Taking a Baby’s Temperature: Choosing and Using a Thermometer

A thermometer reading is most useful when you know how it was taken. Age, device type and placement affect which method is appropriate. Keep the instructions with the thermometer so anyone caring for your baby can use the same approach.

If your baby is struggling to breathe, turning blue or grey, or difficult to wake, call your local emergency service. Do not delay to locate a thermometer or confirm a fever.

Choose a method recommended for your baby’s age

UK guidance recommends a digital thermometer in the armpit for babies and children under five. US paediatric guidance considers a rectal reading most accurate for young infants, particularly under three months. These are different local recommendations, so ask your baby’s clinician which method they want you to use. If advised to use a rectal thermometer, ask them to explain safe use before trying it.

Do not assume every device sold for a family is suitable for a newborn. US guidance advises against ear thermometers before six months because the narrow ear canal makes accurate positioning difficult. Oral measurement is generally for older children who can cooperate, not babies.

A digital forehead thermometer and a colour changing forehead strip are different products. Strips measure skin temperature and are unreliable for deciding whether a baby has a fever. For a digital forehead device, check its age suitability and follow its positioning and environment instructions. Avoid glass mercury thermometers.

Taking an armpit reading

With a digital thermometer intended for this use, hold your baby securely and place the tip in the armpit as the manufacturer directs. Keep the arm gently against the body until the device signals that it has finished. Stay with your baby throughout.

A recent bath, a warm room or heavy wrapping can affect a reading. If your baby otherwise seems well, allow a few minutes after the bath or after removing excessive wrapping before measuring. Do not make them cold or delay assessment of an unwell baby to obtain an ideal reading.

Write down the displayed temperature, whether it is in Celsius or Fahrenheit, the method and the time. If two readings differ, report that rather than averaging them or adding a fixed number to an armpit reading. Follow the cleaning instructions and keep a thermometer used rectally separate from any used orally.

What the number means for getting help

A temperature of 38°C, or 100.4°F, in a baby in the first three months needs immediate medical assessment. A lower armpit reading does not rule out fever or serious illness if your baby feels hot or seems unwell. Get advice rather than repeatedly changing devices to find a reassuring result.

For babies aged three to six months, UK guidance identifies 39°C or higher as needing immediate medical assessment, while some care plans use lower temperatures. Contact a clinician promptly about fever at this age. Our baby and toddler fever guide explains why feeding, responsiveness and breathing matter alongside age.

Make the reading part of a useful handover

Tell the clinician your baby’s exact age, what prompted you to measure, and any changes in feeding or wet nappies. If you have given a medicine, provide its name, strength, amount and time. Keep the fever medicine record with the temperature notes so another caregiver can see what has already happened.

The aim is clear information for care, not a perfect chart. Seek advice whenever your baby’s condition concerns you, even if the thermometer has not crossed a particular threshold.