A baby’s head contains spaces between the skull bones known as fontanels, or soft spots. These flexible, membrane-covered areas allow the baby’s head to pass through the birth canal and accommodate the rapid growth of the brain during the first year of life. Observing changes in the appearance of these soft spots is a common concern for parents because they can serve as an external indicator of a baby’s internal health status. While a fontanel should typically appear flat or slightly curved inward, a noticeable and deep sinking of this area warrants prompt attention.
Understanding the Fontanels
A newborn typically has two main fontanels: the anterior and the posterior. The anterior fontanel, often called the soft spot, is the larger, diamond-shaped area located toward the front-top of the head. This spot remains open for the longest period, usually closing sometime between seven and nineteen months of age, though the range can extend up to two years.
The posterior fontanel is much smaller, shaped like a triangle, and sits toward the back of the head. This smaller spot generally closes much earlier, often within the first two to three months of life. A normal, healthy fontanel should feel relatively firm and be flat or gently curve inward when the baby is in an upright, calm position. Temporary changes, such as a slight bulge when the baby cries or coughs, are usually normal. A sunken fontanel, however, is a deep indentation that remains even when the baby is relaxed, which is a sign that the baby’s internal fluid volume may be low.
The Primary Cause: Recognizing Dehydration
A sunken fontanel is one of the body’s early physical signals indicating that an infant may be experiencing dehydration. Dehydration occurs when the body loses more fluid than it takes in, which can happen quickly in infants due to their smaller body size and higher metabolic rate. This loss of total body fluid volume also affects the cerebrospinal fluid that surrounds the brain, leading to a decrease in internal pressure within the skull. The lack of sufficient fluid volume causes the membrane of the fontanel to be drawn inward, creating the noticeable depression.
This fluid imbalance is often triggered by common childhood illnesses that cause excessive fluid loss, such as diarrhea, vomiting, or a fever that leads to increased perspiration. Insufficient fluid intake, especially during periods of increased heat or illness, can also quickly lead to this state. When a sunken fontanel is present, parents should look for other confirming symptoms of dehydration. The most reliable early indicator is a significant reduction in the number of wet diapers, often fewer than six in a 24-hour period. Other accompanying signs include a dry mouth and lips, a lack of tears when the baby cries, or an unusual degree of listlessness and reduced activity.
Immediate Steps for Rehydration and Monitoring
Upon noticing a sunken fontanel, and if the baby is otherwise alert and not in severe distress, the immediate focus must be on increasing fluid intake. For breastfed infants, this means offering the breast more frequently than usual. For formula-fed infants, parents should offer additional bottles of formula. These fluids provide the necessary hydration and electrolytes.
Parents should carefully track the number of wet diapers, aiming for the normal minimum of six per day, as this is the best objective measure of successful rehydration. Observing the baby’s general behavior, such as their level of alertness and playfulness, provides additional insight into their hydration status. If the sunken fontanel persists despite increased feeding, or if there is concern about the baby’s ability to keep fluids down, contact the pediatrician promptly. Administering plain water is not recommended for infants, as it can disrupt the body’s electrolyte balance. The pediatrician may recommend a specific oral rehydration solution (ORS) to replenish lost electrolytes and fluids effectively.
Warning Signs Requiring Emergency Care
While many cases of sunken fontanel relate to mild or moderate dehydration, certain symptoms signal a medical emergency requiring immediate attention. Severe fluid loss can rapidly lead to serious complications. If a baby is extremely lethargic, unresponsive, or unusually difficult to rouse, this indicates a dangerous level of dehydration affecting brain function.
Immediate emergency care is necessary if the baby exhibits:
- Pale, mottled, or cold skin on the hands and feet, which suggests poor circulation.
- A rapid or weak pulse, along with fast breathing.
- Refusal of all fluids.
- Persistent, projectile vomiting or diarrhea containing blood.
These combined symptoms indicate a medical situation that cannot be safely managed at home and requires professional intravenous fluid replacement.