A rectal temperature of 100.4°F (38°C) or higher is considered a fever in babies. Whether that fever is “too high” depends almost entirely on your baby’s age. For any infant under 3 months old, a temperature of 100.4°F or above is an emergency that needs immediate medical attention, regardless of how your baby seems to be acting. For older babies, the number on the thermometer matters less than how your child looks and behaves.
Why Age Matters More Than the Number
A fever of 101°F in a 2-month-old and the same fever in a 10-month-old are very different situations. Newborns and young infants have immature immune systems, which means a fever can be the only early sign of a serious bacterial infection. Their bodies don’t always show the obvious signs of illness that older babies do, so doctors treat any fever in this age group as potentially dangerous until proven otherwise.
The American Academy of Pediatrics has specific clinical guidelines for evaluating infants between 8 and 60 days old who develop a fever at or above 100.4°F. These guidelines call for blood work, urine testing, and sometimes a spinal fluid sample to rule out infections like meningitis or bloodstream infections. That’s why pediatricians are so insistent about bringing in very young babies with any fever at all.
Temperature Thresholds by Age
Under 3 Months
Any rectal temperature of 100.4°F (38°C) or higher is a reason to go to the emergency department. Don’t give fever-reducing medication first and wait to see what happens. Don’t wait for a callback from your pediatrician’s office. This is the one age group where the temperature alone, even without other symptoms, warrants urgent care. Acetaminophen should not be given to infants under 8 weeks old, and ibuprofen is not safe for babies under 6 months.
3 to 6 Months
Call your pediatrician if your baby has a temperature up to 100.4°F and seems unwell, or if the temperature rises above 100.4°F regardless of how they’re acting. At this age, your baby’s behavior starts to become a more reliable signal. A baby who is feeding well, making eye contact, and wetting diapers normally is less concerning than one who is listless or refusing to eat, but any fever in this range still warrants a phone call.
6 to 24 Months
Call your pediatrician if the fever is above 100.4°F and lasts more than one day. At this age, fevers are common and usually caused by viral infections that resolve on their own. You can use acetaminophen every 4 to 6 hours (no more than 5 doses in 24 hours) or ibuprofen every 6 to 8 hours (no more than 4 doses in 24 hours). Always dose by your baby’s weight, not their age, for the most accurate amount. Give ibuprofen with food or milk to avoid stomach irritation.
How to Get an Accurate Reading
The method you use to take your baby’s temperature affects the number you get. A rectal reading is the gold standard for infants and gives the most reliable core body temperature. Ear and temporal artery (forehead) thermometers also use the 100.4°F threshold for fever. Oral thermometers define fever at 100°F (37.8°C), and armpit readings count as a fever at 99°F (37.2°C).
Armpit temperatures are the least accurate of all methods. If you get a concerning armpit reading, confirm it with a rectal or temporal artery thermometer before deciding on next steps. For babies under 3 months, a rectal thermometer is strongly preferred because even small inaccuracies matter when the stakes are higher.
Warning Signs That Matter More Than Temperature
Once your baby is past the 3-month mark, how they act with a fever tells you more than the number itself. A baby with a 103°F fever who is still drinking, playing between bouts of fussiness, and making wet diapers is generally in less danger than a baby with a 101°F fever who can’t be roused from sleep.
Get emergency care right away if your baby:
- Has difficulty breathing or is breathing much faster than normal
- Looks unusually pale, blotchy, or has blue-tinged skin around the lips
- Is extremely drowsy and hard to wake up or keep alert
- Has a rash that doesn’t fade when you press on it (this can signal a serious infection)
- Has a seizure
- Has a bulging soft spot on top of the head
- Cries inconsolably and cannot be comforted
These signs warrant immediate attention at any temperature, even if the fever itself doesn’t seem particularly high.
Febrile Seizures
Febrile seizures are convulsions triggered by fever, most common in children between 6 months and 5 years old, with the highest risk between ages 1 and 3. They’re frightening to witness but usually not harmful. The most common type lasts a few seconds to 15 minutes and happens only once within a 24-hour period.
If your child has a seizure with a fever, call an ambulance if it lasts longer than 5 minutes or if your child doesn’t seem to recover quickly after a shorter one. Any first febrile seizure should be evaluated by a doctor. Febrile seizures can happen as a temperature is rising, so they sometimes occur before you even realize your child has a significant fever. They are not caused by a specific temperature threshold, and giving fever reducers does not reliably prevent them.
Keeping Your Baby Comfortable and Hydrated
Fever increases fluid loss, so dehydration is the most practical concern when your baby has a fever that you’re managing at home. Babies under 6 months should get all their fluids from breast milk or formula, not water. Babies 7 to 12 months need about 0.8 liters of fluid daily from a combination of breast milk, formula, and food, with only small amounts of water. Offer feeds more frequently than usual rather than trying to increase the volume at each feeding.
Watch for signs of dehydration as the fever continues. Mild dehydration looks like slightly fewer wet diapers than normal. Moderate dehydration brings a dry mouth and lips, cool hands and feet, darker urine, and a baby who is quieter and less active than usual. Severe dehydration causes sunken eyes, mottled or patchy skin, extreme sleepiness, and very few or no wet diapers. Severe dehydration needs emergency care.
Dress your baby in light clothing and keep the room at a comfortable temperature. Bundling a feverish baby in blankets can trap heat and push the temperature higher. A lukewarm (not cold) sponge bath can provide some comfort, but if it makes your baby shiver, stop. Shivering actually raises body temperature.