The infant skull is composed of several separate bony plates joined by flexible, fibrous connections called cranial sutures. Where multiple sutures meet, there are wider, membrane-covered openings known as fontanelles, or soft spots. This unique, segmented design serves two primary functions for early development. First, the flexibility allows the bony plates to slightly overlap and mold during the birthing process, helping the head pass safely through the narrow birth canal. Second, this flexibility accommodates the rapid growth of the infant’s brain, ensuring the skull expands outward to provide sufficient space for developing brain tissue.
Timeline for Fontanelle Closure
Fontanelle closure is the initial stage of ossification, where fibrous membrane tissue is replaced by bone, effectively closing the soft spot. An infant is born with six fontanelles, but the anterior and posterior fontanelles are the most clinically significant and widely monitored. This closure process is distinct from the complete fusion of the sutures, which happens much later.
The posterior fontanelle is located at the back of the head, positioned at the junction of the two parietal bones and the occipital bone. This fontanelle is typically triangular and much smaller than the one at the front. Closure of the posterior fontanelle is the first to occur, often happening very early in life.
In many infants, the posterior fontanelle may already be closed at birth, or it will close within the first two to three months of life. If it remains open beyond three months, it may warrant further investigation by a healthcare professional.
The anterior fontanelle is the largest and most recognizable soft spot, positioned on the top of the head near the forehead. It has a characteristic diamond shape, formed where the two frontal bones and the two parietal bones meet. This fontanelle remains open for a significantly longer period than the posterior one, reflecting the continued need for rapid brain growth. The typical closure window occurs between 9 months and 18 months of age. Pediatricians routinely assess this soft spot, as its tension and size can offer insight into the internal pressure within the skull.
Timeline for Cranial Suture Fusion
The process of cranial suture fusion, known as synostosis, follows the closure of the fontanelles and involves the fibrous joints themselves hardening into solid bone. Even after the fontanelles close, the sutures remain flexible and are still capable of growth for several years to accommodate the remaining brain development. The complete, irreversible bony fusion of the major sutures is a multi-decade process, not an event of infancy.
The metopic suture, which runs vertically down the center of the forehead between the two frontal bones, is the exception to this long timeline. This suture normally begins to close relatively early, with fusion starting around three months of age. Complete fusion is typically achieved by the time a child reaches nine months to two years of age, making it the first major suture to solidify.
For the other major sutures—the coronal, sagittal, and lambdoid sutures—the timeline for complete fusion is much later and extends far beyond childhood. These sutures remain patent and flexible throughout the years of most significant brain growth. Although they become functionally interdigitated and immovable in early childhood, they do not undergo complete osseous fusion.
Full bony obliteration of the coronal, sagittal, and lambdoid sutures usually begins in early adulthood. The sagittal suture often begins its final fusion process around the third decade of life. Coronal and lambdoid sutures follow a similarly delayed schedule, with complete fusion often occurring between 30 and 40 years of age.
Deviations from Normal Development
Abnormalities in the timing of fontanelle or suture closure can signal an underlying health issue, which is why pediatricians monitor the head circumference and soft spots closely. The most significant deviation is premature closure, a condition termed craniosynostosis. Craniosynostosis occurs when one or more cranial sutures fuse too early, sometimes even before birth.
This premature fusion restricts growth perpendicular to the affected suture, causing the skull to grow abnormally in other directions, resulting in a misshapen head. If multiple sutures close early, the rapidly growing brain may not have enough space, which can lead to increased pressure inside the skull. This increased intracranial pressure can potentially affect brain development.
Conversely, fontanelles or sutures that remain open significantly past their expected timeline may also indicate a health concern. Delayed closure of the anterior fontanelle, for instance, can be a symptom of conditions like congenital hypothyroidism, which affects hormone-regulated growth. Nutritional deficiencies, such as rickets due to a lack of Vitamin D, can also interfere with the normal process of bone mineralization and delay closure.
Other causes for delayed closure include certain genetic syndromes or conditions that cause elevated pressure within the skull, such as hydrocephalus. Because the closure timeline is variable, a single measurement is rarely diagnostic, but consistent monitoring of head circumference and the physical characteristics of the fontanelle are important indicators for when a medical consultation is necessary.