The fontanelle, or “soft spot,” is a naturally occurring gap between the bony plates of a newborn’s skull. This feature exists because the skull bones have not yet fully fused. Fontanelles are covered by a tough, fibrous membrane that protects the underlying brain tissue. They are a normal and necessary part of infant anatomy, facilitating both the birthing process and early brain development.
Anatomical Location and Identification
An infant’s skull typically has six fontanelles, though only two are commonly known and easily palpable. The most prominent is the anterior fontanelle, the large, diamond-shaped soft spot found on the top of the head toward the front. It is formed where the two frontal and two parietal bones meet. At birth, the anterior fontanelle averages about 2.1 centimeters in width and is the largest of the soft spots.
The second major soft spot is the posterior fontanelle, located at the back of the head. This smaller, triangular fontanelle is situated where the two parietal bones meet the occipital bone. While four other smaller fontanelles exist on the sides of the head, the anterior and posterior ones are the most important for clinical assessment.
Biological Purpose of the Fontanelles
Fontanelles serve two primary biological functions for the newborn. Primary, they are crucial for childbirth, allowing the baby’s head to successfully navigate the narrow birth canal. The flexible, fibrous sutures permit the skull plates to overlap slightly—a process called molding—which temporarily reduces the head’s circumference.
The second function relates to the rapid expansion of the brain during the first years of life. A baby’s brain grows quickly, reaching about half its adult size by one year of age. The fontanelles and flexible sutures act as expansion joints, preventing the rigid skull from compressing the developing brain.
Developmental Timeline for Closure
Fontanelles close (ossify) as the infant grows and the need for cranial flexibility decreases. The smaller posterior fontanelle is the first to close, typically sealing up within the first two to three months after birth, and sometimes is already closed at delivery. The larger anterior fontanelle remains open significantly longer to accommodate continued rapid brain growth.
This soft spot usually closes between 9 and 18 months of age, though the normal range extends from 4 to 26 months. Pediatricians routinely monitor the size and tension of the fontanelle during check-ups, as the exact timing varies widely between children.
Gentle Handling and Recognizing Health Indicators
Despite the perception that the soft spot is fragile, the brain is protected by a tough, durable membrane beneath the skin. Parents should not fear normal handling, such as gently washing the baby’s hair or placing a hat on the head. The fontanelle acts as a direct indicator of a baby’s internal health and pressure within the skull.
A normal fontanelle should feel relatively flat and firm to the touch. It is common for a slight, gentle pulse to be visible or felt due to underlying blood flow, which is not a cause for concern. Parents should monitor for two specific changes that signal a need for medical attention.
A sunken or depressed fontanelle, where the spot dips inward, is a common sign of dehydration. This is particularly worrisome if accompanied by signs like a lack of tears or decreased wet diapers. Conversely, a bulging or tense fontanelle that remains raised when the baby is calm may indicate increased intracranial pressure.
While crying or vomiting can temporarily cause a slight bulge, persistent tension may signal a serious condition, such as fluid buildup or infection. Any noticeable, lasting change in the soft spot’s appearance should be immediately discussed with a healthcare provider.