A temperature of 99.9°F (37.7°C) in a baby is generally considered slightly elevated but not a significant fever on its own. A baby’s normal body temperature can fluctuate, ranging from 96.8°F (36°C) to 100.3°F (37.9°C) when measured rectally. This natural variation means a single reading of 99.9°F might not indicate an issue unless it is a rectal measurement or accompanied by other concerning signs. The method used to take the temperature greatly influences what is considered a fever.
Defining Fever in Babies
A fever in babies is defined by specific temperature thresholds, which vary depending on the measurement method. The most accurate method for infants is a rectal temperature, where a reading of 100.4°F (38°C) or higher is considered a fever. Other methods, such as an axillary (armpit) temperature, might indicate a fever at 99°F (37.2°C) or higher, while oral temperatures for older children define a fever at 100°F (37.8°C) or higher.
For very young infants, especially those under three months of age, any temperature reaching the fever threshold, even a lower reading like 99.9°F rectally, should prompt a call to a healthcare provider. This is because their immune systems are still developing, and a fever can signal a serious infection that requires prompt evaluation. Overdressing or a warm environment can also temporarily elevate a baby’s temperature.
How to Take a Baby’s Temperature
Taking a baby’s temperature accurately requires using the appropriate method and a reliable digital thermometer. For infants under three months of age, a rectal temperature is recommended as the most accurate way to measure their core body temperature. To perform this, lubricate the tip of a digital thermometer with petroleum jelly. Gently insert the lubricated tip about 1/2 to 1 inch (1.3 to 2.5 cm) into the rectum.
Hold the thermometer until it beeps, then remove and read the display. Clean the thermometer after each use. While axillary (armpit) and temporal (forehead) thermometers are easier to use, they may not be as accurate as rectal measurements, especially in very young infants. Ear (tympanic) thermometers are not accurate for babies younger than six months due to the small size of their ear canals.
When to Consult a Doctor
When to seek medical attention for a baby with a fever depends on their age and accompanying symptoms. For any infant younger than three months, a rectal temperature of 100.4°F (38°C) or higher warrants an immediate call to a healthcare provider or a visit to an emergency department. This age group has an immature immune system, and a fever could indicate a serious underlying infection. Even if the baby appears well, medical evaluation is advised.
For babies between three and six months, contact a doctor if their temperature reaches 100.4°F (38°C) or higher, or 102.2°F (39°C) or higher. Beyond temperature, observe other signs such as unusual drowsiness, difficulty waking, inconsolable crying, refusal to feed, a rash that does not fade when pressed, difficulty breathing, stiff neck, or signs of dehydration like fewer wet diapers or no tears when crying. The baby’s overall behavior and appearance are often more telling than the temperature reading alone.
Managing a Baby’s Fever at Home
If a healthcare provider has ruled out a serious condition, several measures can help comfort a baby with a low-grade fever at home. Ensure the baby stays well-hydrated; continue offering breast milk or formula frequently. For babies over six months, small amounts of water can be offered in addition to their regular milk intake.
Dress the baby in lightweight clothing to allow heat to escape, and keep the room at a comfortable temperature, ideally between 64-71°F (18-22°C). A lukewarm sponge bath can help reduce discomfort, but avoid cold water or alcohol baths, which can cause shivering and raise body temperature.
Fever-reducing medications like acetaminophen can be considered for babies over two months, while ibuprofen is suitable for those over six months. Always administer medication based on the baby’s weight, not age, and use the provided dosing device to ensure accuracy. Never give aspirin to children due to the risk of Reye’s syndrome.