Soft spots, or fontanelles, are the flexible, membrane-covered gaps between a baby’s skull bones. These features are a temporary and normal part of infant anatomy. While they may feel delicate, the fontanelles are protected by tough, fibrous membranes and skin. A newborn typically has six fontanelles, but healthcare providers regularly monitor the two most prominent ones: the anterior and posterior.
What Are Fontanelles and Their Purpose
The two main soft spots are identified by their location and shape. The anterior fontanelle is the larger, diamond-shaped spot located toward the front of the head, near the top. The posterior fontanelle is much smaller, triangular, and situated at the back of the head. These membranous areas exist because the bony plates of the skull are not yet fused together at birth.
This structural separation serves two important biological functions. First, the fontanelles and the flexible sutures connecting the skull plates allow the skull bones to overlap slightly during birth. This molding facilitates the baby’s passage through the relatively narrow birth canal.
Second, these open spaces accommodate the rapid growth of the brain during the first years of life. The infant’s brain grows quickly, reaching about half of its adult size by the first year. The fontanelles remain open, allowing the skull to expand as the brain volume increases.
The Timeline for Soft Spot Closure
The closure of the soft spots is a gradual process of ossification, where fibrous membranes are replaced by bone tissue. The two main fontanelles close at distinctly different times. The posterior fontanelle, the smaller one at the back of the head, closes much sooner than the anterior one.
This spot is often closed at birth or finishes fusing within the first two to three months of life. The anterior fontanelle takes significantly longer to close. Closure is typically complete between 12 and 18 months of age, though a range of 7 to 19 months is considered normal.
Pediatricians routinely check the size and tension of these spots during well-child visits to ensure the baby’s skull is growing appropriately. Once the anterior fontanelle is fully closed, the skull bones have grown together, and the soft spots are no longer palpable.
When to Seek Medical Attention
A normal fontanelle should feel flat and slightly firm, but changes in appearance can indicate a medical issue. A noticeably sunken or concave fontanelle is a classic sign of severe dehydration. This is concerning if accompanied by symptoms like a dry mouth, fewer wet diapers, or unusual lethargy.
Conversely, a bulging fontanelle that curves outward when the baby is calm can signal increased pressure inside the brain. Temporary bulging is normal when a baby cries, strains, or vomits. However, a persistent bulge may be associated with serious conditions like hydrocephalus, meningitis, or encephalitis.
Immediate medical attention is required if bulging is accompanied by fever, excessive sleepiness, or vomiting. Abnormal timing of closure also warrants medical evaluation.
If the anterior fontanelle closes much earlier than nine months, craniosynostosis may be present. This is the premature fusion of skull sutures that can restrict brain growth.
If the soft spot remains open significantly past 18 months, it may suggest underlying conditions. Delayed closure can be associated with issues such as congenital hypothyroidism, Down syndrome, or rickets. A medical professional will assess the fontanelle in the context of the baby’s overall growth and development.