The term “soft spot,” or fontanelle, refers to the areas on an infant’s head where the skull bones have not yet fused together. These temporary gaps are a normal feature of a newborn’s anatomy, accommodating fundamental needs during development. While they feel soft to the touch, these areas are protected by a tough, resilient membrane. Healthcare providers routinely monitor these openings to ensure development is progressing as expected.
Anatomy and Location
The skull of a newborn is composed of several separate bony plates, not a single solid structure. The soft spots are the membrane-covered spaces located at the intersections of the skull’s fibrous joints, known as sutures. While a baby is born with six fontanelles, only two are commonly discussed because they remain open the longest and are easily palpable.
The most prominent soft spot is the anterior fontanelle, situated on the top of the head near the front. This space has a characteristic diamond shape and measures about 2.1 to 2.5 centimeters on average at birth. A second, smaller opening, the posterior fontanelle, is located at the back of the head. This smaller fontanelle is triangular in shape and is often only about 0.5 to 0.7 centimeters in diameter at the time of birth.
Essential Roles of the Soft Spots
The primary function of the fontanelles relates directly to the birth process. The flexible nature of the skull plates allows them to overlap slightly, a process called molding, as the baby passes through the narrow birth canal. This temporary compression reduces the overall size of the infant’s head, which facilitates a safer delivery.
Following birth, the fontanelles accommodate the extremely rapid growth of the brain during the first year of life. The brain volume nearly doubles in the first 12 months, and the open spaces permit the skull to expand accordingly. Additionally, the fontanelles provide an accessible window for physicians to assess brain development and monitor intracranial pressure.
The Closure Timeline
The two main fontanelles follow distinctly different timelines for closure as the skull bones begin to harden and fuse together. The smaller posterior fontanelle, located at the back of the head, typically closes first. This closure usually occurs within the first one to three months after birth.
The anterior fontanelle, the larger soft spot on the top of the head, remains open for a significantly longer period. While some begin to close around six months, the anterior fontanelle is usually fully closed anytime between 7 months and 18 months of age, with a mean closure time around 13 to 14 months.
Monitoring and Warning Signs
Normal Appearance
A normal fontanelle should feel soft, flat, or slightly concave when the baby is calm and upright. You may sometimes observe a subtle pulsation in the area, which is simply the normal blood flow coinciding with the baby’s heartbeat.
Sunken Fontanelle
A fontanelle that appears significantly sunken or depressed below the surrounding skull is often a sign of dehydration. This is especially concerning if accompanied by other symptoms like a lack of tears when crying, fewer wet diapers, or excessive sleepiness. A severely sunken soft spot requires prompt medical evaluation to address the infant’s fluid balance.
Bulging Fontanelle
In contrast, a bulging fontanelle that remains tense and protrudes outward, even when the baby is calm and sitting upright, can signal increased pressure inside the skull. While the soft spot may temporarily bulge when a baby is crying, straining, or vomiting, a persistent bulge can indicate serious conditions like hydrocephalus, meningitis, or intracranial hemorrhage. This finding, particularly when accompanied by fever or extreme drowsiness, is a medical emergency that requires immediate attention.
Closure Issues
Premature closure of a fontanelle, known as craniosynostosis, happens when one or more of the skull sutures fuses too early. This condition can restrict brain growth and lead to an abnormally shaped head, requiring timely intervention. Conversely, a significantly delayed closure beyond two years may suggest underlying issues like hypothyroidism or rickets, warranting further medical investigation.