The fontanelle, commonly known as the soft spot, is a normal anatomical feature present in newborns that often causes concern for new parents. These membranous gaps between the skull bones are a necessary part of an infant’s anatomy. Parents often monitor this area closely as a physical cue to their baby’s well-being.
Understanding the Fontanelles
The infant skull consists of several bony plates, leaving fibrous joints called sutures and the wider spaces known as fontanelles. There are two primary fontanelles: the anterior fontanelle, located on the top of the head, and the smaller, triangular posterior fontanelle, found near the back of the head. This temporary flexibility is crucial during childbirth, allowing the skull plates to temporarily overlap and the head to safely navigate the birth canal.
After birth, the fontanelles continue to serve a major purpose by accommodating the rapid expansion of the infant’s brain, which nearly triples in size during the first year of life. A normal fontanelle should feel relatively soft and flat against the head when the baby is calm and upright. Occasionally, a slight inward curve is normal, and it may gently pulsate in rhythm with the baby’s heartbeat due to the underlying blood flow.
Identifying a Sunken Fontanelle
A sunken fontanelle is defined as a noticeable dip or concave appearance in the soft spot, set lower than the typical slight inward curve. The fontanelle functions as a pressure sensor, and its appearance is highly sensitive to the body’s fluid volume. When there is a significant reduction in circulating blood and cerebrospinal fluid, the pressure that keeps the fontanelle flat decreases, causing it to visibly depress.
The most common cause of a sunken fontanelle is dehydration. This fluid deficit is frequently a result of illness, such as persistent vomiting or diarrhea, fever, or inadequate fluid intake. While a slight indentation might occur when a baby is lying down or has not fed recently, a persistently sunken soft spot is a strong physical indication of a fluid imbalance.
Accompanying Symptoms and When to Worry
A sunken fontanelle alone can sometimes be a normal variation, but it becomes a serious cause for concern when accompanied by other signs of dehydration or illness. Caregivers should seek immediate medical attention if the sunken fontanelle is observed alongside lethargy, which is unusual sleepiness or difficulty waking the infant. This combination suggests a severe level of dehydration.
Other red flags include a decreased number of wet diapers, typically fewer than six in a 24-hour period, or the absence of tears when the baby is crying. A dry mouth and lips, sunken eyes, rapid breathing, or a fast heart rate are also signs of fluid loss. If the sunken fontanelle appears in conjunction with persistent vomiting, diarrhea, or a high fever, caregivers should contact a pediatrician or seek emergency care.
The Fontanelle Closure Timeline
The fontanelles are temporary structures whose closure marks a developmental milestone in the process of skull ossification. The posterior fontanelle typically closes first, usually within the first two to three months after birth. This earlier closure is normal and generally goes unnoticed by parents.
The anterior fontanelle, located at the top of the head, remains open to allow for the brain’s substantial growth. It begins to close around six months of age and is fused completely between seven and nineteen months of age. While premature or delayed closure can occasionally signal an underlying medical condition, the closing of the fontanelles is a normal part of infant development.