A temperature of 100.4°F (38°C) or higher is considered a fever in babies when measured rectally. That single number is the standard threshold used by pediatricians and the American Academy of Pediatrics, but what you should do about it depends heavily on your baby’s age.
Fever Thresholds by Measurement Method
Not all thermometers give the same reading, so the number that counts as a fever shifts depending on where you take the temperature. A rectal, ear, or forehead (temporal artery) reading of 100.4°F (38°C) or higher is a fever. An oral reading of 100°F (37.8°C) or higher qualifies. An armpit reading of 99°F (37.2°C) or higher is considered elevated.
For babies under 3 months, rectal thermometers are the gold standard because they give the most accurate core body temperature. Armpit readings are convenient but tend to run lower, which can mask a real fever. If you get a concerning armpit reading in a young infant, it’s worth confirming rectally.
Why Your Baby’s Age Changes Everything
The same fever that’s manageable in an older baby can be genuinely dangerous in a newborn. Here’s how the urgency breaks down:
- Under 3 months: Any rectal temperature of 100.4°F or higher warrants an immediate call to your baby’s doctor, even if your baby looks fine otherwise. Young infants have immature immune systems, and a fever can be the only visible sign of a serious infection. The AAP’s clinical guidelines specifically address the evaluation of otherwise well-appearing infants between 8 and 60 days old who develop a fever at this threshold.
- 3 to 6 months: A temperature up to 101°F (38.3°C) is worth a call if your baby seems unusually irritable, lethargic, or uncomfortable. Any temperature above 101°F at this age deserves medical attention regardless of how your baby is acting.
- 6 to 24 months: A temperature above 101°F that lasts longer than one day without other symptoms should prompt a call. If additional symptoms are present, sooner is better.
For any age, a fever lasting more than three days deserves a call if you haven’t already contacted your pediatrician.
Signs That a Fever Is Serious
The number on the thermometer matters, but how your baby looks and behaves often matters more (except in those first 3 months, where the number alone is enough to act). Watch for these warning signs alongside a fever:
- Unusual sleepiness or floppiness: Sleeping more than normal, being hard to wake up, or seeming limp.
- Dehydration: Fewer wet diapers than usual, crying without tears, a dry mouth, or a sunken soft spot on the head.
- Rash: Especially one that appears quickly, blisters, or looks infected.
- Breathing trouble: Rapid, labored, or noisy breathing.
- Color changes: Skin or lips that look blue, purple, or gray.
- Inconsolable crying or worsening fussiness: Pain that doesn’t improve or keeps getting worse.
A baby who is not conscious, acting strangely, or seeming withdrawn and less alert needs emergency care.
Teething Does Not Cause True Fevers
This is one of the most common misconceptions. Teething can raise a baby’s temperature slightly, but it does not push it to 100.4°F or above. If your teething baby has a true fever, something else is going on, likely a coincidental infection. Babies start teething around the same age they lose some of the immune protection passed along from their mother, so infections and new teeth often overlap by timing alone.
Post-Vaccination Fevers
A mild fever after routine immunizations is normal and is a sign that your baby’s immune system is responding to the vaccine. Most post-vaccination fevers show up within a week, though the exact timing varies. Some vaccines (like the Hib vaccine) tend to cause fevers within the first two days, while others can trigger a low-grade fever anywhere in the first week.
What’s worth knowing: a rapid rise in temperature, not the peak of the fever itself, is what can occasionally trigger a febrile seizure. These are frightening to witness but are typically brief and not harmful. If your baby develops a high or rapidly climbing fever after a vaccination and you’re unsure what’s normal, calling your pediatrician is always reasonable.
Managing a Fever at Home
For babies old enough that the fever itself isn’t an emergency, comfort is the main goal. Keep your baby lightly dressed rather than bundled up, offer frequent feedings to prevent dehydration, and let them rest.
Acetaminophen (the active ingredient in infant Tylenol) can be used in babies over 2 months, dosed by weight. Ibuprofen is not safe for babies under 6 months old. Neither medication “cures” the fever. They lower the temperature to help your baby feel more comfortable while their immune system does its work. Always dose by your baby’s weight rather than age, and use the measuring device that comes with the medication rather than a kitchen spoon.
One thing to skip: lukewarm baths are sometimes recommended but can make babies shiver, which actually raises core body temperature. If your baby is uncomfortable, medication is more effective than a bath.