A fever in a baby is a rectal temperature of 100.4°F (38°C) or higher. This is the standard threshold used by pediatricians regardless of what caused the fever, and rectal readings are considered the most accurate method for infants. Even a reading right at 100.4°F counts as a true fever in a baby under 12 months old.
Why the Measurement Method Matters
Babies have a normal body temperature that fluctuates throughout the day, generally ranging from about 97°F to 100.3°F depending on activity, clothing, and time of day. A reading in the late afternoon will often be slightly higher than one taken in the morning. That’s completely normal.
The 100.4°F threshold applies specifically to rectal temperature, which reflects core body temperature more reliably than other methods. Armpit (axillary) readings run about 1°F lower than rectal, so an armpit reading of 99.4°F could actually mean your baby has a true fever. Ear and forehead thermometers can be convenient for older babies, but they’re less reliable in infants under 6 months because of the small size of the ear canal and the way skin temperature shifts with the environment. For babies under 3 months, a rectal thermometer is the only method pediatricians trust for making clinical decisions.
How to Take a Rectal Temperature Safely
Use a digital thermometer labeled for rectal use. Lubricate the tip with petroleum jelly, then gently insert it half an inch to one inch into the rectum. Hold the thermometer in place and never leave your baby unattended during the reading. If you feel resistance, stop and do not force it. Most digital thermometers beep within 10 to 20 seconds when they have a final reading.
It helps to lay your baby belly-down across your lap or on a firm surface, keeping one hand on their lower back to prevent sudden movement. Afterward, clean the thermometer with rubbing alcohol or soap and water, and label it so it isn’t accidentally used orally.
Age Makes a Big Difference
The number on the thermometer matters, but your baby’s age changes how urgently you need to respond. Young infants have immature immune systems, which means a fever can signal a serious infection even when a baby doesn’t look particularly sick.
- Under 3 months: Any fever of 100.4°F or higher warrants an immediate call to your pediatrician, day or night. At this age, a fever can be the only sign of a bacterial infection that needs prompt treatment. Don’t give fever-reducing medication first and wait to see what happens.
- 3 to 6 months: Call your pediatrician if the temperature reaches 100.4°F or higher, or if your baby has a lower temperature but seems unusually fussy, lethargic, or unwell.
- 6 to 24 months: A fever above 100.4°F that lasts longer than one day, or that comes with concerning symptoms, is worth a call. A brief fever in an otherwise playful, well-hydrated baby in this age range is often managed at home.
Fever-Reducing Medication by Age
Acetaminophen (Tylenol) is the only fever reducer approved for babies under 6 months, and even then, it should not be given to infants younger than 8 weeks old. Babies under 8 weeks with a fever of 100.4°F or higher need to be evaluated by a doctor rather than treated at home. For babies old enough for acetaminophen, doses are based on weight, not age, and can be given every 4 to 6 hours as needed, with no more than 5 doses in 24 hours.
Ibuprofen (Motrin, Advil) is not recommended for babies under 6 months. Once your baby is old enough, ibuprofen can be given every 6 to 8 hours, up to 4 times in 24 hours. Always use the syringe or dropper that comes with the product rather than a kitchen spoon, and dose by your baby’s current weight. If you’re unsure about the right amount, your pediatrician’s office can walk you through it over the phone.
Never give aspirin to a baby or child. It’s linked to a rare but serious condition that affects the brain and liver.
Signs That Matter More Than the Number
A fever is a symptom, not a disease. Your baby’s behavior often tells you more than the thermometer does. A baby with a 101°F fever who is still nursing well, making eye contact, and wetting diapers normally is in a very different situation than a baby with 100.5°F who is limp, refuses to eat, or is difficult to wake.
Watch for these warning signs at any temperature:
- Unusual drowsiness or difficulty waking: Going beyond normal sleepiness into a state where your baby doesn’t respond to your voice or touch the way they usually do.
- Poor feeding: Refusing the breast or bottle repeatedly, or taking much less than normal over several feedings.
- Fewer wet diapers: Fewer than 4 wet diapers in 24 hours in a baby over a week old can signal dehydration.
- A rash that doesn’t fade when pressed: Tiny purple or red dots that stay visible when you press the skin nearby.
- Breathing changes: Flaring nostrils, grunting, or skin pulling in between the ribs with each breath.
- Inconsolable crying: A high-pitched or unusual cry that doesn’t improve with holding, feeding, or comforting.
What a Fever Actually Does
Fever is the body’s way of fighting infection. When your baby’s immune system detects a virus or bacteria, it raises the internal thermostat to create an environment that’s harder for germs to thrive in. This means a fever on its own isn’t dangerous and doesn’t need to be eliminated completely. The goal of treatment is comfort, not getting the number back to 98.6°F.
Overdressing a feverish baby or bundling them in heavy blankets can actually push the temperature higher. Light clothing and a normal room temperature are usually enough. Lukewarm sponge baths can help if your baby seems uncomfortable, but avoid cold water or rubbing alcohol on the skin, both of which can cause shivering and paradoxically raise core temperature.
Most fevers in babies over 3 months are caused by common viral infections and resolve within 2 to 3 days. Teething can cause a very slight rise in temperature, but true fevers above 100.4°F are not caused by teething alone. If your baby’s temperature crosses that threshold, an illness is the more likely explanation.