The “soft spot” on a baby’s head, medically known as a fontanelle, is a normal and necessary feature of early development. This area of unfused bone plates in the skull is a temporary part of a baby’s anatomy. The presence of these gaps is essential for two significant physical processes of early life. Understanding the function and expected changes of the fontanelle can provide reassurance that this delicate-looking feature is actually a well-protected part of the baby’s skull.
Anatomy and Function
The fontanelles are membranous gaps between the bony plates of the skull that have not yet fused. While a newborn has several smaller fontanelles, the two most notable are the anterior and posterior fontanelles. The anterior fontanelle is the larger, diamond-shaped soft spot located on the top of the head toward the front. The posterior fontanelle is a smaller, triangular spot found closer to the back of the head. These spaces are covered by a tough protective membrane, meaning the underlying brain tissue is safe and not exposed.
The primary function of these soft spots begins during birth, where the gaps allow the skull plates to overlap slightly in a process called molding. This flexibility enables the baby’s head to compress and change shape, which is essential for safely navigating the birth canal. After birth, the fontanelles accommodate the rapid growth of the infant brain. The open fontanelles ensure the skull can expand to keep pace with this development without creating excessive pressure.
When the Soft Spots Close
The closure of the fontanelles is a gradual process of ossification, where the membranous gaps are replaced by bone tissue, forming permanent sutures. The two main fontanelles close on different timelines. The smaller posterior fontanelle typically closes first, often within the first two or three months after birth.
The larger anterior fontanelle remains open for much longer. This delayed closure is necessary to allow for the brain’s substantial growth during infancy. The anterior fontanelle generally closes anytime between 9 and 18 months of age, though a range up to 24 months is sometimes considered normal. A baby’s health care provider monitors the closure during regular checkups to ensure the process is proceeding within the expected range.
Interpreting Changes in Appearance
A healthy fontanelle should appear relatively flat and soft when the baby is calm. It is common to observe a slight pulsation, which is the reflection of the baby’s heartbeat and blood flow. Changes in the appearance of the fontanelle act as a natural indicator of the baby’s internal state and should be monitored by parents.
A depressed or sunken fontanelle, appearing concave or dipped inward, is a primary sign of dehydration. This is caused by insufficient fluid intake or fluid loss from conditions like vomiting or diarrhea. If the soft spot is noticeably sunken and accompanied by signs like fewer wet diapers or a dry mouth, seek medical advice promptly.
A tense or bulging fontanelle indicates increased pressure inside the skull. It is normal for the fontanelle to briefly bulge when a baby is crying or straining, but it should return to flat once the baby is calm. A bulging soft spot that remains tense and curves outward even when the baby is resting can signal a more serious underlying issue. This can be associated with conditions such as fluid buildup in the brain or infections like meningitis, which require immediate medical attention.