The human skull functions as a robust bony vault, offering protection for the brain, yet this structure is not fully fused at birth. The softest parts of the skull, commonly known as soft spots, apply exclusively to infants. These areas of flexibility are technically called fontanelles, which are membranous gaps located between the individual bony plates of the skull. This temporary biological feature allows the skull to accommodate rapid brain growth and facilitate delivery.
Identifying the Soft Spots
There are six fontanelles present in the newborn skull, though two are significantly more prominent and recognized as the primary soft spots. The most notable one is the anterior fontanelle, located on the top of the head where the frontal and parietal bones intersect. This area is relatively large, possessing a distinctive diamond shape, and measures about one to three centimeters across at birth.
The second major area is the posterior fontanelle, which sits toward the back of the head at the junction of the parietal and occipital bones. This spot is much smaller than the large anterior fontanelle and typically takes on a triangular shape. Fontanelles are not unprotected openings; they are covered by a layer of tough, fibrous connective tissue, providing a significant degree of protection to the underlying brain. Smaller paired fontanelles, the sphenoidal and mastoid, also exist on the sides of the head but close quickly after birth.
The Role of Fontanelles in Development
Fontanelles serve two primary biological functions linked to human development and birth. The first is to allow the skull to be temporarily flexible, which is an adaptation for labor and delivery. During passage through the narrow birth canal, the soft spots and the fibrous joints between the skull plates, known as sutures, permit the bony plates to overlap slightly, a process called molding. This overlapping action temporarily reduces the head’s diameter, facilitating a safer birth and preventing damage to the infant’s brain.
The second function is to accommodate the rapid growth of the brain that occurs during the first year of life. The brain nearly triples in weight by the time a child reaches two years of age. The presence of these unfused areas allows the skull to expand uniformly to keep pace with this growth. Without the fontanelles and flexible sutures, the expanding brain would face dangerously high pressure within a rigid bony case.
When the Skull Hardens
The temporary nature of the fontanelles is defined by the process of ossification, where the membranous tissue gradually turns into solid bone. The timeline for this closure varies between the different soft spots, with the posterior fontanelle closing first. This smaller fontanelle usually closes within the first two to three months following birth.
The anterior fontanelle, being the largest, remains open for a much longer period to allow for continued brain growth. Its closure typically occurs between 12 and 18 months of age, though 7 to 19 months is considered within the normal developmental window. Once closed, the spaces become rigid, interlocking joints called sutures, which firmly unite the bony plates of the skull. Pediatricians monitor this timeline, as premature or delayed closure can indicate underlying medical conditions.
Clinical Monitoring and Significance
The fontanelles provide pediatricians with a non-invasive way to assess an infant’s health, offering insight into the brain’s condition and hydration status. A normal fontanelle should feel flat and firm to the touch, or possibly slightly curved inward. It may also exhibit a slight pulse that aligns with the baby’s heartbeat, which is a normal observation.
Any significant change in the fontanelle’s appearance can signal a health issue. A sunken or depressed fontanelle often suggests dehydration, especially if the baby has symptoms like fewer wet diapers. Conversely, a tense or bulging fontanelle that remains prominent when the baby is calm can indicate elevated intracranial pressure. This bulging may be a sign of conditions like hydrocephalus or meningitis, making the fontanelle a valuable tool for early detection.