A temperature of 98.4°F is not a fever for a baby. It falls squarely within the normal range. A fever in infants is defined as a rectal temperature of 100.4°F (38°C) or higher, so 98.4°F sits a full two degrees below that threshold.
What Counts as a Fever in Babies
The exact number that defines a fever depends on how you take the temperature. A rectal reading of 100.4°F or higher is the standard cutoff used by the American Academy of Pediatrics and most pediatric guidelines. For an oral reading, the threshold drops slightly to 100°F. An armpit (axillary) reading of 99°F or higher also suggests a fever, though armpit measurements tend to be less reliable in babies.
At 98.4°F, your baby’s temperature is completely normal regardless of which method you used to measure it.
Normal Temperature Fluctuates Throughout the Day
A healthy baby’s body temperature isn’t locked at one number. It shifts naturally over the course of a day by nearly 2°F. The average sits around 98.8°F, but temperatures tend to be lower in the morning and peak in the late afternoon or early evening. So a reading of 98.4°F in the morning and 99.5°F after a nap can both be perfectly normal for the same baby on the same day.
External factors also play a role. Babies are not as adaptable to temperature changes as adults, and their readings can shift based on how they’re dressed, whether they’ve been swaddled, or how warm the room is. A baby who’s been bundled in blankets may register slightly higher without actually being sick. If you get a borderline reading, try removing a layer, waiting 15 to 20 minutes, and checking again.
Why the Measurement Method Matters
For babies, rectal temperatures are considered the most accurate. Armpit readings are convenient but can miss actual fevers. Research comparing the two methods in newborns found that when a baby truly had an elevated rectal temperature, an armpit reading only caught it about 65% of the time. Individual readings between the two sites can differ by up to 1.8°F in either direction.
This means an armpit reading of 98.4°F could, in rare cases, correspond to a rectal temperature that’s somewhat higher. If your baby seems unwell and you’ve only taken an armpit reading, a rectal check gives you a more trustworthy number. Temporal artery (forehead) and ear thermometers fall somewhere in between for accuracy, but rectal remains the gold standard for infants.
When a Low or Normal Temperature Still Deserves Attention
A normal temperature doesn’t automatically mean everything is fine. Babies, especially newborns, can be seriously ill without running a fever. What matters just as much is how your baby looks and acts. Watch for these signs even when the thermometer reads normal:
- Feeding changes: missing two or more feedings in a row, or eating noticeably less than usual
- Unusual sleepiness: harder to wake up than normal, or seeming floppy and limp
- Dehydration clues: fewer wet diapers, dry mouth, no tears when crying, or a sunken soft spot on the head
- Breathing trouble: fast or labored breathing, flaring nostrils, or a persistent cough
- Forceful vomiting: not just spit-up, but vomit that shoots out after feedings or an inability to keep liquids down for eight hours
A baby who is alert, feeding well, making eye contact, and has normal skin color is generally doing fine, regardless of a slightly higher or lower number on the thermometer.
Temperatures That Do Require Action
If your baby’s temperature does reach 100.4°F rectally and your baby is younger than 3 months old, that warrants an immediate call to your pediatrician. If you can’t reach them, head to the emergency room. Fevers in very young infants can signal serious infections that need prompt evaluation, even if the baby appears well. Do not give fever-reducing medication to babies under 3 months unless specifically directed by a doctor.
For babies 3 months and older, a fever alone is less alarming. A child who still wants to play, drinks well, stays alert, and looks like themselves when the temperature comes down is generally handling the illness well. Contact your pediatrician if the fever lasts more than two to three days, keeps climbing above 104°F, or comes with symptoms like a rash, persistent diarrhea, repeated vomiting, or signs of dehydration.
At 98.4°F, though, none of these fever protocols apply. Your baby’s temperature is normal.