The right way to take your baby’s temperature depends on their age. For newborns up to 3 months old, a rectal reading with a standard digital thermometer is the most accurate method. As your child grows, you can switch to forehead, ear, or underarm options. A temperature of 100.4°F (38°C) or above counts as a fever at any age.
Which Thermometer to Use at Each Age
Not every thermometer works well for every age group. Babies under 3 months should have their temperature taken rectally with a digital thermometer. Recent evidence suggests temporal artery (forehead) thermometers may also give accurate readings in newborns, but rectal remains the standard. Starting at 6 months, you can add a digital ear thermometer to your options. Most children can hold an oral thermometer under their tongue by age 4.
Underarm (axillary) readings work at any age, but they’re the least precise. Research published in Frontiers in Pediatrics found that underarm temperatures averaged about 0.2°F lower than rectal readings, and the margin of error was wide enough that an underarm reading could be off by more than a full degree in either direction. That makes underarm a reasonable quick check for older babies and toddlers, but if the number seems borderline, follow up with a more reliable method.
Avoid glass mercury thermometers entirely. If one breaks, the mercury is toxic, and most childhood mercury exposures at home come from broken thermometers. Digital thermometers are inexpensive, faster, and safer.
How to Take a Rectal Temperature
This sounds intimidating, but it’s straightforward once you’ve done it. Clean the thermometer tip with rubbing alcohol and apply a small amount of petroleum jelly or water-based lubricant to the sensor end.
Lay your baby face up with their legs bent toward their chest, the same position as a diaper change. You can also place them face down across your lap on a soft surface. Gently insert the thermometer tip about half an inch to one inch (1.5 to 2.5 cm) into the rectum. Never force it. Hold the thermometer in place with two fingers near the base, close to your baby’s body, not at the far end of the thermometer. Keep it there for about one minute or until it beeps.
Remove it gently, read the display, and clean the thermometer with alcohol before storing it. Label this thermometer so it’s only used rectally.
How to Use a Forehead Thermometer
Temporal artery thermometers use an infrared sensor to measure the temperature of blood flowing near the skin’s surface. Most models require you to press the sensor flat against the center of the forehead and slide it across to the hairline while holding down the scan button. Some no-touch models just need to be held an inch or two from the forehead.
For the best reading, make sure your baby’s forehead is dry. Sweat, hats, or lying face down on a pillow can all throw off the number. Follow your specific model’s instructions, since the scanning technique varies between brands. These thermometers are fast, usually giving a reading in about two seconds, which makes them practical for squirmy babies and nighttime checks.
How to Use an Ear Thermometer
Ear (tympanic) thermometers measure infrared heat from the eardrum, but they only work if the sensor can “see” the eardrum clearly. The ear canal in babies and young children curves differently than in adults, so you need to straighten it first. For children, gently pull the outer ear backward and slightly downward. This aligns the ear canal so the sensor tip can reach the right spot.
Insert the thermometer tip snugly into the ear canal, press the button, and wait for the beep. Don’t use an ear thermometer if your child has an ear infection, significant earwax buildup, or has been lying on that ear, as all of these can skew the result. Wait until your baby is at least 6 months old before using this method, since the ear canal is too small for an accurate reading before that.
How to Take an Underarm Temperature
Place the thermometer tip in the center of your baby’s armpit, directly against skin (not over clothing). Fold their arm down against their body and hold it gently in place for about a minute or until the thermometer beeps. Because underarm readings run lower than core body temperature, add roughly 1°F (0.5°C) to get a closer estimate of your baby’s true temperature. Keep in mind this is still a rough estimate. If the reading is near 99°F or above, take a rectal or forehead temperature to confirm.
Getting an Accurate Reading
A few things can make your reading unreliable regardless of the method. Taking a temperature right after a bath, after your baby has been bundled in heavy clothing, or right after a feeding can push the number up or down. Wait about 15 to 20 minutes for your baby’s body to settle before measuring.
If a reading seems off, take it again. Digital thermometers occasionally give a bad read if the sensor doesn’t make proper contact. Two readings within a few minutes that are close together give you a number you can trust. Always clean the thermometer with rubbing alcohol both before and after each use to prevent spreading germs.
What the Numbers Mean
A fever is 100.4°F (38°C) or higher. Normal body temperature for babies typically falls between 97°F and 100.3°F depending on the time of day, activity level, and where you measured. A slightly elevated reading on its own isn’t automatically a problem, but the context matters.
For babies 3 months old or younger, any fever of 100.4°F or above warrants a call to your pediatrician right away, even if your baby seems fine otherwise. Young infants can’t fight infections the same way older children can, so fevers in this age group are taken seriously. For older babies and toddlers, a fever lasting more than four days without a clear cause, signs of dehydration, or a child who appears unusually drowsy or ill are all reasons to check in with your pediatrician.
Seek emergency care if your child has a fever along with any of these signs: extreme sleepiness, a stiff neck, a rash, trouble breathing, inconsolable crying, difficulty swallowing, blue lips or nails, or a seizure lasting longer than five minutes.