How to Take Your Baby’s Temperature Safely

Your baby feels warm. Is it just the blanket, or is it a fever? Every parent faces this moment, usually at 2 a.m., usually while second-guessing everything. Taking a baby’s temperature sounds straightforward — until you’re holding a squirming infant, staring at three different thermometers, and wondering which number to trust.

Getting an accurate reading matters because fever guidance for babies is age-specific, and for very young infants it’s genuinely urgent. This guide covers what counts as a fever at different ages, which thermometers work best and when, step-by-step instructions for each method, and exactly when to call your doctor. No guesswork, no panic — just a clear plan for the next time your baby feels warm.

What Counts as a Fever in a Baby?

The number to memorize: 100.4°F (38°C) or higher is considered a fever in babies and young children, regardless of how the temperature was taken. (Rectal readings run slightly higher than armpit readings, but the 100.4°F threshold is the standard reference point pediatricians use.)

But the number alone isn’t the whole story — age changes everything:

• Under 3 months: A rectal temperature of 100.4°F (38°C) or higher is a reason to call your pediatrician immediately or go to urgent care/emergency — even if your baby seems fine otherwise. Young infants are evaluated urgently for fever because their immune systems are immature and serious infections can present subtly. Do not wait to see if it comes down.
• 3 to 6 months: Call your pediatrician promptly for a fever of 100.4°F or higher, especially if your baby seems uncomfortable, is feeding poorly, or the fever lasts more than a day or two.
• Over 6 months: Fever itself is a sign the immune system is working. Call if the fever is very high, lasts more than a couple of days, or comes with concerning symptoms (see the “when to call” section below).

Also worth knowing: a baby’s temperature naturally runs a bit higher than an adult’s, and it fluctuates through the day — typically lower in the morning, higher in the late afternoon and evening. Being bundled up or sitting in a warm room can nudge the number up without any illness at all. If the reading surprises you, remove a layer, wait 15–20 minutes, and recheck.

Thermometer Types: What Works Best at Each Age

Not all thermometers are equal for babies. Here’s the honest breakdown:

• Digital rectal thermometer: The gold standard for infants under 3 months — the most accurate reflection of core body temperature. This is what pediatricians recommend for young babies, and what they’ll ask about when you call.
• Digital armpit (axillary) thermometer: Less accurate than rectal but useful for quick screening in babies of any age. If an armpit reading shows a fever, confirm with a rectal reading in a young infant.
• Temporal artery (forehead) thermometer: Convenient and reasonably accurate in babies over 3 months when used correctly — sweep across the forehead as directed. Less reliable in newborns.
• Tympanic (ear) thermometer: Can work from around 6 months, but tiny ear canals make readings tricky and less reliable in young infants.
• Mercury thermometers: Don’t use these — they’re breakable and the mercury is hazardous. Digital is safer and just as accurate.

Practical advice: buy a dedicated digital thermometer for rectal use and label it clearly (a piece of tape marked “rectal” prevents mix-ups). Keep it with your baby supplies so you’re not hunting for it at midnight. And check the battery occasionally — a dying thermometer gives readings you can’t trust.

How to Take a Rectal Temperature: Step by Step

This is the method your pediatrician wants for babies under 3 months. It’s quick — usually under a minute — and with the right technique it’s not distressing for your baby.

1. Wash your hands and clean the thermometer with soap and water or rubbing alcohol. Rinse and dry it.
2. Apply a small amount of lubricant (petroleum jelly) to the tip of the thermometer.
3. Position your baby: Lay them tummy-down across your lap or on a firm surface, or on their back with legs lifted toward the chest (like a diaper change). Have someone help hold them still if possible.
4. Insert gently: Turn the thermometer on, then insert the tip about half an inch to one inch into the rectum — no further. Hold the thermometer in place; never leave it unattended or let go while it’s inserted.
5. Wait for the beep, then remove and read the temperature.
6. Clean the thermometer thoroughly afterward and wash your hands again.

If your baby is very wiggly or you’re nervous, it’s okay to ask your pediatrician’s office to walk you through it at your next newborn checkup — nurses demonstrate this routinely and are happy to coach you.

How to Take an Armpit Temperature

Armpit readings are the easiest to take and a fine first check for babies over 3 months — or for a quick screen at any age before confirming with a more accurate method.

1. Turn on the digital thermometer and place the tip in the center of the armpit, against bare skin (not over clothing).
2. Hold your baby’s arm snugly against their side to keep the thermometer in place.
3. Wait for the beep, then read.

Armpit readings tend to run lower than core temperature — sometimes by a degree or more — so they’re better at ruling fever in than ruling it out. If the armpit reading is normal but your baby feels hot and looks ill, recheck with a rectal reading (for young infants) or a forehead thermometer.

How to Take a Forehead (Temporal Artery) Temperature

For babies over about 3 months, a temporal artery thermometer is a good balance of accuracy and convenience — no undressing, no waiting, and it works on a sleeping baby.

1. Place the sensor flat on the center of the forehead.
2. Press the button and sweep slowly across the forehead toward the hairline/temple, following your model’s instructions.
3. Read the displayed temperature.

Accuracy depends on technique: sweep too fast and you’ll get a low reading; measure right after your baby’s been lying on that side of their head and the skin may be artificially warm. When the reading will drive a medical decision — especially in a baby under 6 months — confirm an elevated forehead reading with a rectal temperature.

When to Call the Doctor

Call immediately (don’t wait for morning) if:

• Your baby is under 3 months old with a rectal temperature of 100.4°F (38°C) or higher — this is always urgent
• Your baby has a fever and is unusually sleepy, limp, or difficult to wake
• There’s difficulty breathing, persistent vomiting, or signs of dehydration (few wet diapers, no tears when crying, sunken soft spot)
• A rash appears with the fever
• Your baby has a seizure
• You’ve just got a bad feeling — parental instinct is a valid reason to call

Call promptly (same day) if:

• A baby 3–6 months old has a fever of 100.4°F or higher
• Fever lasts more than 24–48 hours in a baby under 2
• Your baby is feeding poorly, has fewer wet diapers than usual, or seems miserable despite comfort measures
• Fever follows immunizations and you’re unsure what’s expected (your pediatrician will tell you the threshold for concern)

Changes in responsiveness matter too — a baby who isn’t startling, grasping, or rooting as usual may be telling you they’re unwell. Our guide to newborn reflexes explains what’s typical at each stage.

Save your pediatrician’s after-hours number in your phone now — before the 2 a.m. moment arrives. It’s one of the most useful things you can do as a new parent, and our newborn checkup guide lists the other questions worth asking at your next visit.

Caring for a Baby With a Fever

While you arrange medical guidance, comfort measures help:

• Keep fluids coming. Offer feeds frequently — breast milk or formula. Dehydration is the bigger risk with fever, so intake matters more than the number on the thermometer.
• Dress lightly. Don’t bundle a feverish baby — one light layer in a comfortable room is enough. Overdressing traps heat.
• A lukewarm sponge bath can provide comfort. Never use cold water or rubbing alcohol — both can be harmful.
• Rest. Keep things calm and quiet; your baby needs energy for recovery, not stimulation.
• Medication: Don’t give fever reducers to a baby under 3 months without explicit pediatrician direction. For older babies, ask your pediatrician about whether medication is appropriate and the correct dose for your baby’s weight — never guess at dosing, and never use aspirin in children.

Remember that fever itself is a symptom, not the disease — it’s the immune system doing its job. Your role is monitoring, comfort, and knowing when to get professional help, not wrestling the number down at all costs.

The two numbers that matter most: 100.4°F (38°C) is a fever — and in a baby under 3 months, it’s an immediate call to your pediatrician, no matter how well your baby seems.

This article is for general information and isn’t a substitute for professional medical advice. When in doubt about your baby’s temperature or symptoms, contact your pediatrician.

Frequently Asked Questions

What is considered a fever in a newborn?

A rectal temperature of 100.4°F (38°C) or higher is considered a fever. In babies under 3 months, any fever at or above this threshold requires immediate contact with your pediatrician or a visit to urgent care — even if the baby otherwise seems well.

What’s the most accurate way to take a baby’s temperature?

Rectal measurement with a digital thermometer is the most accurate method, especially for babies under 3 months — it’s what pediatricians rely on. Armpit readings are convenient for screening but run lower than core temperature; forehead thermometers work reasonably well from about 3 months with correct technique.

Are forehead thermometers accurate for babies?

Reasonably, from around 3 months onward, when used correctly (slow sweep across the forehead). They’re less reliable in newborns. For any reading that will drive a medical decision in a young infant, confirm with a rectal temperature.

When should I call the doctor for my baby’s fever?

Immediately for any fever (100.4°F+) in a baby under 3 months. Promptly for fever in a 3–6-month-old, fever lasting more than a day or two, or fever with concerning symptoms at any age: lethargy, poor feeding, breathing difficulty, rash, dehydration signs, or seizures.

My baby feels warm but the thermometer reads normal. What should I do?

First, remove a layer of clothing, wait 15–20 minutes, and recheck — overdressing and warm rooms commonly cause warm skin without true fever. If the reading stays normal but your baby seems ill (poor feeding, unusual sleepiness, inconsolable crying), trust your instincts and call your pediatrician; the thermometer is a tool, not the final word.

Temperature-taking is one of several hands-on skills that make the newborn weeks less intimidating. Two more worth mastering: burping effectively after feeds (a comfortable baby sleeps better and fusses less) and learning to soothe a fussy baby — because a calm parent with a clear plan handles midnight fevers far better than a panicked one. And if your baby is in the teething window rather than the newborn window, our guide to teething symptoms and remedies sorts out the persistent myth that teething causes true fevers.