The most accurate way to take a baby’s temperature is rectally, especially for infants under 3 months old. A rectal reading is considered the gold standard because it measures core body temperature, while other methods (armpit, forehead, ear) can be less reliable in very young babies. A normal temperature for a baby is around 98.6°F (37°C), and a fever is defined as 100.4°F (38°C) or higher.
Rectal Temperature: The Most Accurate Method
For babies under 3 months, a rectal thermometer is the one method pediatricians trust for a definitive reading. You’ll need a standard digital thermometer (not glass) and a lubricant like petroleum jelly. Here’s how to do it:
- Turn on the thermometer and apply a small amount of petroleum jelly to the tip.
- Position your baby on their back with their thighs lifted, or belly-down across your lap. If belly-down, place one hand on their lower back to keep them still.
- Insert the tip gently about half an inch to one inch (1.3 to 2.5 centimeters) into the rectum. Never force it past any resistance.
- Hold it in place until the thermometer beeps, which usually takes 10 to 20 seconds with a digital model.
It sounds intimidating the first time, but it’s quick and painless for the baby. Mark this thermometer clearly so it’s never accidentally used orally.
Armpit (Axillary) Temperature
An armpit reading is the easiest method and works well as a quick screening, though it’s less precise than rectal. Place the tip of a digital thermometer in the center of your baby’s armpit, then hold their arm snugly against their side until the thermometer beeps. Make sure there’s no clothing between the tip and the skin.
Armpit readings run lower than core body temperature. Add half a degree to one full degree Fahrenheit (0.3 to 0.6°C) to get a closer estimate of your baby’s true temperature. If the adjusted number is near 100.4°F or above, follow up with a rectal reading to confirm.
Forehead (Temporal Artery) Thermometers
Forehead thermometers use infrared sensors to read the blood vessels just beneath the skin. They’re fast and non-invasive, which makes them popular for squirmy babies. Most work by either pressing a sensor to the temple or holding a no-touch device a short distance from the forehead.
The accuracy of forehead thermometers can be affected by sweat, direct sunlight, hats, or a room that’s very hot or cold. If your baby has been bundled up or playing in a warm environment, wait a few minutes before scanning. For babies under 3 months, a forehead reading that looks elevated should always be confirmed rectally before calling your pediatrician, because treatment decisions at that age hinge on precise numbers.
Ear (Tympanic) Thermometers
Ear thermometers measure infrared heat from the eardrum and give results in about a second. They’re convenient for older babies and toddlers, but they aren’t recommended for newborns or very young infants. A baby’s ear canals are too narrow and curved for the probe to get an accurate reading. Most manufacturers and pediatricians suggest waiting until a baby is at least 6 months old before relying on ear readings.
Even in older babies, earwax buildup or a recent ear infection can throw off the number. Pull the ear gently back and up to straighten the canal before inserting the probe for the best result.
Cleaning Your Thermometer
Clean your thermometer before and after every use with cool, soapy water, or wipe the tip with an alcohol swab. Never use hot water, bleach, or household cleaners, and don’t put it in the dishwasher. Hot water can damage the sensor or warp the plastic. Let it air dry or pat it with a clean cloth before storing.
Fever Thresholds by Age
What counts as a concerning fever depends on how old your baby is. The numbers below are based on rectal readings:
- Under 3 months: Any temperature of 100.4°F (38°C) or higher warrants an immediate call to your pediatrician, even if your baby seems fine otherwise. At this age, a fever can signal a serious infection that young immune systems can’t fight well on their own.
- 3 to 6 months: Call if your baby reaches 100.4°F and seems unwell (unusually fussy, sleepy, or not feeding), or if the temperature goes above 100.4°F regardless of how they’re acting.
- 6 to 24 months: A temperature above 100.4°F that lasts more than one day needs medical attention.
- Any age: A temperature that repeatedly climbs above 104°F (40°C) requires a call right away.
If a fever lasts more than 72 hours in a child 2 or older, that also warrants a check-in with your pediatrician.
Signs That Accompany a Fever and Need Attention
The number on the thermometer matters, but how your baby is acting matters just as much. Watch for these warning signs alongside a fever:
- Unusual sleepiness: Sleeping far more than normal, hard to wake, or seeming floppy when you pick them up.
- Dehydration: Fewer wet diapers than usual, crying with few or no tears, a dry mouth, or a sunken soft spot on the head.
- Rash: Particularly one that appears suddenly, blisters, or looks infected.
- Breathing trouble: Rapid breathing, flaring nostrils, or visible effort with each breath.
- Color changes: Skin or lips that look blue, purple, or gray.
Any of these paired with a fever, or even on their own, call for prompt medical attention. Trust your instincts. If your baby just doesn’t seem right, that’s reason enough to pick up the phone.