A temperature of 98°F is not a fever for a baby. It falls well within the normal range. The medical threshold for fever in infants is 100.4°F (38°C) when measured rectally, which is the standard method for babies. At 98°F, your baby’s temperature is completely normal.
What Counts as a Fever in Babies
The exact number that defines a fever depends on where you take the temperature. A rectal, ear, or temporal artery (forehead) reading of 100.4°F or higher is a fever. An armpit reading of 99°F or higher is considered a fever, since armpit readings run lower than internal body temperature. An oral reading of 100°F or higher qualifies, though oral thermometers aren’t practical for infants.
At 98°F, your baby is more than two full degrees below the rectal fever threshold. There’s no gray area here.
Why 98°F Can Still Vary Throughout the Day
Babies don’t hold a perfectly steady temperature. Their bodies are more sensitive to temperature shifts than adults because they have a larger skin surface relative to their size and their internal thermostat is still maturing. A healthy baby’s temperature can fluctuate based on the time of day, how warmly they’re dressed, the room temperature, whether they’ve just been fed, or whether they recently had a bath.
You might get 97.8°F one hour and 99.1°F a few hours later, and both readings can be perfectly normal. Late afternoon and evening temperatures tend to run slightly higher than morning readings. If your baby feels warm but the thermometer reads 98°F, the warmth you’re feeling is likely from bundling, a warm room, or recent activity like crying.
Why the Measurement Method Matters
For babies under three months, rectal temperature is the gold standard. It gives the most accurate reading of core body temperature. Forehead (temporal artery) thermometers are convenient and reasonably reliable for older infants, but in very young babies, even a small measurement error can matter because the stakes of a true fever are higher in that age group.
Armpit temperatures are the least accurate. They typically read about one degree lower than rectal temperatures, which is why the fever cutoff for armpit readings is set lower at 99°F. If you’re using an armpit thermometer and see 98°F, that’s solidly in the normal zone regardless of method.
When a Real Fever in a Baby Is Serious
While 98°F is nothing to worry about, it’s worth understanding why parents are right to be vigilant about fever in infants. For babies under two months old, a true fever of 100.4°F or higher is treated as a medical emergency. Their immune systems are too immature to reliably fight off infections, and a fever at that age can signal something serious that needs immediate evaluation.
The American Academy of Pediatrics has specific guidelines for febrile infants broken down by age: 8 to 21 days, 22 to 28 days, and 29 to 60 days. Younger babies within those windows get more extensive testing. The good news is that serious infections are rare. More than 10% of feverish infants turn out to have a urinary tract infection, which is treatable, while fewer than 0.05% develop meningitis.
For babies older than three months, fever is still worth monitoring but is less likely to signal something dangerous on its own. The fever itself is a sign the immune system is doing its job.
Signs That Matter More Than Temperature
If your baby’s temperature is 98°F but something still feels off, trust your instincts and pay attention to behavior. Several warning signs are more meaningful than any number on a thermometer.
- Lethargy: Your baby stares into space, won’t smile, barely responds to you, or is unusually hard to wake up.
- Poor feeding: Refusing to eat or drinking significantly less than usual, especially in a newborn under one month.
- Trouble breathing: Fast breathing, grunting, bluish lips, or skin pulling in between the ribs with each breath.
- Dehydration: No wet diaper in eight hours, no tears when crying, dry mouth, or a sunken soft spot on the head.
- Inconsolable crying: Constant, nonstop crying that isn’t soothed by holding, feeding, or any of your usual tricks.
- Vomiting bile: Bright green vomit can indicate a blocked intestine and needs immediate attention.
- Color changes: Bluish or gray lips, tongue, or gums suggest low oxygen.
These symptoms warrant a call to your pediatrician or a trip to the emergency room regardless of what the thermometer says. A baby can be seriously ill with a normal or even low temperature. The number on the thermometer is one data point, not the whole picture.
What to Do With a 98°F Reading
Nothing. Your baby’s temperature is normal. If you were checking because your baby seemed warm to the touch, consider whether the room is too warm or your baby is overdressed. Babies lose heat through evaporation and air currents, so keeping them dry and in a comfortable room temperature (around 68 to 72°F) helps them regulate. If your baby is acting normally, eating well, and sleeping in their usual pattern, a reading of 98°F simply confirms what healthy looks like.