A rectal temperature of 100.4°F (38°C) or higher is considered a fever in infants, and the most accurate way to check is with a digital rectal thermometer. The method you use matters, though, because different thermometer types vary in reliability depending on your baby’s age. Here’s how to get an accurate reading and what the numbers mean.
Which Thermometer to Use at Each Age
For babies under 3 months, a digital rectal thermometer is the standard. Most research establishing the risk of serious infections in young infants has relied on rectal temperatures, making them the benchmark for detecting fever in this age group. This isn’t the most pleasant option for parent or baby, but it’s the most reliable one when accuracy counts most.
Once your baby reaches 6 months, you can start using a digital ear (tympanic) thermometer. Before that age, a baby’s ear canal is too small and curved for the probe to get a good reading. Even after 6 months, earwax or an unusually narrow canal can throw off results.
Temporal artery thermometers (the kind you swipe across the forehead) and non-contact infrared thermometers work at any age and are the least disruptive option, especially for sleeping babies. A study published in JAMA Pediatrics found that temporal artery thermometers detected 66% of fevers confirmed by rectal reading, compared to just 49% for ear thermometers. For higher fevers, the temporal artery thermometer caught 94% of cases. These are useful for quick screening, but if the number looks borderline, follow up with a rectal reading for babies under 3 months.
Armpit (axillary) readings are the least accurate method. They consistently read lower than core body temperature. If you use one, add 0.5 to 1°F (0.3 to 0.6°C) to the displayed number to approximate your baby’s actual temperature.
How to Take a Rectal Temperature
Lay your baby face-down across your lap or on a firm, flat surface. Apply a small amount of petroleum jelly to the tip of a clean digital thermometer. Gently insert the tip about half an inch to one inch into the rectum. Don’t force it. Hold the thermometer in place with one hand while keeping your other hand on your baby’s back or bottom so they can’t roll or squirm off the surface.
Wait for the thermometer to beep, which typically takes 10 to 30 seconds with modern digital models. Remove it gently and read the display. Clean the thermometer tip with rubbing alcohol (70% isopropyl) or soap and warm water after every use, and label it so it’s never accidentally used in the mouth.
How to Use a Forehead or Non-Contact Thermometer
These thermometers use infrared sensors, and environmental conditions can significantly affect their accuracy. The FDA recommends using them in a draft-free space, out of direct sunlight, and away from radiant heat sources like space heaters. The room should be between roughly 61°F and 104°F with humidity below 85%. If you’ve just brought the thermometer in from a car or garage, let it sit in the room for 10 to 30 minutes before using it.
Make sure your baby’s forehead is clean, dry, and uncovered. Hats, headbands, or even a recently applied facial wipe can alter the skin temperature enough to skew the reading. For temporal artery models, swipe the sensor straight across the forehead following the manufacturer’s directions. For non-contact models, hold the device at the recommended distance (usually 1 to 2 inches) and aim at the center of the forehead.
How to Read an Armpit Temperature
Place the thermometer tip in the center of your baby’s armpit and hold their arm snugly against their body. Keep the arm pressed down until the thermometer beeps. The reading will be lower than your baby’s core temperature, so add 0.5 to 1°F to get a closer estimate. An armpit reading is fine for a general check when your baby seems well, but it shouldn’t be the method you rely on if you suspect a real fever, especially in a newborn.
What Counts as a Fever
A rectal temperature of 100.4°F (38°C) or above is a fever. This threshold applies to all infants, but the urgency depends heavily on age. For babies under 3 months, any fever at or above 100.4°F is treated seriously because their immune systems are still developing and infections can escalate quickly. For older babies, a fever in the same range is more common and often less concerning on its own.
Temperature alone doesn’t tell the whole story. A baby at 100.2°F who is limp, difficult to wake, or refusing to eat may need attention more urgently than a baby at 101°F who is fussy but alert and drinking normally.
Signs That Need Immediate Attention
For babies under 3 months with a fever, contact your pediatrician right away. Beyond the number on the thermometer, watch for these warning signs at any age:
- Inconsolable crying that doesn’t respond to comfort or feeding
- Difficulty waking up or unusual drowsiness
- A stiff neck, which can signal a serious infection
- Purple spots on the skin that don’t fade when pressed
- Labored breathing that doesn’t improve after clearing the nose
- Inability to swallow, with drooling instead of eating
If your baby looks or acts very sick but the thermometer shows a normal or borderline reading, trust your instincts. Recheck the temperature in an hour, and if something still feels off, seek care.
Keeping Your Thermometer Accurate
Never use a glass mercury thermometer. Mercury is a toxic substance, and a broken thermometer creates both a poisoning risk and a cleanup hazard. The EPA has been working to phase mercury thermometers out entirely, and NIST stopped calibrating them back in 2011. Digital thermometers are inexpensive, faster, and safer.
Clean your digital thermometer before and after each use by wiping the probe with a cotton ball or cloth soaked in 70% isopropyl alcohol. For non-contact thermometers, gently wipe the sensor lens with an alcohol wipe, making sure no excess liquid drips into the device. Store thermometers in their protective case at room temperature to keep the sensors functioning properly. If your thermometer gives readings that seem consistently off, replace the battery first, as a low battery is the most common cause of inaccurate digital readings.