GUIDE
Rectal is the gold standard for babies under 3 months. Forehead works well after that.
Taking your baby's temperature sounds straightforward until you are standing in the baby aisle staring at five different thermometer types, or worse, standing over a feverish infant at midnight wondering if you are doing it right. This guide walks you through every method, step by step.
The short answer: if your baby is under 3 months old, use a digital rectal thermometer. Full stop. The American Academy of Pediatrics is clear on this, and there is a good reason — when your baby is that young, the difference between 100.2°F and 100.5°F can be the difference between "monitor at home" and "go to the ER right now." No other method is accurate enough to make that call.
After 3 months, you have more options. Temporal artery (forehead) thermometers are the most popular alternative, and for good reason — they are fast, non-invasive, and reasonably accurate. A quick swipe across the forehead takes two to three seconds, and you can do it without waking a sleeping baby. For most fever checks in babies over 3 months, this is perfectly adequate.
Armpit (axillary) thermometers are the least accurate of all methods. They consistently read 1 to 2 degrees Fahrenheit below rectal, and the exact offset varies. They are fine as a screening tool — "my baby feels warm, let me check if there is actually a fever" — but if the armpit reading is anywhere near the fever threshold, you should confirm with a more accurate method.
Ear (tympanic) thermometers are not recommended for babies under 6 months because the ear canal is too small for reliable positioning. Even in older babies, they can be finicky — earwax, ear infections, and incorrect angle all affect the reading. They become more practical for toddlers and older children.
| Age | Recommended | Acceptable Alternative | Not Recommended | Notes |
|---|---|---|---|---|
| 0-3 months | Rectal (digital) | None — rectal is the only reliable method | Forehead, armpit, ear | At this age, clinical decisions depend on precise readings. Only rectal is accurate enough. |
| 3-6 months | Rectal (digital) | Temporal artery (forehead) | Ear, armpit (screening only) | Rectal remains the gold standard. Forehead is a reasonable alternative if rectal is not feasible. |
| 6-12 months | Rectal or temporal artery | Ear (tympanic) with caution | Armpit (screening only) | Ear thermometers become more reliable as ear canals grow, but positioning still matters. |
| 12-24 months | Rectal, temporal artery, or ear | Armpit (screening only) | Oral (too young to cooperate) | Multiple methods now work well. Use whichever your child tolerates best, confirmed rectally if borderline. |
| 24+ months (toddler) | Temporal artery, ear, or oral (if cooperative) | Rectal, armpit | None at this age | Many toddlers resist rectal by this age. Forehead or ear are practical choices. |
| Type | Accuracy | Speed | Ease of Use | Best For | Limitations |
|---|---|---|---|---|---|
| Digital rectal | Most accurate — measures core temperature | 10-30 seconds | Moderate — requires some technique | All ages, essential under 3 months | Baby may resist. Requires lubrication. Must clean after each use. |
| Temporal artery (forehead) | Good — slightly less precise than rectal | 2-3 seconds | Easy — swipe across forehead | Babies 3+ months, sleeping babies, quick checks | Sweat on forehead can give falsely low readings. Technique matters — must follow the hairline correctly. |
| Axillary (armpit) | Least accurate — reads 1-2°F below rectal | 30-60 seconds | Easy — place under arm, hold in place | Quick screening to decide if a more accurate method is needed | Not precise enough for clinical decisions in young infants. Ambient temperature and arm positioning affect results. |
| Tympanic (ear) | Moderate — depends heavily on positioning | 2-3 seconds | Easy when positioned correctly | Babies 6+ months, toddlers, quick reads | Not reliable under 6 months. Earwax, ear infections, and incorrect angle affect accuracy. |
Taking a rectal temperature is easier than most parents expect, and it causes less distress than you might imagine — especially if you are calm and confident. Here is the process.
What you need: a digital rectal thermometer (not glass, not mercury), petroleum jelly or water-based lubricant, rubbing alcohol or soap and water for cleaning, and a diaper nearby for afterward.
Step 1: Clean the thermometer. Wipe the tip with rubbing alcohol or wash with soap and warm water. Dry it.
Step 2: Apply lubricant. Put a small amount of petroleum jelly or water-based lubricant on the silver tip.
Step 3: Position your baby. You have two options. Face-down: lay your baby across your lap, belly-down, with one hand firmly on the lower back. Face-up: lay your baby on a firm surface (changing table, bed) and hold both ankles with one hand, gently lifting the legs like you are changing a diaper. Either position works — use whichever feels more stable.
Step 4: Insert gently. With your other hand, gently insert the thermometer tip into the rectum about one-half to one inch. Do not force it. You will feel mild resistance and then it will slide in easily. Hold the thermometer in place between two fingers — never let go of it, as babies can wiggle and it could shift.
Step 5: Wait for the beep. Most digital thermometers beep within 10 to 30 seconds. Keep your baby still and calm. Talk to them softly.
Step 6: Remove and read. Gently slide the thermometer out, read the number, and note the time. Clean the thermometer with rubbing alcohol or soap and water before storing.
Step 7: Record everything. Write down the temperature, time, and "rectal" so you have it when you need it. This is the information your pediatrician will want.
A common worry: will this hurt my baby? No. The thermometer tip is small and smooth, and with proper lubrication, the insertion is gentle. Your baby may fuss or squirm — that is a normal reaction to an unfamiliar sensation, not pain. Many babies actually stay calm through the whole process.
Temporal artery thermometers are the most parent-friendly option after the newborn period. They work by measuring the infrared heat radiating from blood flowing through the temporal artery, which runs across the forehead just below the skin.
Step 1: Turn on the thermometer and wait for the ready signal.
Step 2: Place the sensor flat against the center of the forehead, just above the eyebrow.
Step 3: Slowly swipe the thermometer across the forehead toward the hairline while holding down the scan button. Follow the manufacturer's instructions — some models require a straight swipe across the forehead, while others need you to also swipe behind the ear.
Step 4: Release the button and read the display.
Tips for accuracy: Make sure the forehead is dry — sweat can cause falsely low readings. Do not take a reading right after your baby has been wearing a hat, lying face-down on a surface, or pressing their forehead against you. Wait a few minutes for the skin to normalize. If the reading seems unexpectedly low, try again.
The biggest advantage of forehead thermometers is speed and non-invasiveness. You can check a sleeping baby without waking them. You can get a reading in under three seconds. For routine fever checks in babies over 3 months, this is an excellent everyday tool. For that borderline reading where you need precision — say, your 10-week-old feels warm — switch to rectal.
When you are taking temperatures every few hours during an illness, details blur together by morning. Log each reading in tinylog so you have a clear fever timeline. 'Rectal, 101.8°F at 10 PM' and 'Forehead, 100.2°F at 2 AM' tells your pediatrician exactly what is happening.
Armpit temperature is the easiest method but the least accurate. It is best used as a quick screening tool — a way to confirm that the warmth you feel when you touch your baby's forehead is real — rather than a definitive measurement.
Step 1: Make sure the armpit is dry. Pat it gently with a cloth if needed.
Step 2: Place the thermometer tip in the center of the armpit, pointing toward your baby's head.
Step 3: Hold your baby's arm snugly against their body, keeping the thermometer in position.
Step 4: Wait for the beep — this usually takes 30 to 60 seconds, longer than rectal or forehead.
Step 5: Read and record, noting "axillary" or "armpit" as the method.
Remember: armpit readings run 1 to 2 degrees Fahrenheit lower than rectal. A reading of 99.0°F under the arm could correspond to 100.4-101.0°F rectally. If the armpit reading is 99°F or higher and your baby is under 3 months, confirm with a rectal temperature. Do not simply add a degree — the offset is not consistent enough for that to be reliable.
Ear thermometers measure the infrared heat from the eardrum. They are fast and well-tolerated, but they have more accuracy limitations than other methods, especially in young babies.
Do not use this method on babies under 6 months. The ear canal is too narrow and curved for the sensor to reach the eardrum properly.
Step 1: Gently pull the ear. For babies 6-12 months, pull the earlobe gently down and back. For children over 1 year, pull the outer ear up and back. This straightens the ear canal.
Step 2: Insert the probe tip snugly into the ear canal, aiming toward the opposite eye (this helps angle toward the eardrum).
Step 3: Press the button and hold until it beeps — usually 2 to 3 seconds.
Step 4: Read and record.
Accuracy issues: earwax can block the infrared sensor and give a low reading. An ear infection can falsely elevate the reading in the infected ear. If your child has been lying on one side, that ear may read higher. Take the reading from both ears and use the higher number, or confirm with another method if the results seem off.
Most temperature-taking errors lead to underestimating fever, not overestimating it. When accuracy matters — and in young infants, it always matters — use rectal.
For babies over 3 months, these behavioral signs are as important as the number on the thermometer.
This deserves its own section because the "add a degree" myth causes real confusion.
The old conventional wisdom was: add a degree to an armpit reading, subtract a degree from a rectal reading, and you will get the "true" temperature. This is no longer recommended by the AAP, and here is why — the offset between measurement methods is not consistent. For one baby, the difference between armpit and rectal might be 1.2 degrees. For another, it might be 2.1 degrees. Applying a fixed adjustment gives a false sense of precision.
The correct approach is simple: report the exact number and the method. "100.2 rectal." "99.4 armpit." "100.6 forehead." Your pediatrician knows how to interpret each method's reading in context. They do not need you to do math on it first.
The one situation where adjustments matter is when you need to decide whether your baby has crossed the 100.4°F threshold. If your armpit reading is 99.0°F on a baby under 3 months, that could be 100.4°F or higher rectally. Do not assume the armpit reading is fine — confirm with rectal. At that age, the stakes are too high for approximation.
The worst time to learn how to take a rectal temperature is at midnight with a screaming, feverish baby. Practice the technique a few times when your baby is calm and healthy. You will build muscle memory and confidence, and your baby will get used to the sensation. Keep the rectal thermometer, lubricant, and rubbing alcohol together in one kit so you are not fumbling in a dark room.
If you have both a rectal thermometer and one for forehead or armpit use, label them clearly. You do not want to mix them up. Some parents use a permanent marker on the case or buy a rectal thermometer in a different color. The rectal thermometer should never be used orally or under the arm.
One of the real advantages of a temporal artery thermometer is that you can use it on a sleeping baby without waking them. A gentle swipe across the forehead takes two seconds. If the reading is normal, everyone goes back to sleep. If it is elevated, you can decide whether to confirm rectally or monitor. This alone makes it worth having one in the house.
If a reading seems unexpectedly high or low, take it again. Wait a minute, make sure the thermometer is positioned correctly, and repeat. If the two readings are significantly different, take a third. Outliers happen — thermometers can misread if the positioning is off, the sensor is dirty, or the baby moved at the wrong moment.
When your pediatrician asks 'What has the fever been doing?' — you want data, not guesses. Track every reading in tinylog with the temperature, method, and time. Over the course of an illness, this creates a clear picture of whether the fever is climbing, cycling, or resolving.
This guide is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. If you are unsure whether your baby has a fever or how to respond, contact your pediatrician. For any fever in a baby under 3 months old, seek immediate medical attention. Call 911 if your baby is having difficulty breathing, is unresponsive, or is having a seizure.
