A temperature of 100.4°F (38°C) or higher, measured rectally, is considered a fever in a baby. This threshold applies from newborns through toddlerhood, but your baby’s age changes how urgently that number matters and what you should do about it.
Fever Thresholds by Age
The 100.4°F cutoff is consistent across pediatric guidelines, but the response it calls for depends on how old your baby is.
- Under 3 months: Any rectal temperature of 100.4°F or higher needs immediate medical attention. At this age, a baby’s immune system is still immature, and fever can be one of the only signs of a serious infection. More than 10% of feverish infants in this age group turn out to have a urinary tract infection, and while rare (fewer than 0.05%), some develop meningitis. Doctors take fever in this age group seriously because it’s difficult to tell a minor illness from a dangerous one just by looking at the baby.
- 3 to 6 months: A temperature up to 101°F (38.3°C) warrants a call to your pediatrician if your baby also seems unusually irritable, sluggish, or uncomfortable. A temperature above 101°F at this age deserves a call regardless of how your baby is acting.
- 6 to 24 months: A temperature above 101°F that lasts longer than one day, even without other symptoms, is worth contacting your pediatrician about. If your baby seems to be acting normally and the fever is brief, you can often manage it at home.
How to Take an Accurate Temperature
Rectal temperature is the gold standard for babies. It gives the most accurate core body temperature reading, which is why pediatric guidelines are built around it. Ear and temporal artery (forehead) thermometers also use the same 100.4°F fever threshold, but rectal readings are preferred for infants under 3 months because accuracy matters most when the stakes are highest.
Armpit (axillary) readings run lower than rectal temperatures, sometimes by a full degree or more. If you take your baby’s temperature under the arm and get a concerning number, follow up with a rectal reading before deciding what to do. Forehead strip thermometers and “feeling warm to the touch” are not reliable enough to rule a fever in or out.
Common Causes of Fever in Babies
Most infant fevers come from the same everyday illnesses that affect older kids and adults: colds, flu, ear infections, and roseola. These viral infections are by far the most common trigger, and while the fever itself can look alarming, it’s actually your baby’s immune system working as designed.
Vaccines can also cause a low-grade fever, typically within 24 to 48 hours of a shot. This is a normal immune response and usually resolves on its own. Febrile seizures after vaccination are uncommon.
Teething Does Not Cause Fever
This is one of the most persistent myths in parenting. Teething can nudge your baby’s body temperature slightly above its normal baseline, but it will not push it to 100.4°F or beyond. If your baby has a true fever, something else is going on. Attributing a fever to teething and ignoring it can delay treatment for an actual infection.
Keeping a Feverish Baby Comfortable
If your baby is over 3 months old and the fever doesn’t hit the thresholds above, you can focus on comfort at home. Dress your baby in one layer of lightweight clothing. Even if your baby has the chills, resist the urge to pile on blankets, as bundling can trap heat and drive the temperature higher. One lightweight blanket for sleep is enough. Keep the room at a comfortable temperature, and use a fan if the air feels stuffy.
Hydration is the priority. Babies should drink breast milk or formula, not fruit juice. If your baby is vomiting, an oral electrolyte solution like Pedialyte can help replace lost fluids. A lukewarm sponge bath can bring temporary relief, but it works best when paired with fever-reducing medication. Skip cold baths, ice, or alcohol rubs, as these can trigger shivering, which actually raises body temperature.
For medication, acetaminophen (Tylenol) is safe for most babies. Ibuprofen (Advil, Motrin) should not be given to infants under 6 months old unless specifically directed by a doctor. Always dose by your baby’s weight, not age, and use the measuring device that comes with the medication rather than a kitchen spoon.
Signs That Need Immediate Attention
Beyond the temperature number itself, certain symptoms alongside a fever signal something more serious. These include difficulty breathing, skin or lips that look blue, purple, or gray, seizures, loss of consciousness, unusual withdrawal or decreased alertness, and pain or fussiness that keeps getting worse or won’t let up. Any of these alongside a fever, at any age, calls for emergency care.
For babies under 3 months, the fever alone is reason enough to call. You don’t need to wait for additional symptoms. The younger the baby, the lower the bar should be for making that call.