The most accurate way to take a baby’s temperature is rectally, using a digital thermometer. For babies under 3 months old, a rectal reading is the recommended method because it measures core body temperature and gives the most reliable result. A temperature of 100.4°F (38.0°C) or higher taken rectally counts as a fever.
Why Rectal Thermometers Are the Standard
Rectal thermometry is considered the gold standard for children younger than 3 years. Other methods exist, but they’re less reliable in young babies. Forehead (temporal artery) thermometers come closest, with readings typically within 0.4°F of a rectal reading. But they still miss fevers more often than you’d expect. In one large study published in the Annals of Emergency Medicine, forehead thermometers detected only about 62% of true fevers at the standard 100.4°F cutoff. Armpit readings performed far worse, catching just 11.5% of fevers.
That gap matters most in the first few months of life, when even a low fever can signal something serious. An armpit or forehead reading that says “normal” could be masking a real fever. For babies under 3 months, stick with rectal.
How to Take a Rectal Temperature
Use a digital rectal thermometer with a short, flexible probe. Label it clearly so it’s only used for rectal readings. Here’s the process:
- Lubricate the tip. Apply a small amount of petroleum jelly to the end of the thermometer.
- Position your baby. Lay your baby on their back and gently lift their thighs toward their chest. You can also place them belly-down across your lap, resting one hand on their lower back to keep them still.
- Insert gently. Slide the thermometer in about half an inch to one inch (1.3 to 2.5 centimeters). Never force it past any resistance.
- Wait for the beep. Hold the thermometer in place until the alarm sounds, which usually takes 10 to 20 seconds with a modern digital model.
- Remove and read. Slide it out gently and check the display.
The whole process takes under a minute. Babies often fuss, but a thermometer designed for rectal use has a short probe that minimizes any risk of injury.
Other Methods for Older Babies
Once your baby is past 3 months, you have more options, though rectal remains the most accurate.
Forehead (temporal artery) thermometers are quick and non-invasive. You swipe or hold the device across the forehead, and it reads the temperature of blood flowing through a surface artery. These correlate more closely with rectal readings than armpit thermometers do. If you use one as a screening tool, know that a reading of 99.9°F or above is a better cutoff for flagging a possible fever. At that threshold, sensitivity jumps to about 81%, meaning it catches most true fevers.
Ear (tympanic) thermometers are not accurate before 6 months of age. A baby’s ear canals are too small and curved for the sensor to get a reliable reading. After 6 months, they can be a convenient option. Gently pull the ear back and up to straighten the canal before inserting the probe tip.
Armpit (axillary) readings are the least reliable method at any age. They measure skin temperature rather than core temperature, and they miss the vast majority of fevers in young children. An armpit temperature of 99°F (37.2°C) or higher suggests a fever, but if the reading concerns you at all, follow up with a rectal check.
What Counts as a Fever
The fever threshold depends on where you measure:
- Rectal, ear, or forehead: 100.4°F (38.0°C) or higher
- Mouth: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
For babies under 3 months, any rectal temperature of 100.4°F or above needs prompt medical evaluation. At that age, a fever can be the only visible sign of a serious infection, even if the baby otherwise looks fine. Don’t wait to see if it resolves on its own.
Tips for an Accurate Reading
A few things can throw off your results. Bundling your baby in heavy blankets or swaddling can artificially raise skin temperature, making armpit and forehead readings unreliable. If your baby has been tightly swaddled, loosen the layers and wait a few minutes before measuring. The same goes for a recent warm bath.
Always use a digital thermometer. Glass mercury thermometers are no longer recommended because of the risk of breakage and mercury exposure. Digital models are faster, safer, and easier to read. Clean the thermometer with rubbing alcohol or soap and water before and after each use.
Signs That Need Immediate Attention
A fever number alone doesn’t tell the full story. Pay attention to how your baby is acting. Contact your pediatrician or seek care right away if your baby has a fever along with any of these:
- Excessive sleepiness. Sleeping more than usual, hard to wake up, or seeming floppy and unresponsive.
- Signs of dehydration. Fewer wet diapers, crying without tears, a dry mouth, or a sunken soft spot on the head.
- A new rash. Especially one that appears quickly, blisters, or looks infected.
- Breathing trouble. Rapid breathing, grunting, or flaring nostrils.
- Color changes. Skin or lips that look blue, purple, or gray.
- Inconsolable crying. Pain or fussiness that keeps getting worse and won’t let up.
For babies under 3 months, a rectal temperature of 100.4°F or higher warrants a call to your doctor regardless of other symptoms. These babies need to be evaluated even if they seem otherwise comfortable.