A temperature of 100.4°F (38°C) or higher is considered a fever in babies, and for any infant under 3 months old, that number requires an immediate call to your pediatrician. For older babies between 3 months and 3 years, the threshold for concern is higher: 102°F (38.9°C) when measured rectally. These aren’t arbitrary cutoffs. A young baby’s immune system is still developing, and a fever that would be routine in a toddler can signal a serious infection in a newborn.
Fever Thresholds by Age
The number that counts as “too high” depends entirely on how old your baby is.
For babies under 3 months, a rectal or forehead reading of 100.4°F (38°C) or higher is a fever, and it’s treated as urgent regardless of how your baby looks or acts. Even if your baby seems fine, this temperature in a newborn warrants a same-day call. An armpit reading of 99°F (37.2°C) or higher in this age group also qualifies as a fever, though armpit readings are less reliable and your doctor will likely want a rectal confirmation.
For babies 3 months to 36 months, the concern level shifts. A rectal, forehead, or ear temperature of 102°F (38.9°C) or higher is the threshold that typically warrants medical attention. An armpit reading of 101°F (38.3°C) or higher falls into the same category. Fevers below these numbers in older babies are common with viral illnesses and often resolve on their own within a few days.
How to Get an Accurate Reading
Rectal thermometers are the gold standard for babies, especially those under 3 months. They give the most accurate core body temperature, and pediatricians rely on rectal readings when making decisions about a young infant’s care. A standard digital thermometer works fine for this.
Forehead (temporal artery) thermometers are a reasonable alternative for older babies and are much easier to use on a squirming child. Ear thermometers also work for babies over 3 months, though earwax or improper positioning can throw off the reading.
Armpit temperatures are the least reliable method. They tend to read lower than the actual core temperature, which means they can miss a fever or underestimate its severity. If you take an armpit reading and it seems borderline, follow up with a rectal measurement to be sure. This matters most in young infants, where a single degree can change the medical response.
Teething Fever vs. Real Fever
Teething gets blamed for a lot of fevers, but the research doesn’t support it as a cause of high temperatures. A study published by the American Academy of Pediatrics tracked infants through tooth eruptions and found that teething was associated with only a mild temperature elevation, not a true fever. None of the teething babies in the study developed a temperature of 104°F, and fever over 102°F was not significantly linked to tooth emergence at all.
Teething does cause real symptoms: increased drooling, biting, gum rubbing, irritability, and sometimes a slight rise in temperature. But if your baby has a temperature above 102°F, something other than teething is almost certainly responsible. Attributing a genuine fever to teething can delay treatment for an infection that needs attention.
Overheating vs. Fever
Babies regulate their body temperature poorly, so a warm room, too many blankets, or overdressing can push their temperature up without any illness being involved. If your baby feels hot, try removing a layer of clothing and waiting 15 to 20 minutes before retaking the temperature. A baby who was simply overdressed will cool down quickly. A baby with a fever won’t.
The ideal room temperature for a sleeping baby is generally between 68°F and 72°F. Dress your baby in one layer more than what you’d wear comfortably in the same room. If you’re checking because your baby feels warm after a nap in a hot room or car seat, give their body a chance to normalize before assuming the reading reflects illness.
What You Can Give for a Fever
Acetaminophen (Tylenol) is safe for babies 8 weeks and older. It can be given every 4 to 6 hours as needed, up to 5 times in 24 hours. Dosing is always based on your baby’s weight, not age, so check the packaging or ask your pediatrician for the correct amount.
Ibuprofen (Advil, Motrin) is not recommended for babies under 6 months old. After 6 months, it can be given every 6 to 8 hours, up to 4 times in 24 hours. Ibuprofen should not be given to babies who are vomiting, dehydrated, or have stomach pain, as it can irritate the stomach lining and mask symptoms of a more serious condition.
For babies under 8 weeks with a fever of 100.4°F or higher, do not give any fever-reducing medication. These infants need to be evaluated by a doctor, and the fever itself is important diagnostic information. Masking it with medication before the visit can complicate the assessment.
Warning Signs Beyond the Number
Temperature alone doesn’t tell the whole story. A baby with a 101°F fever who is nursing well, making eye contact, and wetting diapers normally is in a very different situation than a baby with the same number who is limp, refusing to eat, or difficult to wake. Pay attention to how your baby is acting, not just what the thermometer says.
Signs that warrant urgent attention alongside a fever include unusual sleepiness or difficulty waking, persistent vomiting, a rash that doesn’t fade when you press on it, a weak or high-pitched cry, fewer than normal wet diapers (a sign of dehydration), and a soft spot on the head that looks swollen or sunken. Difficulty breathing, including flaring nostrils or skin pulling in between the ribs with each breath, also calls for immediate evaluation.
For any baby under 3 months, the threshold is simple: any fever, any time, call your doctor. You don’t need to wait for additional symptoms or try to bring the temperature down first. The age alone makes the fever significant enough to warrant professional guidance.