How to Take a Baby's Temperature: Rectal, Forehead, Armpit or Ear

Which method at which age

• Birth to 3 months: rectal is the most accurate and the one pediatricians want for decisions. A forehead (temporal artery) reading can be a quick screen; confirm any high or low reading rectally.
• 3 months to 3-4 years: rectal, forehead or armpit. Ear thermometers work from about 6 months.
• 4 years and up: mouth (once they can hold it under the tongue), ear or forehead.
• Armpit readings are the least accurate. Use them as a screen and recheck if the number matters.
• Use a digital thermometer. Never use a mercury or glass thermometer. Strip and pacifier thermometers and touching the forehead aren't reliable.
• Label a thermometer for rectal use only and keep it separate.

Rectal: step by step

1. Clean the tip with soap and water or rubbing alcohol, then rinse.
2. Put a little petroleum jelly on the tip.
3. Lay the baby on their back with knees bent toward the chest, or tummy-down across your lap.
4. Gently insert the tip about 1/2 to 1 inch (1.5-2.5 cm), no further. Stop if you feel resistance.
5. Hold it still with your hand cupped around the bottom until it beeps.
6. Read, clean it, and write down the number and time.

Forehead, armpit, ear and mouth

• Forehead: follow the device's directions, usually a slow sweep across the forehead or a single point. Dry sweat first, and skip it right after a bath or with a hat on.
• Armpit: tip in the center of a dry armpit, arm held snugly against the side until it beeps.
• Ear: gently pull the outer ear back and up to straighten the canal, then aim toward the eye. Earwax or a small canal can lower the reading.
• Mouth: wait 15 minutes after eating or drinking anything hot or cold; tip under the tongue, lips closed.

What counts as a fever

• A fever is 100.4°F (38°C) or higher by any method. Armpit readings run a bit lower, so an armpit number near that line deserves a rectal recheck in young babies.
• Under 3 months: a rectal temperature of 100.4°F (38°C) or higher, or below about 97.5°F (36.4°C), always needs a call to the pediatrician right away, day or night, even if the baby seems fine, and before giving any medicine. See the newborn red flags guide.
• 3-6 months: call for about 101-102°F (38.3-38.9°C) or higher, or sooner if your baby seems unwell; ask your pediatrician which number they use.
• Older babies and children: how they look and act matters more than the number. See the colds and fevers guide for when a fever has gone on long enough to check in.

Common mistakes

• Taking a temperature right after a bath or while bundled.
• Treating a fever before checking it, which hides the true number.
• Converting or adding degrees for armpit readings. Just report the method and number.
• Waking a sleeping, comfortable older child to check.

When to check in

• Tell the nurse line which method you used, the number, the time, and any medicine given.
• Any fever under 3 months: call the pediatrician right away. A child of any age who is hard to wake, struggling to breathe or blue needs 911.