The baby’s soft spot, or fontanelle, is a frequent source of worry for parents, especially when its appearance seems to change. It is reasonable to feel concerned when the soft spot appears to dip inward, as this fluctuation can sometimes signal a change in your baby’s health. However, the fontanelle is designed to be dynamic, and small variations in its firmness and contour are a common part of normal infant development. Learning to observe these changes allows you to distinguish between everyday fluctuations and signals that require closer attention.
What the Soft Spot Is and Why It Exists
The soft spot is not a single feature but a membrane-covered gap between the bones of the skull, known as a fontanelle. A newborn has several of these, but the two most prominent are the diamond-shaped anterior fontanelle on top of the head and the smaller, triangular posterior fontanelle at the back. These spaces exist because an infant’s skull is made up of multiple bony plates held together by flexible sutures that have not yet fused completely.
The first function of these gaps is to allow the skull plates to overlap and compress slightly during passage through the narrow birth canal. This temporary molding is an accommodation for a safe delivery. The second function is to provide necessary space for the brain, which grows rapidly during the first two years of life. The smaller posterior fontanelle typically closes within the baby’s first two to three months. The larger anterior fontanelle remains open longer, often closing between seven and twenty-six months of age.
The Primary Reason for a Sunken Appearance
The most common reason a fontanelle appears noticeably sunken is dehydration, which is a state where the body is losing more fluid than it is taking in. The fontanelle offers a direct, visible indication of the fluid status within the skull. The brain and spinal cord are cushioned by cerebrospinal fluid, a clear liquid that helps maintain pressure within the enclosed space.
When an infant becomes dehydrated, the overall volume of fluid in the body decreases, and this includes the cerebrospinal fluid. As the pressure and volume of this fluid drop, the membrane covering the fontanelle loses its underlying support. This reduction in internal pressure causes the soft spot to dip inward, creating the sunken appearance.
Mild dehydration can happen due to prolonged exposure to heat or increased perspiration. More significant fluid loss often occurs due to common childhood illnesses, including fevers, vomiting, or diarrhea. Even if the baby is feeding normally, these conditions can cause fluid loss to outpace intake, leading to a temporary, but visible, depression of the soft spot.
Differentiating Normal Changes from Urgent Concerns
It is important to recognize that the fontanelle is not static, and its appearance changes based on the baby’s position and activity level. It is normal for the soft spot to appear slightly concave when your baby is sitting upright or calm. Conversely, it might bulge slightly when the baby is crying vigorously, straining during a bowel movement, or lying down, due to a temporary, harmless increase in internal pressure.
A soft spot that is significantly sunken and persistent, however, is a sign that the underlying fluid loss has reached a level of concern. If the deeply sunken fontanelle is accompanied by other specific symptoms, it suggests moderate to severe dehydration requiring immediate medical attention.
Signs of Severe Dehydration
Accompanying indicators of more serious dehydration include:
- Fewer wet diapers than usual, perhaps going six or more hours without one.
- Producing dark, concentrated urine.
- A lack of tears when the baby is crying.
- Noticeable dryness in the mouth and lips.
- A general change in behavior such as lethargy or excessive sleepiness.
- Sunken eyes and cool, mottled skin.
When a sunken fontanelle is observed alongside any combination of these symptoms, it is a signal for prompt consultation with a healthcare provider.