A fever in a baby is a rectal temperature of 100.4°F (38°C) or higher. That number is the same regardless of your baby’s age, but what you should do about it changes significantly depending on whether your baby is a newborn, a few months old, or closer to toddlerhood. Normal body temperature sits around 98.6°F, so anything at or above that 100.4°F mark is considered a true fever.
How to Get an Accurate Reading
For babies under 3 months, a rectal thermometer is the most accurate way to check. It’s not the most pleasant option, but other methods can give misleading results at this age. Forehead (temporal artery) thermometers are nearly as accurate as rectal ones and cause less discomfort, and newer research suggests they may be reliable even in newborns. Ear thermometers, however, aren’t considered accurate until a baby is at least 6 months old because an infant’s ear canal is too small and curved for a reliable reading.
Armpit readings are the least accurate of all methods. If you take your baby’s temperature under the arm and the number looks concerning, follow up with a rectal reading to confirm. The small difference in accuracy matters most when your baby is very young, because a confirmed fever in a newborn triggers a very different response than one in an older infant.
Why Age Changes Everything
A fever of 100.4°F in a 2-week-old baby is treated as a medical emergency. A fever of 102°F in a 9-month-old is usually manageable at home. The reason comes down to immune system development and the risk of serious infection.
If your baby is younger than 3 months and has any rectal temperature of 100.4°F or above, call your pediatrician immediately or go to the emergency room. At this age, babies don’t have the immune defenses to fight off infections the way older children do, and a fever can be the only visible sign of something serious. More than 10% of febrile infants in this age group turn out to have a urinary tract infection, though the risk of more dangerous infections like meningitis is very low (under 0.05%).
For babies between 3 and 6 months, a fever of 102°F or higher, or a lower fever that lasts more than a day, generally warrants a call to your pediatrician. After 6 months, fever itself becomes less of a concern than how your baby is acting. A baby who has a fever but is still drinking, making eye contact, and responsive is in a very different situation than one who is limp, inconsolable, or refusing to eat.
Teething Does Not Cause a True Fever
This is one of the most common misconceptions. Teething can nudge your baby’s temperature slightly above normal, but it won’t push it to 100.4°F or beyond. If your baby’s temperature hits the fever threshold, something else is going on, most likely a viral or bacterial infection. Dismissing a real fever as teething can delay care that your baby actually needs.
Viral vs. Bacterial: What the Fever Might Mean
Most fevers in babies are caused by common viral infections. These tend to produce “all over” symptoms: runny nose, cough, mild body aches, fatigue, and a low-grade fever. Your baby might be fussier than usual and sleep more, but the illness runs its course in a few days.
Bacterial infections tend to be more localized. Instead of general cold symptoms, you might notice a single red or tender area on the skin, ear pain, or a severely sore throat in older babies. One important pattern to watch for: a viral illness that seems to improve and then suddenly gets worse, with a higher fever or new pain. That can signal a secondary bacterial infection that may need treatment.
Keeping Your Baby Comfortable
Fever is your baby’s immune system doing its job, so the goal isn’t necessarily to eliminate the fever entirely. It’s to keep your baby comfortable and hydrated while their body fights off the infection.
Dress your baby in light clothing. Bundling them up traps body heat and can push the temperature higher. A lukewarm bath can help bring some relief, but don’t use cold water. If your baby starts shivering, the shivering itself raises body temperature and makes things worse. Trying to “sweat out” a fever doesn’t work either and only increases the risk of dehydration.
Push fluids. For babies under 6 months, that means breast milk or formula. For older babies, water and diluted juice can supplement their usual feeds. Watch for signs of dehydration: fewer than usual wet diapers (or none for three hours), a dry mouth, no tears when crying, sunken eyes or a sunken soft spot on the skull, and skin that stays pinched up instead of flattening back when you gently squeeze it. Dehydration during a fever can escalate quickly in small bodies.
When Fever Reducers Are Safe
Acetaminophen (Tylenol) can be given to babies as young as 2 to 3 months, but check with your pediatrician for the right dose since it’s based on weight, not just age. Ibuprofen (Motrin, Advil) is not approved for babies under 6 months old and should not be given before that age unless your doctor specifically tells you to. After 6 months, ibuprofen can be given every 6 to 8 hours as needed.
Never give aspirin to a baby or child of any age. Use your baby’s current weight to determine the correct dose of any fever reducer. If you don’t know their weight, age-based dosing on the package is a backup, but weight is always more accurate.
Febrile Seizures
Some babies and young children experience seizures triggered by fever, called febrile seizures. These are most common between 6 months and 5 years of age, with the highest risk between ages 1 and 3. During a febrile seizure, your baby may lose consciousness, shake or jerk uncontrollably in the arms or legs, roll their eyes, or lose bladder or bowel control.
Febrile seizures look terrifying but are usually not dangerous and don’t cause lasting harm. They can happen even with a relatively low fever and often occur as the temperature is rising rapidly rather than at its peak. If your baby has a seizure, place them on a safe surface away from hard objects, turn them on their side, and time the seizure. If it lasts longer than five minutes, call emergency services. Even for shorter seizures, let your pediatrician know so they can evaluate what caused the fever.