The human skull begins life not as a single, solid structure but as a collection of separate bone plates. This flexible design is a feature of early human development, setting the stage for the tremendous growth that occurs shortly after birth. Newborns have several gaps and seams in their skulls, composed of fibrous, elastic tissue rather than hard bone. This initial flexibility allows the infant skull to be pliable for two distinct purposes: navigating the birth canal and accommodating the rapidly expanding brain.
The Function of Cranial Sutures and Fontanelles
The specialized structures that provide the skull’s flexibility are the cranial sutures and the fontanelles. Cranial sutures are fibrous joints that connect the six separate bone plates of the infant skull, acting like seams. These sutures are not fused at birth, allowing the bones to overlap slightly under pressure. This overlapping is essential for the first function: facilitating passage through the narrow birth canal during labor.
Fontanelles, commonly known as soft spots, are the wider, membrane-covered spaces where multiple sutures intersect. The newborn skull typically has six fontanelles, with the anterior and posterior fontanelles being the most prominent. These soft spots accommodate the second function: rapid brain growth in infancy. Without this flexibility, the brain, which nearly doubles in size during the first year, would be constrained. Fontanelles also serve as a window for medical providers to assess pressure inside the brain, as a sunken or bulging soft spot can indicate issues like dehydration or increased intracranial pressure.
The Standard Timeline for Suture Fusion
The eventual closure of these fontanelles and sutures is a normal part of development, where the fibrous tissue is replaced by bone in a process called ossification. The posterior fontanelle, located toward the back of the head, is typically the first to close. This smaller soft spot usually closes completely within the first two to three months after birth. The anterior fontanelle, the largest soft spot located on the top-front of the head, remains open for a much longer period.
The closure of the anterior fontanelle generally occurs between 7 and 19 months of age, with 12 to 18 months being the most commonly cited average range for complete fusion. This extended timeline reflects the period of most rapid brain growth. While these timelines represent the typical average, slight variations in the timing of closure are not uncommon and are generally considered normal developmental differences. The smaller sphenoidal and mastoid fontanelles on the sides of the head usually close within the first few months, following the posterior fontanelle.
The cranial sutures follow a fusion timeline far longer than that of the fontanelles. While sutures become functionally closed and stable in childhood, true bony fusion, known as obliteration, continues well into adulthood. Major sutures, such as the sagittal, coronal, and lambdoid sutures, may not fully obliterate until the third decade of life or later. Although the soft spots disappear relatively early, the lines where the skull bones meet remain somewhat flexible for many years.
Consequences of Premature Suture Closure
When one or more cranial sutures fuse or close too early, a condition known as craniosynostosis occurs. This is a congenital birth defect that restricts the growth of the skull perpendicular to the affected suture line. The premature closure prevents the skull from expanding normally in that direction, which can lead to an abnormally shaped head and, in some cases, pressure on the developing brain. The overall incidence of craniosynostosis is estimated to be about one in every 2,000 to 2,500 live births.
The specific head shape that results depends entirely on which suture is affected by the early fusion. For example, sagittal synostosis, the most common type, involves the premature closure of the sagittal suture running along the top of the head. Since growth is restricted side-to-side, the head compensates by growing long and narrow, resulting in a shape called scaphocephaly. If the coronal suture closes too early, the forehead on the affected side may appear flattened, and the skull can grow broad and short.
Restricted growth can increase pressure inside the skull, potentially harming brain development if left unaddressed. The presence of craniosynostosis usually necessitates medical intervention to allow the brain sufficient space to grow. This intervention is most often surgical, aiming to reopen the fused suture and reshape the skull. Early diagnosis and treatment are important to prevent potential complications like developmental delays or vision problems that can arise from sustained intracranial pressure.