Think your baby might have a fever? Learn how to take their temperature with different types of thermometers, including rectal, forehead, ear, and armpit.

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An image of a mother's hand holding a thermometer with her baby in the back.
Credit: Getty Images.

Does your baby seem warm and fussy? It's important to take their temperature, because fever can be dangerous in little ones. In fact, if your baby is less than 3 months old, temperatures above 100.4 degrees Fahrenheit can cause threatening infections like bacterial meningitis and pneumonia. That's because newborns don't have functioning immune systems yet.

Fevers are less worrisome in those between 3 months and 6 months, but you should still call your doctor if their temperature is greater than 101 degrees F. And if your baby is older than 6 months, you can wait until their temperature surpasses 103 degrees F—but let your pediatrician know if they have a fever of 102 degrees F that's accompanied by worrisome symptoms like body aches, cough, fatigue, headache, chills, etc.

Keep reading to learn how to take a baby's temperature with different types of thermometers, including rectal, forehead, ear, and armpit.

How to Take a Baby's Rectal Temperature

Like it or not, rectal thermometers are the gold standard for children 3 years of age and younger. Especially for those under 3 months old, "a rectal thermometer is best," says Paula Elbirt, M.D., author of Dr. Paula's House Calls to Your Newborn. "It's accurate, easy to use, and reliable." Here's how to take a baby's rectal temperature:

1. Remove a layer or two of your baby's clothing, and wait several minutes before taking their temperature. (Overdressing can nudge the gauge artificially higher, and even a few tenths of a degree can make a big difference in a newborn.)

2. Read the directions that came with the thermometer. Clean the tip of the thermometer with rubbing alcohol or soap and water, then rinse it with cool water. If you have one available, cover the thermometer with a disposable single-use plastic sleeve.

3. Lubricate the tip of the thermometer with a water-soluble jelly.

4. Place your baby across your lap, face down. Don't forget to support their head. For an older child or a very squirmy baby, lay them down on a firm, flat surface (such as a changing table) or on top of a blanket on the floor. You can also place your baby face-up on the changing table, and holding both ankles with one hand, lift their knees to their chest.

5. Press the palm of one hand firmly against your baby's lower back to hold them still. With the other hand, turn on the thermometer switch and insert the thermometer about 1/2 inch into your baby's anus. Always stop if you feel resistance.

6. Hold the thermometer in place loosely with two fingers, keeping your hand cupped around your child's bottom, until you hear the beep.

7. Remove the thermometer to check the digital reading. Record the temperature and the time of day it was taken, in case you need to inform the pediatrician.

8. If you used a disposable plastic sleeve, discard it right away. Clean the thermometer.

How to Take a Baby's Temporal (Forehead) Temperature

Temporal artery (forehead) thermometers are fast and fairly accurate. Many parents like using them for squirmy babies, although they work on children of any age, according to the American Academy of Pediatrics (AAP). Here's how to take a baby's forehead temperature:

1. Read the directions to ensure you're using the thermometer properly. Forehead thermometers work by measuring heat waves from the temporal artery.

2. For some thermometers, you'll place the infrared sensor at the center of the forehead. Pressing on the scan button, slide the thermometer across the forehead to the temporal artery, which is located between the hairline and outside edge of the eye. Always keep contact with the skin. Stop when you reach the hairline.

3. Other thermometers will have you aim the sensor directly on the temporal artery, then press the scan button. Make sure to follow the directions for your particular thermometer.

4. Release the scan button and read the results on the thermometer's screen.

How to Take a Baby's Tympanic (Ear) Temperature

Tympanic (ear) thermometers are easy and give fast results, as long as your baby doesn't have a buildup of earwax. But according to the AAP, parents should "avoid using an ear thermometer for babies under 6 months old, since their ear canals are too small to allow an accurate reading." Here's how to take a baby's ear temperature:

1. Read the directions that came with the thermometer, and make sure your child's ear doesn't have excess earwax in it. If so, try cleaning it with a cotton swab without entering the ear canal.

2. Gently put the end of the thermometer in your baby's ear and press the "start" button.

3. Remove the thermometer when you hear the beep or series of beeps (which should be within seconds).

4. Remove the thermometer to check the reading. Record the temperature and the time of day it was taken in case you need to inform the pediatrician.

5. Be aware that tympanic readings have a tendency to be off. If your baby's temperature is above 102 degrees F, it's recommended that you take it again rectally for a more accurate reading.

How to Take a Baby's Temperature Under the Arm

Armpit (axillary) temperatures are certainly preferable (for both Mother and Baby) to a rectal reading, but the results are usually not as accurate. If you're in a pinch, or if your baby isn't cooperating, you might consider the axillary method. Then, if the reading indicates a fever, you can double check by taking their rectal temperature.

Note that doctors often recommend adding 1 degree F to the axillary reading for accuracy. That's because underarm temperature tends to be lower than that at the core, especially if the room is chilly. Here's how to take a baby's temperature under the arm:

1. If your infant just had a bath, wait 15 minutes before measuring temperature.

2. Place the baby on your lap.

3. Remove your baby's shirt and place the probe of the thermometer in the center of their armpit. The probe must not touch any clothing, only skin.

4. Gently hold your baby's arm down and across the chest to hold the probe in place.

5. Wait to hear a series of beeps that indicate the reading is complete.

6. Remove the thermometer and check the results. Write down the time of day and the temperature reading.

Other Baby Temperature Considerations

Once your child is 4 or 5 years old, you can start taking their temperature orally. This method isn't recommended for babies, according to the AAP. Also, the organization doesn't recommend pacifier thermometers or fever strips. "While parents can use them for quick screening, they do not work as well as measuring your child's rectal, forehead, ear, or oral temperature with a digital thermometer," the AAP says.

Never use a glass mercury thermometer with your baby. The glass can break, leaking mercury in the mouth or on the skin, causing serious injury. Call your doctor or the local health department to find out how to dispose of a glass mercury thermometer.