Baby Fever Chart by Age

A fever is defined as a rectal temperature of 100.4°F (38°C) or higher. Rectal readings are the most accurate for infants. For babies under 3 months old, any rectal temperature of 100.4°F or above is considered an emergency and requires immediate medical evaluation.

The American Academy of Pediatrics recommends rectal temperatures for the most accurate reading in babies under 3 years. Ear (tympanic) and temporal artery (forehead) thermometers are more convenient but can give different readings. Axillary (underarm) measurements run about 1°F lower than rectal. Always tell your doctor which method you used.

Yes. For babies under 3 months of age, a rectal temperature of 100.4°F (38°C) or higher requires immediate medical evaluation — even if your baby appears to be acting normally. Young infants can deteriorate rapidly and may have serious infections that aren't immediately apparent. Do not wait for symptoms to worsen.

Generally, no. If your baby is sleeping comfortably (breathing normally, not unusually pale or flushed), letting them sleep is okay. Fever itself is not dangerous in most cases — it's the body's immune response. The main reason to give fever reducers is comfort. Wake your baby if they haven't fed or had fluids for an unusually long time, or if they appear very distressed.

Some pediatricians recommend alternating acetaminophen (Tylenol) and ibuprofen (Motrin/Advil) for more consistent fever control, but this increases the risk of dosing errors. Ibuprofen should only be given to babies 6 months or older. Always use weight-based dosing, never exceed recommended doses, and check with your pediatrician before alternating medications.

A fever's temperature isn't always the best predictor of illness severity. Watch for these warning signs regardless of temperature: difficulty breathing, persistent vomiting, severe headache, stiff neck, sensitivity to light, a purple/red spotted rash that doesn't fade when pressed, extreme lethargy (not waking up), or inconsolable crying. These symptoms require immediate care.