A baby’s soft spot, medically known as a fontanelle, is a naturally occurring gap between the plates of the skull that have not yet fused together. These membranous spaces are a normal feature of an infant’s anatomy and serve an important function in development and delivery. The soft spots are covered by a tough, protective membrane, meaning they are not as fragile as they might seem.
The Major Fontanelles: Location and Shape
The newborn skull contains six fontanelles, but the two most prominent are the anterior and posterior fontanelles. The anterior fontanelle is the soft spot most people recognize, located at the top and front of the baby’s head. This is the largest gap and has a characteristic diamond shape where the frontal and parietal bones meet. The posterior fontanelle is situated toward the back of the head, positioned between the occipital and parietal bones. It is considerably smaller than the anterior one, often measuring less than a quarter-inch at birth, and is triangular in shape.
Why Soft Spots Exist: Function in Birth and Growth
Fontanelles are necessary for two distinct processes in early life. The first occurs during labor, where the spaces allow the skull bones to be flexible. This flexibility permits the bones to overlap slightly (molding), which temporarily reduces the head’s circumference. This compression allows the baby’s head to pass more easily through the narrow birth canal.
The second function is to accommodate the rapid growth of the brain after birth. Since the brain grows quickly during the first two years of life, the open fontanelles ensure the skull has room to expand without exerting pressure on the developing brain tissue.
What is Normal: Appearance and Closure Timeline
A healthy fontanelle should feel soft and flat against the contour of the baby’s head, or sometimes slightly curved inward. A subtle pulsation is often visible or palpable in the anterior fontanelle; this is simply the baby’s heartbeat transmitted through the connective tissue and is normal. The fontanelles close at different times as the skull bones begin to fuse. The smaller posterior fontanelle closes relatively quickly, typically between one and three months after birth. The anterior fontanelle, being larger, remains open much longer to allow for continued brain growth, usually closing between 12 and 18 months of age, sometimes as late as 24 months.
Practical Care and Urgent Warning Signs
The soft spots are protected by a durable membrane, so everyday care activities like gently washing a baby’s hair are safe. Parents should avoid applying direct, strong pressure, but routine gentle handling will not cause harm. The anterior fontanelle is routinely checked by a pediatrician during well-child visits because it acts as an external indicator of a baby’s health status. Parents should watch for two specific changes that require prompt medical attention.
Sunken Fontanelle
A noticeably sunken fontanelle, which dips inward, is often a sign of dehydration. This is a concern if the baby also has fewer wet diapers or shows other signs of low fluid intake.
Bulging Fontanelle
A bulging or tense fontanelle, especially one that remains raised when the baby is calm and upright, may indicate increased pressure inside the skull. While a slight bulge during crying or vomiting is normal, persistent bulging can be a sign of a serious medical issue. If the fontanelle is consistently bulging and is accompanied by symptoms like fever, vomiting, or lethargy, emergency medical advice should be sought immediately.