The area commonly referred to as a baby’s soft spot is known medically as a fontanelle. These are the normal gaps between the separate, bony plates that form the skull. The skull is not a single, solid structure at birth; these fibrous, membrane-covered spaces are a temporary, yet entirely normal, part of infant anatomy. They are a necessary feature that allows for the physical changes a baby undergoes in its earliest stages of life.
Identifying the Soft Spots
An infant is born with several fontanelles, but parents typically notice two. The most prominent is the anterior fontanelle, the larger soft spot located on the top, front portion of the head. This spot is diamond-shaped, forming where the two frontal and two parietal skull bones meet.
The second, smaller soft spot is the posterior fontanelle, situated toward the back of the head. It is typically triangular and less noticeable than the anterior fontanelle. Although they may appear vulnerable, these gaps are covered by a tough, protective layer of connective tissue that safeguards the underlying brain tissue. A healthy fontanelle should feel flat or very slightly curved inward when the baby is calm.
Crucial Roles in Early Development
The fontanelles serve two primary functions related to a newborn’s development and survival. The first is accommodating the pressure and shape changes required during a vaginal delivery. These gaps allow the flexible skull plates to overlap and mold, temporarily reducing the circumference of the head as it passes through the narrow birth canal. This process, called molding, helps prevent injury to both the mother and the baby’s brain.
The second, longer-term function is to allow for the rapid expansion of the brain following birth. A human infant’s brain triples in size during the first year. If the skull bones were rigidly fused at birth, this rapid growth would be restricted, potentially causing severe developmental issues. The soft spots and the flexible sutures connecting the skull plates provide the necessary space for the brain to grow without being compressed.
The Timeline of Fontanelle Closure
The closure of the fontanelles is a process of ossification, where the membranous gaps are gradually replaced by bone. The smaller posterior fontanelle is the first to close, typically occurring within the first one to three months after birth.
The anterior fontanelle remains open significantly longer to support continued rapid brain development. While the timing can vary widely among healthy infants, this largest soft spot usually closes between nine and eighteen months of age. Medical providers monitor the timing of this closure during routine checkups to ensure the skull is developing at an appropriate rate.
When to Seek Medical Advice
Parents should monitor the appearance and tension of the anterior fontanelle, as changes can signal underlying health issues. A bulging or tense fontanelle, especially when the baby is upright and not crying, can be a sign of increased intracranial pressure. This may indicate conditions like hydrocephalus or a serious infection such as meningitis, and requires immediate medical attention.
Conversely, a noticeably sunken fontanelle is often an indicator of dehydration, particularly if accompanied by signs like a reduced number of wet diapers. Parents should also consult a pediatrician if the anterior fontanelle closes much earlier than the nine-month range (a sign of craniosynostosis), or if it remains widely open past two years (suggesting a developmental or metabolic issue).