The question of when a baby’s skull completely fuses often stems from the presence of “soft spots.” The infant skull is structured differently from an adult skull, designed to be flexible and adaptive during rapid development. This temporary flexibility allows for necessary changes before the bony plates eventually knit together. Understanding the anatomy, timeline, and function of this process demystifies this normal stage of growth.
The Flexible Design: Sutures and Fontanelles
The skull of a newborn is not a single, solid bone; instead, it is composed of several bony plates held together by fibrous joints called cranial sutures. These sutures are seams of flexible, elastic tissue that provide the necessary give between the plates. This design is distinct from the fused, rigid structure of an adult skull.
Where several sutures intersect, they create wider membranous gaps known as fontanelles, or soft spots. While a newborn has six fontanelles, the two most prominent are the anterior and posterior fontanelles. The anterior fontanelle is the largest, diamond-shaped space located toward the front and top of the head.
The posterior fontanelle is smaller and triangular, located toward the back of the head. These open spaces are covered by a tough, protective membrane. This allows healthcare providers to assess the infant’s development and the pressure inside the brain.
The Standard Timeline of Closure
The timeline for fontanelle closure is staggered, with the posterior fontanelle closing first, typically within the first few months after birth. The posterior soft spot often closes between one and two months of age, and sometimes may be closed at birth. This early closure reflects its smaller size and location at the junction of the parietal and occipital bones.
The anterior fontanelle, the main soft spot at the top of the head, remains open for a much longer period. This larger fontanelle usually closes between nine and eighteen months of age. The normal range extends from seven to nineteen months, with the median time for closure being approximately 13.8 months.
While the fontanelles close within the first two years of life, complete skull fusion continues long after. The sutures connecting the bony plates do not fully ossify—meaning they do not turn into solid bone—until early adulthood, often late into the third decade of life. Therefore, while the soft spots disappear in infancy, the “complete fusion” of the entire skull is a slow process spanning many years.
Why the Skull Needs to Remain Open
The flexible skull structure serves two necessary functions for the developing infant. The first is accommodating passage through the birth canal during labor and delivery. The open sutures and fontanelles allow the skull plates to temporarily overlap and mold, which slightly reduces the head’s diameter.
This molding process prevents excessive pressure on the brain as the infant passes through the maternal pelvis. The second function is to allow for the rapid growth of the brain after birth. The human brain grows fastest during infancy, reaching about 66% of its adult size by the age of two.
The flexible sutures act like expansion joints, permitting the skull to enlarge evenly as the brain grows. Without this ability to expand, the brain would not be able to develop fully, potentially leading to neurological damage. The delayed fusion supports this rapid postnatal brain growth.
When Closure Doesn’t Follow the Schedule
Deviations from the typical closure schedule can signal an underlying medical condition, which is why pediatricians routinely check the fontanelles. If the fontanelles close too early, craniosynostosis may be present. This condition involves the premature fusion of one or more cranial sutures, restricting growth perpendicular to the fused seam.
This restriction can lead to an abnormal head shape and limit space for the growing brain. Early closure can also be associated with conditions like hyperthyroidism. Conversely, a significantly delayed closure of the anterior fontanelle, typically past two years of age, can also signal concerns.
Delayed closure is often linked to conditions such as congenital hypothyroidism, Down syndrome, or rickets (softening of bones due to vitamin D deficiency). A bulging or sunken fontanelle also warrants immediate attention. A sunken appearance commonly indicates dehydration, while a bulging fontanelle suggests increased pressure within the brain due to issues like hydrocephalus or infection. Consulting a pediatrician is advised if the soft spots appear to be closing too soon, too late, or if they feel abnormally firm, sunken, or swollen.