The most accurate way to take an 8-month-old’s temperature is rectally, using a digital thermometer. A rectal reading of 100.4°F (38°C) or higher means your baby has a fever. Forehead (temporal artery) thermometers are the next best option for accuracy, followed by ear thermometers, which become reliable at 6 months of age. Armpit readings work as a quick screen but are the least accurate method.
Best Methods Ranked by Accuracy
At 8 months old, your baby is eligible for every thermometer type except oral, which isn’t recommended until age 4. Here’s how they stack up:
- Rectal: The gold standard for infants. Gives the most reliable core body temperature reading.
- Forehead (temporal artery): Second most accurate. Works by scanning across the forehead and is much better tolerated than rectal. However, studies show it misses about one-third of fevers that a rectal thermometer would catch.
- Ear (tympanic): Safe to use starting at 6 months. Less accurate than forehead in infants, and misses about half of true fevers compared to rectal readings.
- Armpit (axillary): Least accurate. Useful for a quick check, but if it shows a borderline number, follow up with a rectal or forehead reading.
A forehead thermometer is a good everyday tool since it’s fast and causes no fussing. But if your baby seems genuinely sick and you need a number you can trust, rectal is the way to go.
How to Take a Rectal Temperature
Use a standard digital thermometer with a flexible tip. Label it clearly so it’s only used rectally. Apply a small dab of petroleum jelly or water-based lubricant to the tip before you begin.
You have two positioning options. The easiest for most parents is to lay your baby on their back on a firm surface, then gently lift both thighs toward the belly with one hand. The other option is to place your baby belly-down across your lap, resting one hand on their lower back to keep them still.
Insert the lubricated tip gently into the rectum, no more than half an inch to one inch deep. Hold the thermometer in place between two fingers (like a pencil) with your palm resting against your baby’s bottom so the thermometer can’t accidentally push in further. Wait for the beep, which usually takes 10 to 20 seconds, then slide it out and read the display.
Never force the thermometer if you feel resistance, and never leave your baby unattended with it inserted. Clean the thermometer afterward with rubbing alcohol or soap and warm water, then let it dry completely before storing it.
How to Use a Forehead Thermometer
Temporal artery thermometers scan the blood vessel that runs across the forehead. Turn the device on, place the sensor flat against the center of your baby’s forehead, and press the button while sweeping it across to the hairline. Some models require a straight hold without sweeping, so check your specific instructions.
These thermometers are fast (often under two seconds) and cause almost no discomfort. Research published in JAMA Pediatrics found that infants scored significantly lower on discomfort scales with forehead readings compared to rectal. The tradeoff is precision: forehead thermometers track more closely with rectal temperatures than ear thermometers do, but they can still read lower than the true core temperature. If a forehead reading comes back normal but your baby is acting sick, consider confirming with a rectal check.
How to Use an Ear Thermometer
Ear thermometers are accurate enough for an 8-month-old, since they become reliable at 6 months. Pull the ear gently back and up to straighten the ear canal, then insert the probe tip snugly into the opening. Press the button and wait for the beep.
A few things can throw off the reading: earwax buildup, a recent ear infection, or if your baby has been lying on that ear. Try the other ear if you get a reading that doesn’t match how your baby looks or feels. Ear thermometers are convenient but miss roughly half of fevers that rectal would detect, so they’re better for routine monitoring than for confirming a suspected fever.
How to Take an Armpit Temperature
Make sure the armpit is dry first. Place the tip of a digital thermometer in the center of the armpit, then hold your baby’s elbow snug against their chest to keep the tip fully covered by skin. Wait for the beep, typically about 10 seconds.
An armpit temperature above 99.0°F (37.2°C) suggests a fever, but this method consistently reads lower than core body temperature. Think of it as a screening tool. If the number is elevated or your baby seems unwell, switch to rectal or forehead for a definitive reading.
Tips for an Accurate Reading
Wait at least 15 to 20 minutes after a warm bath before taking your baby’s temperature, since warm water can temporarily raise skin temperature and skew forehead, ear, or armpit readings. The same applies after your baby has been bundled in heavy clothing or blankets. Undress them to a single layer and give it a few minutes before checking.
If you get a reading that surprises you, take it again. Digital thermometers can occasionally misread if the sensor wasn’t positioned correctly, especially with the ear and forehead methods. Two consistent readings a minute apart are more reliable than a single number.
Always use a digital thermometer. Glass mercury thermometers are no longer recommended because of the risk of breakage and mercury exposure.
What Temperature Means a Fever
The threshold depends on where you measured:
- Rectal: 100.4°F (38°C) or higher
- Forehead: 100.4°F (38°C) or higher (though readings may run slightly low)
- Ear: 100.4°F (38°C) or higher
- Armpit: 99.0°F (37.2°C) or higher
When reporting your baby’s temperature to a doctor, always mention which method you used. A rectal reading of 100.4°F carries more weight than an armpit reading of 99.0°F, even though both suggest a fever.
Signs That Need Medical Attention
For babies between 6 and 24 months old, a temperature above 100.4°F that lasts more than one day warrants a call to your pediatrician. Call sooner if your baby has any of these alongside a fever: skipping two or more feedings in a row, sleeping much more than usual or being difficult to wake, seeming unusually floppy or hard to console, breathing fast or with visible effort, producing fewer wet diapers than normal, or having a dry mouth and crying without tears. Loose, watery stools (three or more) or a rash that appears quickly, blisters, or looks infected also warrant a prompt call.
A fever by itself in an 8-month-old is usually the immune system doing its job. The number matters less than how your baby is acting. A baby at 101°F who is drinking, making eye contact, and playing is generally less concerning than a baby at 100.5°F who is limp, refusing to eat, and inconsolable.