The presence of a “soft spot” on an infant’s head, commonly known as a fontanelle, is a normal feature of early development. These areas are gaps between the bony plates of the skull that have not yet fused together. The fontanelles are essential for two distinct phases of an infant’s life. A healthy fontanelle should feel flat and soft, sometimes exhibiting a slight, visible pulse that mirrors the baby’s heartbeat.
The Anatomy and Function of Fontanelles
A newborn’s skull is not a single, solid bone structure but is composed of several separate bony plates joined by fibrous seams called sutures. The fontanelles are the wider, membranous spaces where multiple sutures intersect. These spaces are important because they allow the skull to change shape dynamically in response to pressure.
The most prominent of these spaces is the anterior fontanelle, a diamond-shaped area located on the top of the head toward the front. There is also the posterior fontanelle, which is much smaller and triangular, situated near the back of the head. These temporary openings are covered by a tough, protective membrane, not merely exposed skin, which shields the brain beneath.
The fontanelles serve a dual function that aids in both birth and growth. The first role is during childbirth, where the flexibility of the fontanelles and sutures allows the bony plates to overlap, a process called molding. This slight compression decreases the overall diameter of the head, enabling the infant’s skull to pass more easily through the narrow birth canal.
The second function is to accommodate the rapid growth of the brain during the first year of life. The brain undergoes a growth spurt, and the open fontanelles ensure that the hard skull does not restrict this expansion. Without this flexibility, the brain’s development would be severely hindered.
Practical Care and Safety Around the Soft Spots
Despite the common term “soft spot,” the fontanelles are protected by a durable membrane. Normal activities like gently touching the baby’s head, washing their hair, or carefully placing a hat on them are safe. It is important to handle the baby’s head with the same general caution used for the rest of their body, avoiding any direct, hard pressure or trauma.
The fontanelles act as an important indicator of an infant’s health, which is why pediatricians routinely examine them. A fontanelle that appears noticeably sunken or depressed often signals dehydration. This can occur if the baby is not taking in enough fluids, perhaps due to illness involving vomiting or diarrhea.
Conversely, a bulging or tense fontanelle, particularly when the baby is calm and upright, can be a sign of increased pressure inside the skull. This may indicate a serious medical issue such as a brain infection, fluid buildup, or head trauma, and requires immediate medical attention. If the fontanelle bulges only when the baby is crying, coughing, or lying down, and then returns to a flat, normal state, it is usually not a cause for concern.
The Timeline for Fontanelle Closure
The fontanelles eventually close as the fibrous tissue is replaced by bone in a process called ossification. The two main fontanelles close at different, predictable rates, which allows for continued brain growth while the skull simultaneously gains structural integrity.
The posterior fontanelle is the first to close. This closure typically occurs within the first two or three months after birth. In some instances, this fontanelle may be already closed at the time of birth.
The anterior fontanelle remains open for much longer to accommodate the significant brain growth during infancy. It is generally the last to fully close, with the process typically concluding anytime between 9 and 18 months of age. Pediatricians monitor this timeline closely, as extremely early closure can restrict brain growth, while significantly delayed closure may be linked to certain underlying medical conditions.