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

> Which thermometer to use at each age, how to take a rectal temperature step by step, and what counts as a fever.

Stage: newborn - Topic: Health - Page: https://www.askcladle.com/guides/newborn/how-to-take-a-baby-temperature

Grounded in: AAP HealthyChildren ("How to Take a Child's Temperature", "Fever and Your Baby") and FDA guidance on thermometers. Summarized in Cradle's own words; none of these sources endorse Cradle. General information, not medical advice.

Reviewed: 2026-11

## TLDR

- Under 3 months, rectal is the reading that counts. Ear thermometers work from about 6 months.
- A fever is 100.4°F (38°C) or higher by any method. Don't add degrees to armpit readings; just report the method.
- Under 3 months, a fever (or below about 97.5°F) means call the pediatrician right away, before any medicine.

## 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.