A normal body temperature for a baby is 97.7°F to 99.5°F (36.5°C to 37.5°C) when measured rectally. That range applies to both full-term and premature infants. A reading of 100.4°F (38°C) or higher counts as a fever, while anything below 97.7°F signals the body is too cold.
Those numbers sound simple, but the reading you get depends on where you take the temperature, the time of day, and your baby’s age. Here’s what you need to know to interpret your thermometer correctly.
Normal Ranges by Measurement Site
Not every spot on your baby’s body gives the same reading. Rectal temperatures run the highest and are considered the gold standard for infants. Armpit (axillary) readings tend to run lower, sometimes by nearly a full degree Fahrenheit. Ear and forehead (temporal artery) readings fall somewhere in between.
The thresholds that define a fever reflect these differences:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
Armpit readings are the least reliable. In studies comparing armpit and rectal temperatures in newborns, the two methods differed by up to 1°C (1.8°F) in individual babies, and armpit readings only caught about 65% of true fevers detected rectally. If an armpit reading seems off, confirm it with a rectal measurement.
Why Temperature Fluctuates Throughout the Day
Your baby’s temperature isn’t a fixed number. It shifts naturally over a 24-hour cycle, typically lowest in the early morning and highest in the late afternoon or evening. Studies in young children show an average daily swing of about 1.9°F. That means a baby who reads 97.9°F at 7 a.m. and 99.3°F at 4 p.m. could be perfectly healthy both times.
Activity, feeding, warm clothing, and a hot room can also push a reading slightly higher. If your baby feels warm but the thermometer reads just under the fever threshold, wait 15 to 20 minutes, remove a layer of clothing, and check again before worrying.
When a Temperature Is Too Low
The World Health Organization defines infant hypothermia as a core temperature below 97.7°F (36.5°C). Babies lose heat faster than adults because of their larger skin surface relative to body weight, and premature or low-birth-weight babies are especially vulnerable.
Mild cold stress can happen even before the thermometer drops into the hypothermia range. When a baby has to burn extra calories just to stay warm, those calories get diverted from growth. Prolonged low temperatures can lead to low blood sugar, difficulty breathing, and in severe cases, increased risk of infection. Keeping the room between 68°F and 72°F and dressing your baby in one more layer than you’d wear yourself are practical ways to prevent this.
Fever Urgency Depends on Age
A temperature of 100.4°F means very different things depending on how old your baby is. The younger the infant, the more seriously a fever needs to be taken, because young babies have immature immune systems and fewer obvious signs when something is wrong.
- Under 3 months: Any fever at all (100.4°F rectally or higher) warrants an immediate call to your pediatrician, even if your baby seems fine otherwise.
- 3 to 6 months: Call if the temperature reaches 100.4°F, or sooner if your baby seems unusually sleepy, irritable, or unwell.
- 6 to 24 months: A fever above 100.4°F that lasts longer than one day needs medical attention.
- Any age: A fever lasting more than three days should be evaluated regardless of how high it is.
For babies under 3 months, the concern isn’t the number itself. It’s that fever at this age can be the only visible sign of a serious bacterial infection. Pediatricians take this seriously and may want to see your baby the same day.
How to Get an Accurate Reading
For babies under 3 months, a digital rectal thermometer gives the most trustworthy result. Apply a small amount of petroleum jelly to the tip, insert it about half an inch, and hold it in place until it beeps. The process takes under a minute and, despite how it sounds, most babies tolerate it well.
Once your baby is older than 6 months, a digital ear thermometer or temporal artery (forehead) thermometer becomes a reasonable option for everyday checks. These are faster and easier, though slightly less precise. If you get a borderline reading and need certainty, a rectal check is still the best backup.
Forehead strip thermometers and pacifier thermometers are not accurate enough to rely on. They can give a rough sense of whether your baby feels warm, but they shouldn’t be used to rule out a fever.
Managing a Fever at Home
Fever itself is not dangerous in most cases. It’s the body’s way of fighting infection. For babies 3 months and older with low-grade fevers who are eating, sleeping, and behaving mostly normally, the goal isn’t necessarily to eliminate the fever. It’s to keep your baby comfortable.
Acetaminophen (Tylenol) can be given to babies 3 months and older, dosed every 4 to 6 hours, up to five times in 24 hours. Ibuprofen (Advil, Motrin) is only safe for babies 6 months and older, dosed every 6 to 8 hours, up to four times daily. Both are dosed by weight, not age, so check the packaging carefully or ask your pharmacist. Never give aspirin to a baby or child.
Keep your baby in light clothing, offer extra fluids (breast milk, formula, or water if they’re eating solids), and let them rest. Cool baths are no longer recommended because shivering can actually raise body temperature.