A healthy baby’s normal body temperature is around 98.6°F (37°C), with a typical range of 97°F to 100.3°F (36.1°C to 37.9°C) depending on where and when you measure. A reading of 100.4°F (38°C) or higher, taken rectally, is the standard definition of fever in infants. That threshold matters most for babies under 3 months old, where even a single fever reading warrants immediate medical attention.
Normal Temperature by Measurement Method
Where you place the thermometer changes what counts as “normal.” A rectal reading runs about 0.5°F to 1°F higher than an oral reading, while an armpit (axillary) reading runs about 0.5°F to 1°F lower. So a rectal temperature of 99.5°F, an oral reading of 98.6°F, and an armpit reading of 97.8°F can all reflect the same core body temperature.
For babies under 3 months, rectal thermometers are the gold standard. Armpit temperatures are the least accurate of common methods, and ear thermometers can be thrown off by earwax or the small, curved shape of an infant’s ear canal. Forehead (temporal artery) thermometers are convenient for older babies but can be affected by sweat or ambient temperature. If you get a concerning reading from any non-rectal method, confirm it with a rectal temperature before making decisions.
What Counts as a Fever
A rectal temperature of 100.4°F (38°C) or higher is formally defined as a fever in infants. This isn’t an arbitrary number. It’s the threshold used by the American Academy of Pediatrics and most pediatric emergency departments to guide how seriously a fever needs to be evaluated.
The urgency depends heavily on your baby’s age:
- Under 3 months: Any rectal temperature of 100.4°F or higher needs immediate medical evaluation. For babies under 2 months, the Children’s Hospital of Philadelphia recommends going to an emergency department rather than waiting for a clinic appointment.
- 3 to 6 months: Call your pediatrician if your baby reaches 100.4°F and seems unwell (unusually fussy, lethargic, or not feeding), or if the temperature goes higher than 100.4°F regardless of how they seem.
- 6 to 24 months: A fever above 100.4°F that persists for more than a day warrants a call, even if your baby seems comfortable.
The reason newborns get treated so urgently is that their immune systems are immature. A fever in a very young infant can signal a serious bacterial infection, and the usual reassuring signs parents rely on (baby seems alert, is eating okay) aren’t reliable indicators of severity at that age.
When Temperature Is Too Low
Most parents worry about fever, but a temperature that’s too low also matters. The World Health Organization defines hypothermia in newborns as a core temperature below 97.7°F (36.5°C). This is more common in premature or low-birth-weight babies, but it can happen to any newborn who’s been underdressed in a cold room or exposed to drafts after a bath.
Before a baby’s temperature actually drops into the hypothermic range, their body burns extra calories to generate heat. If this goes unrecognized, it can divert energy away from growth and cause drops in blood sugar. Prolonged hypothermia in newborns increases the risk of infection and, in severe cases, can cause neurological damage. Signs to watch for include cool skin (especially on the chest and belly, not just hands and feet), lethargy, weak feeding, and a bluish tint to the skin.
Normal Fluctuations Through the Day
A baby’s temperature isn’t static. It naturally runs a bit lower in the early morning and higher in the late afternoon, with swings of up to 1°F being completely normal. Physical activity like crying, feeding, or being bundled in warm clothing can temporarily push a reading higher. A warm bath can do the same. If your baby feels warm and you get a borderline reading, wait 15 to 20 minutes, remove a layer of clothing, and recheck.
Overheating vs. Actual Fever
A baby who’s been overdressed, swaddled too tightly, or left in a warm car seat may register a high temperature without actually being sick. The difference matters. If you get a high reading and suspect the environment is the cause, remove extra clothing or blankets, move your baby to a cooler spot, and retake the temperature after about 15 minutes. If the reading drops back to the normal range, overheating was likely the issue.
If the temperature stays at 100.4°F or above after you’ve ruled out environmental causes, treat it as a true fever. This is especially important for babies under 3 months, where the safest approach is to seek medical evaluation promptly rather than waiting to see if the temperature comes down on its own.
Tips for Accurate Readings
The most common mistake parents make is relying on touch. Feeling your baby’s forehead gives you a general sense that something might be off, but it can’t tell you whether a temperature is 99.5°F or 101°F, and that difference changes what you should do next. Always confirm with a thermometer.
For rectal readings, use a digital thermometer with a flexible tip. Apply a small amount of petroleum jelly to the tip, lay your baby face-down across your lap or on a firm surface, and insert gently about half an inch to one inch. Hold the thermometer in place until it beeps. Label this thermometer so it’s only used rectally.
For armpit readings, place the thermometer tip in the center of the armpit and hold your baby’s arm snugly against their body until the thermometer beeps. Keep in mind that if this reading is borderline (around 99°F or higher), it’s worth confirming rectally, since armpit readings can underestimate the true core temperature.
Ear thermometers work best for babies over 6 months, once the ear canal is large enough for a reliable reading. Gently pull the ear back to straighten the canal before inserting the probe. If your baby has been lying on one ear, that side may read higher, so use the other ear or wait a few minutes.