones. When sutures are open, impact loads produce higher strains at the suture sites themselves but lower peak stresses on the bone plates, because the flexible sutures act as shock absorbers.20PubMed Central. Stress and strain propagation on infant skull from impact loads during falls: a finite element analysis This is part of why infant skulls are relatively resilient to the minor bumps and tumbles of early life. The flexibility comes at the cost of less rigid protection overall, which is one reason head injuries in infants are taken so seriously.
Fetal cranial bone itself has very different mechanical properties from adult bone. Characterizing these properties has been a challenge, but it is essential for understanding how the skull responds to the forces of labor and to impacts during infancy.21Journal of Biomechanics. The elastic modulus of fetal cranial bone: A first step towards an understanding of the biomechanics of fetal head molding The infant skull is an engineering compromise: flexible enough to pass through the birth canal and accommodate explosive brain growth, but rigid enough to protect the brain from everyday forces.
Intentional Head Shaping Through History
The malleability of the infant skull has not gone unnoticed by human cultures. In pre-Columbian South America, intentional cranial deformation was widely practiced. Newborns’ heads were shaped using boards, pads, adjusted bindings, or specially designed cradle-boards. The motivations varied by culture and region: in some places it signified nobility or social class, in others it emphasized ethnic identity, and in still others it was done for aesthetic, magical, or religious purposes.22PubMed. Artificial cranial deformation in newborns in the pre-Columbian Andes Similar practices have been documented across cultures on every inhabited continent, from Mesoamerica to parts of Africa, Europe, and the Pacific Islands.
These practices worked precisely because the infant skull is so responsive to external forces during the period when sutures are open and fontanelles are present. The bones yield to sustained gentle pressure and grow into the new shape. By the time the sutures begin to close, the reshaped form is permanent. The practice demonstrates, rather dramatically, the same biological principle that modern helmet therapy exploits for craniosynostosis treatment: during the window of rapid brain growth and open sutures, the skull can be guided into a different shape with relatively modest sustained force.