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The back of your hand against a tiny forehead. A sudden warmth that makes your own heart race. Is it just a warm room, or is it a fever? In that moment of uncertainty, knowing how to accurately take your baby’s temperature is your first line of defense, your concrete piece of data in a sea of worry. Yet, with a squirmy, unhappy infant, the thought of a rectal reading can feel daunting, and those quick forehead strips seem unreliable.
The underarm method—simple, safe, and non-invasive—often feels like the most natural choice for parents. But a quiet doubt nags: Is this reading even correct? If the thermometer isn’t positioned just right, or you don’t hold it long enough, you could be missing a crucial sign of illness or stressing over nothing. You need confidence, not guesswork.
This guide is here to replace that doubt with calm, competent skill. We’ll walk through, step-by-step, exactly how to get the most trustworthy underarm temperature reading possible. You’ll learn the best type of thermometer to use, the precise placement technique, how long to wait, and most importantly, how to interpret the number you see and know what to do next. Let’s master this essential parenting skill together.
Why Choose the Underarm Method? Weighing the Pros and Cons
Before we dive into the technique, it’s crucial to understand what the underarm (axillary) method is good for, and where its limitations lie. This isn’t about finding a “perfect” method, but about using the right tool correctly for the situation.
The Advantages: Safety and Comfort
- Non-Invasive & Safe: There is zero risk of injury or discomfort, making it an excellent choice for hesitant parents or fussy babies.
- Universally Acceptable: It’s a socially and medically accepted screening method in all settings.
- Good for Frequent Checks: Ideal for monitoring a temperature trend over time (e.g., before and after medication) without distressing your baby.
- Ease of Use: With practice, it becomes a quick and simple process.
The Limitations: It’s a Screening Tool
- Less Precise: An axillary temperature measures skin temperature, which is influenced by the environment. It is generally 0.5°F to 1°F (0.3°C to 0.6°C) LOWER than a core body temperature taken rectally.
- Easily Affected: The reading can be skewed if your baby is bundled up, just out of a bath, hasn’t had the thermometer positioned correctly in the arm-pit crease, or is moving around.
- Not for Definitive Diagnosis: Pediatricians typically consider underarm readings as a good initial check, but may recommend a rectal temperature for a definitive fever diagnosis in infants under 3 months.
The Bottom Line: The underarm method is a fantastic, safe screening tool. Its true value shines when you combine correct technique with an understanding of its margin of error. For babies under 3 months old, any fever (100.4°F / 38°C rectally or equivalent) is a medical emergency, so when in doubt, confirm with a rectal reading or call your doctor.
Gear Up: Choosing the Right Thermometer for the Job
You can’t build a house without a good hammer, and you can’t take an accurate temperature without the right thermometer. Here’s what you need:
- Digital Multi-Use Thermometer: This is your workhorse. A basic, inexpensive digital thermometer that can be used orally, rectally, or underarm is perfect. Look for one with a flexible tip, a large, easy-to-read display, and a beep to signal completion.
- Avoid: Old glass mercury thermometers (safety hazard), forehead strips (notoriously inaccurate), and pacifier thermometers (unreliable). While temporal artery (forehead) scanners are popular and good for older children, they can be tricky to position correctly on a small infant’s head and are often not recommended as the primary method for very young babies.
Pro Tip: Label your thermometer! If you use one digital thermometer for both underarm and rectal readings, use permanent marker or a sticker to designate one as “ORAL/ARMPIT” and a separate one as “RECTAL ONLY” to maintain hygiene and peace of mind.
The Step-by-Step Playbook: How to Take an Accurate Underarm Reading
Follow these steps precisely to minimize errors and get the best possible reading.
Step 1: Prepare Your Baby and Environment
- Timing is Everything: Wait at least 20-30 minutes after your baby has had a bath, been heavily bundled, or been physically active. These can all temporarily raise skin temperature.
- Create a Calm Setting: Choose a quiet spot. If your baby is crying or fighting, try to soothe them first. A calm baby is easier to handle and provides a more stable reading.
- Undress the Arm: Remove your baby’s shirt or onesie from one arm completely. You need unobstructed access to the armpit.
Step 2: Prepare the Thermometer
- Clean the Tip: Wipe the silver metal tip with a cotton ball dipped in rubbing alcohol or wash it with cool, soapy water. Rinse and dry.
- Turn it On: Press the button to power on the thermometer. Most will display a “Lo” or similar symbol, indicating they’re ready.
Step 3: The Critical Placement and Hold
This is where accuracy is won or lost.
- Find the “Pocket”: Gently pat your baby’s armpit dry if it’s at all sweaty. Lift their arm and locate the natural hollow at the very crest of the armpit—this is where the thermometer tip must sit.
- Position the Tip: Place the silver tip firmly into the center of that armpit hollow, pointing slightly upwards towards the center of the body (towards the opposite shoulder).
- Lower the Arm: Gently but securely lower your baby’s arm down over the thermometer, trapping it in place. The elbow should be tucked snugly against their side.
- Secure and Comfort: Hold your baby close to your chest, with their arm firmly against their body. You can rock, sing, or read a book to keep them still. The thermometer tip must maintain constant, firm contact with the skin in the armpit hollow for the entire reading.
Step 4: Timing and Reading the Result
- Wait for the Beep: Hold the thermometer in place until it beeps or signals completion. This usually takes 10-15 seconds for newer models, but can take longer. Do not remove it early.
- Note the Number: Once it beeps, gently remove the thermometer and read the digital display. Note the number and the time you took it.
- Add the “Buffer” (Crucial Step): Remember, an underarm reading is an estimate of core temperature. Add 1°F (0.6°C) to the displayed number to get a closer estimate of what a rectal temperature would be. This adjusted number is what you should consider when deciding if it’s a fever.
- Example: Thermometer reads 99.0°F underarm. Adjusted estimate = 100.0°F.
Interpreting the Numbers: What Does It All Mean?
Now you have a number. Here’s how to make sense of it, based on the adjusted underarm temperature (actual reading +1°F).
- Normal Range: An adjusted temperature between 97.6°F and 100.3°F (36.5°C to 37.9°C) is generally considered normal for a baby.
- Fever: An adjusted temperature of 100.4°F (38.0°C) or higher is considered a fever in an infant.
- Low Temperature: A persistently low temperature (below 97.0°F adjusted/36.1°C) in a newborn can also be a concern and warrants a call to your pediatrician.
The Age Factor: When to Call the Doctor Immediately
This is the most important rule in infant fever management:
- For a baby under 3 months old: Any fever (adjusted temp 100.4°F / 38°C or higher) is a medical emergency. Call your pediatrician or go to the ER immediately, day or night. Do not wait. Their immune systems are too immature to fight infection without help.
- For babies 3-6 months old: A fever over 101°F (adjusted) warrants a call to the doctor within the day.
- For babies 6+ months old: A fever over 102°F (adjusted) or a lower fever that lasts more than 2-3 days needs a doctor’s evaluation.
Always trust your instincts. If your baby is acting very ill—lethargic, difficult to wake, not wetting diapers, has a stiff neck, or has a rash that doesn’t blanch—seek medical attention regardless of the thermometer reading.
Troubleshooting: Solving Common Underarm Temperature Problems
Problem: “My baby won’t stop wiggling! I can’t hold the thermometer in place.”
Solution: Try taking the temperature while your baby is asleep. Gently slide the thermometer into their armpit and hold their arm close as they sleep. This often yields the most accurate reading of all.
Problem: “I get a different reading every time I take it.”
Solution: Ensure consistency. Use the same thermometer, the same preparation (dry armpit, calm baby), and hold it for the full duration. Re-take it after 10-15 minutes if you get a surprising result. Temperature can fluctuate.
Problem: “The reading seems way too low or way too high.”
Solution: Re-check placement. Was the tip truly in the deep crease of the armpit? Was their arm held tightly down? Was the room very cold or were they just swaddled? If in doubt and concerned, confirm with a rectal temperature using a properly designated thermometer.
Problem: “My thermometer won’t beep.”
Solution: Check the battery. Ensure the tip is making full contact with skin, not fabric. Some models have a longer read time; consult the manual.
A Final Word of Calm Confidence
Taking your baby’s temperature is more than a mechanical task; it’s an act of vigilant love. By mastering the underarm method, you’ve equipped yourself with a safe, reliable way to gather vital information. Remember the formula: Good Technique + The 1-Degree Buffer = Your Best Estimate.
Your observation skills are your most powerful tool. The number on the thermometer is just one piece of the puzzle. How your baby is acting—their energy, their wet diapers, their cry—tells the rest of the story. You are the expert on your child. Use this skill with confidence, and never hesitate to partner with your pediatrician when your parent gut tells you something is wrong. You are doing a great job.
For more on decoding your baby’s health signals, see our guide on how to know if baby is getting enough breast milk.
Your Top 5 Baby Temperature Questions, Answered!
1. Is an underarm temperature as good as a rectal one?
For screening purposes at home, yes—when done correctly. However, a rectal temperature is considered the gold standard for accuracy in infants because it measures core body temperature directly. For any serious concern, especially in a baby under 3 months, a rectal reading is the definitive method. Think of underarm as a helpful first check; rectal is the confirmatory test.
2. How often should I re-take my baby’s temperature when they’re sick?
A good rule of thumb is every 4-6 hours, or about 30-60 minutes after giving fever-reducing medication (like acetaminophen or ibuprofen for older babies) to see if it’s working. Avoid taking it constantly, as it can increase anxiety. Focus more on your baby’s overall behavior and comfort.
3. What’s the difference between a fever caused by teething and an illness fever?
This is a critical myth to bust. Research shows teething may cause a very slight elevation in temperature (around 99-100°F), but it does not cause a true fever (100.4°F+). If your baby has a high fever, assume it is due to an illness (viral or bacterial) and not teething. Blaming a fever on teething can delay treatment for a real infection.
4. My baby feels hot but the thermometer says normal. What gives?
Your touch is sensing skin temperature, which can be raised by crying, being overdressed, or a warm room. The thermometer measures the temperature in a specific spot. Always trust the properly taken thermometer reading over your touch. If your baby seems ill but is afebrile, they still may be unwell—watch for other symptoms and call your doctor if concerned.
5. When is a low temperature in a baby a problem?
A cold environment is the most common cause. But in a newborn, a persistently low body temperature (hypothermia, below 97.0°F rectally) can be serious, as they struggle to regulate their own body heat. It can also be a sign of infection. If your newborn feels cool to the touch and is lethargic, warm them with skin-to-skin contact and call your pediatrician immediately. For older babies, ensuring they are warm and fed is key, but a call to the doctor can provide peace of mind. Understanding their baseline is part of overall care, much like tracking their intake. If feeding is also a concern, our post on newborn cluster feeding schedule can help you understand normal patterns.
