The soft spots on an infant’s head, known medically as fontanelles, are a normal and highly functional feature of the developing skull. These spaces are strong, membrane-covered gaps between the bones of the skull, not merely delicate areas to be avoided. Understanding these anatomical features is important because their presence and eventual closure are linked to two significant developmental needs in early life. Tracking the closure of these soft spots is a routine part of pediatric care, as the timing provides health professionals with information about the baby’s growth and internal health.
What Are Fontanelles and Why Are They Necessary?
Fontanelles are the fibrous, flexible spaces located where the skull bones have not yet fused, acting as junctions between the cranial bones. The six separate bones that form the infant skull are held together by elastic tissues called sutures. Two fontanelles are routinely monitored: the diamond-shaped anterior fontanelle at the top of the head and the smaller, triangular posterior fontanelle at the back.
These soft spots serve two primary functions necessary for safe delivery and healthy development. First, the flexibility of the fontanelles and sutures allows the bony plates of the skull to slightly overlap or “mold” during childbirth. This temporary compression changes the shape of the head, allowing it to pass through the narrow birth canal without damaging the brain.
The second purpose is to accommodate the rapid expansion of the infant’s brain during the first year of life. The brain grows faster than the surrounding bone, and the fontanelles remain open to allow for this massive growth. Once the skull bones fully ossify, no further expansion of the braincase is possible, making this flexibility crucial for neurological development.
The Timeline for Soft Spot Closure
The closure of the soft spots occurs as the fibrous membranes are replaced by bone tissue through a process called intramembranous ossification. The two primary fontanelles close on different timelines, with the posterior fontanelle closing much sooner than the anterior one. This staggered process ensures the skull maintains the necessary flexibility for brain growth over the first two years of life.
The posterior fontanelle, located toward the back of the head, is the first to solidify. This smaller, triangular soft spot typically closes between 6 weeks and 3 months after birth, sometimes being closed at delivery. Caregivers may notice its disappearance early on during the baby’s first few well-child visits.
The anterior fontanelle is the largest and most easily felt soft spot on the top of the head. It has a much wider closure window, generally occurring between 9 months and 18 months of age. Although the average closure time is often cited around 12 to 14 months, it can sometimes remain open until the child is two years old, and a broad range is considered normal.
Significant variation in this timeline warrants a medical evaluation. If the anterior fontanelle closes prematurely, typically before nine months, it is known as craniosynostosis, which can restrict brain growth and may require intervention. Conversely, delayed closure beyond two years may be associated with various underlying conditions, such as congenital hypothyroidism or rickets.
Recognizing Normal and Abnormal Appearance
A healthy fontanelle should feel soft and flat, potentially curving slightly inward relative to the surrounding skull bone. It is normal for the soft spot to exhibit a gentle, visible pulse that corresponds to the baby’s heartbeat, reflecting the blood flow beneath the membrane. Caregivers should know that it is safe to gently touch and wash the area, as the membrane provides adequate protection to the underlying brain.
Two distinct appearances of the soft spot signal a need for medical attention, starting with a sunken fontanelle often indicating dehydration. This causes the soft spot to appear depressed or concave, sinking noticeably below the contour of the skull. Dehydration can result from insufficient fluid intake, excessive heat exposure, or illness involving vomiting or diarrhea.
The second warning sign is a bulging fontanelle, which is generally considered a more urgent concern. A bulging fontanelle appears tense, firm, or raised above the level of the surrounding bone when the infant is calm and upright. It is normal for the soft spot to bulge temporarily while the baby is crying, coughing, or vomiting. However, if it remains tense when the baby is relaxed, it can indicate increased pressure within the skull.
Increased intracranial pressure can be caused by serious conditions such as hydrocephalus (fluid buildup), meningitis (infection of the protective membranes), or hemorrhage. If a bulging fontanelle is accompanied by other signs like fever, excessive sleepiness, or unusual behavior, immediate medical evaluation in an emergency setting is necessary. Observing the soft spot for these changes offers a direct way for caregivers to monitor the baby’s internal health.