A baby has a fever when their rectal temperature reaches 100.4°F (38°C) or higher. This is the standard threshold used by pediatricians and the American Academy of Pediatrics, regardless of the baby’s age. But what counts as a fever can look slightly different depending on how you take the temperature, and your baby’s age changes how urgently you need to respond.
Fever Thresholds by Measurement Method
Not every thermometer reads the same way, because different parts of the body run at slightly different temperatures. Here’s what qualifies as a fever depending on where you measure:
- Rectal: 100.4°F (38°C) or higher
- Ear or forehead (temporal artery): 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit (axillary): 99°F (37.2°C) or higher
Rectal readings are the gold standard for babies under 3 months old because they’re the most accurate. Armpit readings are convenient but tend to run lower than the baby’s true core temperature, which is why the fever threshold for armpit readings is set lower. If you get an armpit reading that seems borderline, taking a rectal temperature gives you a more reliable number to work with.
Why Your Baby’s Age Matters More Than the Number
A temperature of 100.4°F in a 4-month-old and in a 4-week-old mean very different things medically. For any baby younger than 3 months, a rectal temperature of 100.4°F or higher warrants an immediate call to your pediatrician, even if the baby seems fine otherwise. At that age, a young immune system can’t fight infections as effectively, and a fever can be the only early sign of something serious like a urinary tract infection or bacterial infection in the blood.
The AAP’s clinical guidelines break this down even further for very young infants, with specific evaluation steps for babies 8 to 21 days old, 22 to 28 days old, and 29 to 60 days old. The younger the baby, the more thorough the workup a doctor will want to do. This isn’t meant to scare you. It’s simply that fever in the first few months of life gets treated with extra caution because the stakes are higher and the signs of serious illness can be subtle.
For babies older than 3 months, fever alone is less of a red flag. How your baby looks and acts matters more than the exact number on the thermometer. A baby at 101°F who is feeding well, making eye contact, and wetting diapers normally is in a very different situation than a baby at 100.5°F who is limp and refusing to eat.
Signs That Matter More Than the Temperature
A fever is your baby’s immune system doing its job, not a disease in itself. The temperature number tells you something is going on, but your baby’s behavior and appearance tell you how serious it is. Watch for these warning signs, which call for urgent medical attention at any temperature:
- Breathing changes: breathing very fast, working hard to breathe, or pausing between breaths
- Skin color: pale, blotchy, or bluish skin, especially around the lips
- Extreme drowsiness: unusually hard to wake up, or floppy and unresponsive when awake
- Rash that doesn’t fade: if you press on a rash and it stays visible rather than briefly disappearing, that needs emergency evaluation
- Seizure: any shaking or convulsing episode during a fever
- Dehydration signs: refusing to drink, fewer than half the usual number of wet diapers, or repeated vomiting
- Bulging soft spot: the fontanelle on top of a young baby’s head looks swollen or pushed outward
- Inconsolable crying: nothing calms the baby, or the cry sounds unusual
Any of these symptoms alongside a fever, or even without one, signals that your baby needs to be seen right away.
How to Take a Rectal Temperature
If you haven’t done this before, it’s simpler than it sounds. Lubricate the tip of a digital thermometer with petroleum jelly, lay your baby face-down on your lap or face-up on a firm surface, and gently insert the tip about half an inch into the rectum. Hold the thermometer in place until it beeps. The whole process takes under a minute. Use a thermometer you’ve labeled for rectal use only, so it doesn’t get mixed up with oral thermometers later.
Forehead and ear thermometers are fine for older babies and toddlers as a quick screening tool. But for infants under 3 months, a rectal reading gives you and your pediatrician the most trustworthy number to make decisions with.
Managing a Fever at Home
For babies 3 months and older with a mild fever who are otherwise acting normally, you don’t always need to bring the fever down. Fever itself helps the immune system fight infection. Focus on keeping your baby comfortable and well-hydrated with regular feedings.
Acetaminophen (the active ingredient in infant Tylenol) can be used for babies who are uncomfortable, but dosing is based on your baby’s weight, not age. Your pediatrician or pharmacist can confirm the right amount. Ibuprofen is not recommended for babies under 6 months old because it hasn’t been established as safe for that age group. After 6 months, ibuprofen becomes an option, again dosed by weight.
Avoid bundling a feverish baby in extra layers, which can trap heat and push the temperature higher. Dress them in light clothing and keep the room at a comfortable temperature. Lukewarm sponge baths can help if your baby is visibly uncomfortable, but skip cold water or ice baths, which can cause shivering and actually raise core temperature.