To take your baby’s temperature under the arm, place a digital thermometer’s tip against dry, bare skin in the armpit, gently hold the arm against the chest, and wait for your device’s completion signal. Record the number exactly as shown, including °F or °C. Don’t add a degree. An underarm reading can miss a fever. If a young infant may have a fever, or your baby seems seriously unwell, get medical advice promptly instead of repeatedly checking for a more reassuring number.
I would keep two things together: the thermometer reading and what you are noticing about your baby. A small screen cannot tell you how well they are breathing, feeding or responding to you. Here is how to make the reading useful without asking it to answer every health question.
Before another reading: does your baby need help now?
Call your local emergency number now if your baby is difficult to wake, has serious trouble breathing, has blue or grey lips, or is having a seizure. A rash that does not fade with pressure also needs emergency help. Don’t delay to finish measuring. These warning signs come from MedlinePlus and NHS fever guidance.
For a baby 3 months old or younger: the American Academy of Pediatrics (AAP) says a rectal temperature of 100.4°F (38°C) or higher needs an immediate call to the pediatrician, even if the baby otherwise seems well. If you suspect fever but only have an underarm reading, call for guidance; don’t wait for that reading to reach the rectal cutoff.
Say your baby’s exact age at the start of the call. A baby who is a few weeks old needs a different conversation from an older child. Follow any earlier instructions your care team gave you for prematurity, immune problems or another medical condition.
Take an underarm temperature, step by step
“Axillary” simply means under the arm. Use a digital stick thermometer whose instructions allow that method. An ear or forehead thermometer does not become an underarm thermometer by moving it to a different place.
- Read the device instructions and clean it as directed. Locate the power button, unit setting and signal that means the measurement is finished. Keep the instructions nearby.
- Hold your baby securely. Sit somewhere stable where you can support them and reach the thermometer. Move the sleeve or clothing away from the armpit. The sensor needs skin contact.
- Check that the armpit is dry. Place the sensor tip high inside the armpit, against bare skin. Don’t leave fabric between the tip and the skin.
- Gently close the arm. Support the elbow so the upper arm rests against the body and the tip stays enclosed. Stay with your baby and hold the device; don’t leave it tucked there unattended.
- Wait for the correct signal. Follow your thermometer’s instructions, rather than a universal “10-second” or “one-minute” rule. Then remove it and read the display.
- Write down what it actually says. Include the unit, “underarm,” and the time. Clean and put away the device according to its instructions.
The placement details are supported by Nemours KidsHealth and Seattle Children’s. If the tip slipped out or your baby’s arm opened before the signal, describe that as an uncertain attempt. Don’t present it as a reliable measurement.
With two caregivers, one can support the baby while the other manages the device and note. Alone, put the notebook or phone within reach before you begin. There is no need to turn a brief check into a complicated procedure.
What does the underarm number mean?
There is no single “safe number” that overrides your baby’s age, symptoms and the way you measured. The AAP describes underarm measurement as the least accurate common method, useful for screening. Rectal measurement is its most accurate method for young infants.
Why you may see 99°F and 100.4°F on different pages
Seattle Children’s and Nemours use 99°F (37.2°C) or higher for an underarm fever reading. That is a method-specific screening threshold. It is not interchangeable with the AAP’s 100.4°F (38°C) rectal threshold for urgent assessment in a young infant. If a young baby’s underarm reading is around or above 99°F, contact their clinician about the next step, including whether and how to confirm it. An ill-looking baby needs help regardless of the number.
Tell the person advising you which method you used before discussing thresholds. “My baby is six weeks old; the underarm reading is…” carries more useful information than “It’s only a low fever.” I would leave labels such as “low-grade” out of that first conversation.
Why UK advice may sound different
The NHS recommends digital underarm measurement for children under 5. Its fever advice calls for urgent help for a baby under 3 months with 38°C or more, or if you think they have a high temperature; for ages 3–6 months it lists 39°C or suspected high temperature. US guidance may use different methods and earlier call thresholds. Follow your own clinician’s local instructions, and seek advice when worried rather than waiting to reach a number printed in another country’s guide.
AFTER THE HEALTH QUESTION IS ADDRESSED
The thermometer answers one question. Bedtime can wait.
The SleepBaby.org homepage describes the SleepBaby.org Method and its information about bedtime routines and baby sleep schedules.
When your baby may be ill, getting the right medical advice comes first. A sleep schedule cannot interpret a temperature, treat a fever or tell you that it is safe to stop checking on your baby. Keep following the care plan your clinician gives you.
At another time, when ordinary bedtime planning is the question again, you may want to think through the familiar steps around feeds, pajamas and settling. You can visit the homepage to read about the Method and decide whether its discussion of routines and schedules fits the information you are looking for.
There is no promise here that a routine will prevent illness, speed recovery or produce an uninterrupted night. This is an invitation to explore everyday sleep planning after immediate health needs have been addressed, whenever that is appropriate for your family.
Do you add a degree to an underarm temperature?
No. Report the displayed reading and the measurement site. MedlinePlus explains that the exact relationship between measurement methods is unclear. An approximate population difference does not turn your baby’s underarm reading into a measured rectal temperature.
For example, “99.1°F under the left arm at 8:10 p.m.” is a record of something you measured. “100.1°F after adding one” is a calculated number with a different meaning. Keep the first version. If your clinician asks for another method, record that separately too.
The same applies to averaging. If you obtained two different numbers, retain both with their times and any placement problem you noticed. An average hides the difference you are trying to explain. You don’t need to resolve that uncertainty yourself before asking for advice.
When the reading doesn’t seem right
Your baby feels hot but the underarm reading is low
Touch alone cannot measure a fever, but a lower underarm reading cannot settle every concern either. First consider how your baby is behaving. If they are hard to wake or struggling to breathe, act on that. If a young infant seems feverish, call. If your baby is otherwise responsive and you are checking technique, look for a loose arm, damp skin, clothing under the sensor or a measurement stopped too soon.
The number changes between attempts
Check the unit before comparing: Celsius and Fahrenheit displays look very different. Then check whether you used the same device and method. Note the difference instead of choosing whichever number feels preferable. A photo of each finished display, alongside the time and method, can help you avoid a transcription mistake when you are tired.
Your baby just had a bath or was bundled up
The NHS notes that a bath, heavy clothing or a warm environment can affect a reading. For an otherwise well baby, allow a few minutes in comfortable conditions and follow the device instructions before checking again; don’t make them cold or shivery. This is not a reason to postpone care for a young infant with possible fever or a baby with warning signs.
The display says “Lo,” “Err” or something unfamiliar
Look up that exact message in your model’s manual. An error or startup display is not automatically your baby’s body temperature. Check the battery indicator and whether the measurement cycle completed. If you cannot get a dependable reading and your baby may be ill, tell the clinician that; shopping for a different thermometer can wait.
A genuinely low temperature also deserves attention, especially in an infant who seems unwell. MedlinePlus notes that some serious infections can cause a low temperature rather than fever. Don’t dismiss an unexpectedly low completed reading as “definitely just the thermometer”; describe the reading and your baby’s condition when seeking advice.
A short note to keep beside the phone
You don’t need a polished medical history. Start with the age and the immediate concern, then use these prompts:
- Age: ___ weeks or months; relevant care plan or medical condition: ___.
- Reading: ___ °F / °C; method: underarm; time: ___.
- Other readings: ___, using ___, at ___; any slipping or error message: ___.
- What changed: feeding, wet diapers, breathing, alertness, rash or other symptoms: ___.
- Medicine already given: exact product, strength, amount and time: ___.
Then ask: “Does my baby need to be seen now? Do you want another measurement, and which method should I use? What changes mean I should call back or get emergency help?” Write down the answer. If another caregiver takes over, pass on the advice as well as the thermometer.
I would keep this note short enough to use while holding a baby. A simple phone note or scrap of paper is fine. Recording information should support a call, never become a task you have to finish before making it.
Should you switch to a different thermometer?
Ask your clinician what they want you to use for your baby’s age and situation. Under AAP guidance, rectal readings are especially useful below 3 months; ear thermometers are not reliable for babies younger than 6 months, and oral measurement is for children old enough to cooperate, generally age 4 or older. Forehead devices have their own positioning instructions.
If you have never taken a rectal temperature, ask for instructions rather than improvising from an underarm guide. Some children have medical reasons to avoid that method. Seattle Children’s lists important exceptions, including certain cancers and weakened immune systems. Do not delay an urgent call while trying to learn a new technique.
Keep any thermometer used rectally clearly labeled and separate from one used in the mouth. A more expensive device does not remove the need for correct placement, suitable age guidance and a clear record of the method.
Optional: a digital thermometer with instructions you can find
If you need a replacement, the Vicks SpeedRead V912 is one named option to compare. Its manufacturer lists underarm use. Check the exact model’s current instructions, intended use, cleaning directions and battery requirements. I haven’t tested it, and this is not an accuracy ranking or a claim that its display can decide when a baby needs care.
Already-owned alternative: use a working digital thermometer you already have if its instructions permit underarm measurement. Locate the manual and learn its signals before you need it. Keep the device and any batteries out of children’s reach. If your baby needs urgent advice, making that call comes before buying equipment.
This section has no affiliate purchase link. As an Amazon Associate, SleepBaby.org earns from qualifying purchases through affiliate links elsewhere on the site.
Watch: the AAP on when a fever needs a call
In this American Academy of Pediatrics video, pediatrician Nathan Chomilo discusses fever and contacting your child’s doctor. It provides broader fever context, rather than an underarm-placement demonstration.
If the player doesn’t work: open the official AAP video or read the AAP’s written when-to-call guidance. The written guidance emphasizes your child’s age and condition, including worsening illness, unusual drowsiness and other concerning symptoms. Young infants need prompt assessment for fever. Use your clinician’s advice for your baby’s measurement method and any medicine; don’t apply an older child’s dosing guidance to an infant.
What about feeds, fever medicine and sleep?
Once a clinician has advised that home care is appropriate, follow that plan. Continue offering your baby’s usual milk feeds and keep track of wet diapers and responsiveness. Seek advice if feeding falls off, diapers are less wet than usual or your baby is not acting like themselves. Don’t use an improved thermometer number as the only sign that everything is settled.
If the illness is worsening or fever continues, get back in touch. The AAP advises a call when fever lasts more than 24 hours in a child under 2. That does not mean waiting a day for a young infant, a baby with warning signs or one for whom you were given an earlier call threshold.
Medication depends on age, weight, product and circumstances. For a young infant with possible fever, call before giving fever medicine. Ask for the exact product and dose rather than estimating or alternating medicines on your own. MedlinePlus advises contacting the clinician before medicine for children 3 months or younger; it also says not to give ibuprofen at 6 months or younger. Local guidance differs, so use your baby’s own care instructions.
At night, ask the clinician how to check on your baby and whether they want repeat temperatures. There is no universal alarm schedule to copy from this article. A baby who is difficult to wake needs urgent help; don’t describe that as simply “sleeping through.” Illness is a reason to respond to care needs, not to push a bedtime schedule.
Small questions that come up during a temperature check
Which armpit should I use?
Use an accessible armpit where you can maintain the placement your device requires. If you take another reading, note any change of side or method. Comparing two sides is not a substitute for calling about a young infant or a baby who seems unwell.
Can I check through a sleeve?
No. The sensor needs direct skin contact. Move the clothing clear rather than trying to trap the thermometer between fabric and the upper arm.
Does a beep always mean the same thing?
No universal timing rule applies to every model. Use the manufacturer’s explanation of its signals. Learn the difference between switching on, a stored previous result and a completed new reading.
Should I keep measuring until the number goes down?
No. Follow the checking plan you were given and pay attention to changes in your baby. Keep the readings you already have. If you are worried enough to keep repeating the check, explain that concern to your care team instead of searching for a result that ends the worry.
The useful finish is simple: an actual reading, its method and time, a clear picture of your baby’s condition, and an appropriate next step. You do not have to make the diagnosis yourself.
Sources
- NHS: Taking your baby’s temperature
- NHS: Fever in children
- AAP: How to take your child’s temperature
- AAP: Fever and your baby
- Seattle Children’s: Temperature methods
- MedlinePlus: Temperature measurement
- MedlinePlus: Fever
- Vicks: SpeedRead V912 product support
- Nemours KidsHealth: Underarm temperature
- AAP: When to call the pediatrician
KEEP THE READING. KNOW THE NEXT STEP.
Care first. Ordinary routines when you are ready.
Record the number, unit, method and time, then follow the medical advice appropriate for your baby. For a separate look at ordinary bedtime routines and baby sleep schedules, the SleepBaby.org homepage introduces the Method.
