The posterior fontanelle, the small triangular soft spot near the back of a baby’s skull, typically closes between one and three months after birth. That makes it far shorter-lived than the more familiar anterior fontanelle at the top of the head, which stays open well into toddlerhood. Because of its small size and early closure, the posterior fontanelle gets less attention from parents and even from some clinicians, but it serves real purposes during birth and in the early weeks of life, and its timing can offer subtle diagnostic clues.
Where It Is and How It Differs From the Anterior Fontanelle
A newborn’s skull has six fontanelles, but two get nearly all the clinical attention. The anterior fontanelle sits at the crown of the head, roughly diamond-shaped, and measures around two to three centimeters across at birth. It does not close until somewhere between nine and eighteen months of age, and in some children it remains palpable past two years. The posterior fontanelle, by contrast, sits at the junction of the two parietal bones and the occipital bone, near the back of the head. It is triangular, much smaller, and often only fingertip-sized at birth. Many parents never notice it at all, especially if a baby has a full head of hair.
While the anterior fontanelle is easy to spot and routinely checked at well-baby visits for months, the posterior fontanelle is typically only assessed in the first few weeks. In some healthy full-term newborns, the posterior fontanelle is already nearly closed or barely palpable at birth, which is considered perfectly normal. The window of one to three months is a population average; individual variation is wide, and the clinical significance lies mainly in cases that fall far outside that range.
Why Babies Have Fontanelles in the First Place
Fontanelles exist because a newborn’s skull is not a single fused shell. It is made up of several flat bones connected by fibrous tissue called sutures, with fontanelles at the points where multiple sutures converge. This architecture serves two distinct purposes, one immediate and one long-term.
The immediate purpose is childbirth. During a vaginal delivery, the baby’s head has to navigate through the birth canal, which is a tight fit. The skull bones are able to shift, overlap, and compress because they are not yet fused, a process called molding. The fontanelles and sutures allow the head to change shape under the compressive forces of labor contractions and the resistance of the birth canal, making delivery possible without damaging the brain inside.1Neurology and Neuroscience Reports. Configuration (molding) of the fetal head during labor and related issues Biomechanical modeling of this process confirms that the skull bones, sutures, fontanelles, and underlying soft tissues all participate in accommodating the geometry of the pelvic floor.2PubMed. Study on the influence of the fetus head molding on the biomechanical behavior of the pelvic floor muscles, during vaginal delivery
The long-term purpose is brain growth. A human infant’s brain roughly triples in volume during the first two years, and most of that growth happens in the first year. If the skull were already fused at birth, that expansion could not happen without dangerous pressure buildup. The open sutures and fontanelles give the cranial bones room to grow outward in response to the mechanical push of the expanding brain.3PubMed. A mechanically regulated computational framework for simulating infant cranial growth and craniosynostosis-associated dysmorphologies The posterior fontanelle closes early because the occipital region of the brain grows less rapidly than the frontal and parietal regions in the first months of life, so the bones there can begin fusing sooner without restricting brain development.
What Happens If It Closes Too Early or Too Late
A posterior fontanelle that fuses prematurely, within the first few weeks or even before birth, may signal craniosynostosis, a condition in which one or more cranial sutures close too soon. Craniosynostosis at the lambdoid suture, which runs along the back of the skull near the posterior fontanelle, is relatively rare compared to other forms. When it does occur, it can restrict skull growth in that region and push growth into compensatory directions, sometimes producing a visible asymmetry at the back of the head. Clinicians distinguish this from the far more common positional plagiocephaly (flat-head syndrome caused by sleeping position) through physical examination and, when needed, imaging.
A posterior fontanelle that remains open well past three months is less common but worth evaluating. Delayed closure of fontanelles in general can be associated with conditions that slow bone mineralization. Rickets caused by vitamin D or calcium deficiency is a classic example. In affected infants, softening of the occipital area, enlarged sutures and fontanelles, and delayed fontanelle closure can all be observed.4The Journal of Clinical Investigation. Resurrection of vitamin D deficiency and rickets Hypothyroidism, Down syndrome, and certain skeletal dysplasias can also slow fontanelle closure, though these conditions typically show other signs well before a persistently open posterior fontanelle raises concern on its own.
A single finding of a posterior fontanelle that seems larger or smaller than expected at one visit is rarely meaningful by itself. Pediatricians look at the overall pattern: head circumference growth, developmental milestones, and whether sutures elsewhere are tracking normally. A posterior fontanelle that is still palpable at four months in an otherwise healthy infant with a normal growth curve is unlikely to prompt any workup.
The Posterior Fontanelle as a Diagnostic Window
Before the posterior fontanelle closes, it offers clinicians a brief but valuable opportunity. Cranial ultrasound is one of the primary ways to image a newborn’s brain without radiation, and the fontanelles serve as acoustic windows for the ultrasound transducer. Most routine brain ultrasounds in neonates use the anterior fontanelle, which is larger and stays open longer. But the anterior approach has a significant limitation: structures at the back and base of the brain, including the cerebellum and the posterior portions of the cerebral hemispheres, are poorly visualized from the front.5PubMed Central. Posterior fontanelle sonography: an acoustic window into the neonatal brain
Scanning through the posterior fontanelle dramatically improves the view of these posterior and infratentorial structures. This matters most for premature infants, who are at elevated risk for bleeding in the germinal matrix and cerebellum, conditions that may be missed or underestimated when only the anterior fontanelle is used. Studies correlating posterior fontanelle ultrasound images with anatomical sections of premature infant brains have confirmed that this approach allows detailed visualization of occipital lobe anatomy across a range of gestational ages.6Early Human Development. POSTERIOR FONTANELLE CRANIAL ULTRASOUND – ANATOMIC AND SONOGRAPHIC CORRELATION The catch is timing: because the posterior fontanelle closes so early, this imaging window is available only in the first weeks to couple of months of life, and in very premature infants whose fontanelles may remain open somewhat longer.
The anterior fontanelle also serves as a site for noninvasive pressure monitoring. A device called a fontanelle tonometer, adapted from instruments used in eye pressure measurement, can estimate intracranial pressure by pressing gently against the fontanelle membrane. In a validation study involving 72 healthy infants, mean fontanelle pressure measured about 7 mmHg, with measurements correlating closely with direct intracranial pressure readings.7PubMed. The fontanelle tonometer: a noninvasive method for measurement of intracranial pressure While this technique has mainly been studied with the anterior fontanelle because of its larger size and longer availability, the principle underscores why open fontanelles matter clinically: they are not just gaps waiting to close, but functional diagnostic access points to the brain that disappear permanently once the bones fuse.
Vitamin D, Nutrition, and Fontanelle Timing
The link between nutrition and fontanelle closure is real, though it is most visible at the extremes. Vitamin D deficiency severe enough to cause rickets is the most studied nutritional influence. In rickets, the body cannot properly mineralize bone, and the cranial bones are among the earliest and most visibly affected. Pediatricians in the nineteenth century used a soft, yielding posterior skull (called craniotabes) as one of the earliest physical signs of the disease. Modern cases still show these same features: softened occipital bone, widened sutures, and fontanelles that stay open longer than expected.4The Journal of Clinical Investigation. Resurrection of vitamin D deficiency and rickets
Rickets has declined sharply in countries with routine vitamin D supplementation recommendations for breastfed infants, but it has not disappeared. Exclusively breastfed babies with dark skin who live in northern latitudes and do not receive supplementation remain at meaningful risk. In these cases, a posterior fontanelle that remains open past three or four months could be one early piece of a larger puzzle, alongside bowed legs, poor growth, and characteristic changes on X-rays. The fontanelle finding alone, without other signs, does not diagnose rickets, but it can be the observation that prompts further investigation.
On the flip side, there is no evidence that extra calcium or vitamin D supplementation accelerates fontanelle closure in healthy infants, and attempting to speed closure would not be desirable even if it were possible. The fontanelles are supposed to be open during that early window. They close on their own timetable as the skull bones grow toward each other, driven by the brain growth happening underneath.
Premature Babies and the Posterior Fontanelle
Preterm infants add a layer of complexity to fontanelle timelines. A baby born at 28 weeks has skull bones that are thinner, more pliable, and less mineralized than those of a full-term newborn. The fontanelles tend to be relatively larger, and they remain open longer when measured from birth date rather than from the original due date. Clinicians typically adjust for gestational age when assessing fontanelle size and closure, just as they do for developmental milestones. A premature infant whose posterior fontanelle is still open at four months of chronological age but only one month of corrected age is right on schedule.
The prolonged openness of fontanelles in preterm babies is actually clinically useful, because these infants are the ones who most need serial brain ultrasounds. Intraventricular hemorrhage, periventricular leukomalacia, and cerebellar hemorrhage are all complications of prematurity that can be monitored by ultrasound through the fontanelles. The posterior fontanelle approach is especially important for detecting cerebellar injury, which can be missed entirely when scanning only from the front.5PubMed Central. Posterior fontanelle sonography: an acoustic window into the neonatal brain The fact that preterm babies’ fontanelles stay open longer gives clinicians a wider window to catch problems that develop over days or weeks in the neonatal intensive care unit.
What You Can and Cannot Tell by Touching the Fontanelle
Parents are often told to watch the fontanelle, but rarely told what they are actually watching for. The anterior fontanelle is the one you can realistically monitor at home because of its size and accessibility. A normal fontanelle feels flat or very slightly concave and may pulse visibly with the baby’s heartbeat, which is completely normal and reflects the pulsation of blood vessels beneath the membrane.
A fontanelle that appears sunken or noticeably concave can be a sign of dehydration. When the body’s fluid volume drops, the soft fontanelle membrane sinks inward. This is one of the most practical reasons parents are taught about the fontanelle at all: a sunken soft spot in a baby with vomiting, diarrhea, or poor feeding is a signal to seek medical attention quickly. In contrast, a fontanelle that is persistently bulging or tense, especially when the baby is calm and upright, could indicate increased pressure inside the skull. This is uncommon but warrants prompt evaluation.
For the posterior fontanelle specifically, most parents will never be able to feel it because of its small size, its location under hair, and the fact that it closes so quickly. The posterior fontanelle is primarily a clinical finding rather than something you monitor at home. If your pediatrician mentions it during an early checkup, it is usually in the context of confirming that it is present and appropriately sized, not because you need to keep tabs on it between visits.
Cultural Beliefs and Traditional Practices Around Fontanelles
In many cultures around the world, the fontanelle carries significance that goes beyond Western biomedical understanding. In parts of Ghana, a sunken fontanelle or one that closes late is associated with a condition locally called “asram,” a feared newborn illness characterized by head enlargement, weight loss, and visible veins on the head and abdomen. Traditional belief holds that failure of the fontanelle to close properly may allow air to enter the brain, leading to persistent headaches and other chronic problems lasting into adulthood.8PubMed Central. Hot fomentation of newborn fontanelles as an indigenous practice in Ghana: implications for policy and integrated community-based health care in Covid-19 pandemic and beyond Hot fomentation, where warm compresses or heated substances are applied to the fontanelle, is a common traditional remedy in these communities.
Similar practices exist across West Africa, parts of Latin America, and in some Middle Eastern and South Asian communities, each with their own explanatory frameworks. In Latin American folk medicine, “mollera caÃda” (fallen fontanelle) is a recognized condition treated by traditional healers through methods including palate pressure and herbal applications. These practices occasionally come into conflict with biomedical care, particularly when hot applications cause burns or when traditional treatment delays evaluation for genuine conditions like dehydration or meningitis.
Public health researchers have increasingly argued that rather than dismissing these beliefs, healthcare systems should engage with them. The underlying observation, that a sunken fontanelle signals something wrong with the baby, is often medically accurate (dehydration being the most common cause). The disagreement is over the mechanism and the remedy, not over whether the fontanelle is worth watching. Community health programs that acknowledge traditional knowledge while connecting families to clinical care for concerning fontanelle findings tend to be more effective than those that simply contradict local beliefs.
Common Worries That Usually Are Not Problems
The fontanelle is a source of outsized parental anxiety, partly because the idea of a “hole” in a baby’s skull sounds alarming. A few reassurances are worth spelling out. First, the fontanelle is not fragile in the way most people imagine. The membrane covering it is tough fibrous tissue, not a thin film. Normal handling, gentle shampooing, and even moderate bumps are extremely unlikely to injure the brain through the fontanelle.
Second, pulsation at the anterior fontanelle is normal and does not indicate a problem. Third, a fontanelle that feels slightly different at different times of day, or when the baby is crying versus calm, is behaving exactly as expected. Intracranial pressure fluctuates normally with crying, straining, and position changes, and the fontanelle reflects those shifts because it is flexible. What matters is the baseline state when the baby is calm and upright.
For the posterior fontanelle, the most common parental worry is not being able to find it. Because it is small and closes early, many parents who go looking for it after reading about fontanelles cannot locate it and assume something is wrong. In most cases, the fontanelle is simply too small to feel easily, or has already closed, both of which are perfectly normal in a healthy infant past the first few weeks. If there were a genuine problem with premature fusion of the posterior skull bones, it would show up as a visible asymmetry or a ridge along the lambdoid suture that your pediatrician would notice on examination.