A temperature of 100.4°F (38°C) or higher is considered a fever in babies. But whether that fever requires a phone call or an emergency visit depends heavily on your baby’s age. For infants under 3 months, any fever at or above 100.4°F warrants an immediate call to your pediatrician, no matter how well your baby seems.
The 100.4°F Threshold
The American Academy of Pediatrics uses 100.4°F (38°C) as the standard fever line for infants. That number applies to rectal, ear, or forehead (temporal artery) readings. If you’re using an armpit thermometer, the fever threshold is lower: 99°F (37.2°C). Oral thermometers fall in between at 100°F (37.8°C).
Armpit readings are the least accurate of the three methods. For babies, a rectal thermometer gives the most reliable number. If an armpit reading looks borderline, it’s worth rechecking with a rectal thermometer before deciding what to do.
Why Your Baby’s Age Changes Everything
The urgency of a fever shifts dramatically based on how old your baby is. Here’s how it breaks down:
- Under 3 months: Call your pediatrician for any fever at or above 100.4°F, even if your baby looks fine. Young infants have immature immune systems, and a fever can be the only sign of a serious infection.
- 3 to 6 months: Call if your baby has a temperature above 100.4°F, or if the temperature is at 100.4°F and your baby seems sick (unusually fussy, sleepy, or not feeding well).
- 6 to 24 months: Call if a fever above 100.4°F lasts more than one day.
- Any age: Call if a fever persists beyond three days, regardless of how high it is.
A fever of 105°F or higher in a child of any age needs emergency attention. That said, it takes a temperature of about 107°F to actually damage body tissues, which is extremely rare. A fever itself is not the enemy. It’s your baby’s immune system fighting an infection. The concern is what’s causing the fever, not the fever alone.
Signs That Need Emergency Care
Temperature is just one piece of the picture. Certain symptoms alongside a fever, or even without one, mean you should head to the emergency room:
- Skin or lips turning blue, purple, or gray
- Difficulty breathing
- Unresponsiveness or unusual drowsiness: your baby is hard to wake up, seems limp or floppy, or acts strangely withdrawn
- Pain or fussiness that keeps getting worse and won’t calm down
- Trouble swallowing, which may show up as excessive drooling or spitting
Watching for Dehydration
Fevers cause babies to lose fluid faster than normal, so dehydration is one of the biggest practical risks. Your baby should have at least six wet diapers in a 24-hour period. If you go more than eight hours without a wet diaper, that’s a warning sign.
Other dehydration signals to watch for: crying with fewer or no tears, a dry mouth, and a sunken soft spot on the top of your baby’s head. If you notice any of these during a fever, call your pediatrician.
Febrile Seizures
Seeing your baby have a seizure during a fever is terrifying, but febrile seizures happen in about 3 to 4 out of every 100 children. They’re most common when temperatures reach 102°F (38.9°C) or higher, though they can occur with milder fevers too. Febrile seizures are generally brief and don’t cause lasting harm, but you should call your pediatrician after one happens and seek emergency care if it lasts longer than five minutes.
Treating a Fever at Home
For babies 3 months and older, acetaminophen (the active ingredient in infant Tylenol) can help bring a fever down. Don’t give it to babies younger than 3 months without a doctor’s guidance. Ibuprofen (infant Advil or Motrin) is an option starting at 6 months. Both medications are dosed by weight, not age, so check the packaging carefully or ask your pharmacist to confirm the right amount for your baby.
Keep your baby dressed in light clothing and offer frequent feedings to replace lost fluids. Don’t bundle a feverish baby in extra blankets. A lukewarm (not cold) sponge bath can also help if your baby seems uncomfortable.
Room Temperature and Overheating
Sometimes a baby feels hot not because of illness but because the room is too warm. The recommended nursery temperature is 60 to 68°F (16 to 20°C). Keeping the room in this range helps lower the risk of sudden infant death syndrome (SIDS), since overheating is a known risk factor.
To check whether your baby is too hot, feel their chest or the back of their neck. Hands and feet tend to run cooler naturally, so they’re not the best gauge. If your baby’s chest feels hot or sweaty, remove a layer of clothing or bedding. A simple room thermometer near the crib takes the guesswork out of monitoring overnight temperatures.