A rectal temperature is the most accurate way to check an infant’s temperature, and it’s the method pediatricians recommend for babies under 3 months old. A reading of 100.4°F (38°C) or higher counts as a fever. The process is straightforward once you know the positioning, and it takes less than a minute.
Why Rectal Readings Are the Standard
For infants, a rectal thermometer gives the closest measure of true core body temperature. Other methods, like forehead (temporal artery) and ear (tympanic) thermometers, are faster and easier to use, but they’re less reliable in babies. A study of 304 infants published in JAMA Pediatrics found that temporal artery thermometers only detected 66% of fevers confirmed by rectal readings. Ear thermometers performed even worse, catching just 49%. Both types improved with higher fevers, but for borderline readings in a young infant, rectal is the one that matters.
This is especially important for babies under 3 months. Any fever at that age requires a call to your pediatrician, so you need a number you can trust.
What You Need
Use a basic digital thermometer, not a glass one. Mercury thermometers release toxic vapor if they break and have been banned in many states. Digital thermometers are inexpensive, widely available, and beep when the reading is done. If you plan to use the same type of thermometer for rectal and oral readings later on, buy two and label them so they never get mixed up.
You’ll also need petroleum jelly (or another water-based lubricant) and rubbing alcohol or alcohol wipes for cleaning.
How to Take a Rectal Temperature
Clean the thermometer tip first. Dip a cotton ball in rubbing alcohol (at least 70%), wipe the entire tip, and wait a few seconds for the alcohol to evaporate. Then turn the thermometer on and apply a small dab of petroleum jelly to the tip.
You have two positioning options. The first is to lay your baby on their back on a firm, flat surface, then gently lift both thighs toward the belly. The second is to place your baby belly-down across your lap, with one hand resting on their lower back to keep them steady. Either position works well.
Gently insert the lubricated tip into the rectum about half an inch to one inch (1.3 to 2.5 centimeters). Never force it past any resistance. Hold the thermometer in place between two fingers, keeping your other hand on the baby so they can’t roll or squirm. Wait for the beep, then slowly remove it and read the number.
After you’re done, clean the tip with antibacterial soap and water to remove any residue, then wipe it down with rubbing alcohol. Let it air dry on a clean paper towel before storing it.
Other Methods and When They Work
For babies older than 3 months, an armpit (axillary) reading can serve as a quick screening. Place the thermometer tip in the center of your baby’s armpit, fold their arm snugly against their body, and hold it there until the thermometer beeps. Armpit readings tend to run about 1°F lower than rectal readings, so if the number looks borderline, follow up with a rectal check to be sure.
Forehead thermometers are convenient because you just swipe them across the skin without waking a sleeping baby. They’re better tolerated and reasonably good at ruling out fever when the reading is normal. But they miss about a third of true fevers in infants, so a normal forehead reading in a baby who seems sick shouldn’t fully reassure you. A rectal confirmation is worth the extra step.
Ear thermometers are generally not recommended for infants. A baby’s ear canals are narrow, making it hard to get a proper seal, and the readings tend to be the least reliable of all the options for this age group.
What Counts as a Fever
A rectal temperature of 100.4°F (38°C) or higher is a fever. Normal infant temperature typically sits around 98.6°F (37°C) but can fluctuate slightly throughout the day.
What you do next depends on your baby’s age:
- Under 3 months: Call your pediatrician for any fever, even if your baby seems fine otherwise. At this age, a fever can signal a serious infection that’s hard to detect by appearance alone.
- 3 to 6 months: Call if the temperature reaches 100.4°F or higher, or if it’s below that threshold but your baby seems unusually fussy, sluggish, or unwell.
- 6 to 24 months: Call if the fever is above 100.4°F and lasts more than one day. If your baby also has cold symptoms, a cough, or diarrhea, you may want to call sooner depending on severity.
For any age, a fever lasting more than three days warrants a call if you haven’t already made one.
Signs That Need Immediate Attention
Beyond the number on the thermometer, certain symptoms alongside a fever call for urgent care: difficulty breathing, blue or gray-looking skin or lips, inability to wake up or unusual unresponsiveness, and pain or fussiness that keeps getting worse. Trouble swallowing, especially with drooling or spitting, also falls in this category. These signs matter regardless of the exact temperature reading.
Tips for Easier Readings
Taking a rectal temperature goes more smoothly when your baby is calm. Try right after a diaper change, when you already have access and your baby is used to being handled in that area. Have everything prepped before you start: thermometer on, lubricant applied, clean surface ready. The whole process from insertion to beep typically takes 10 to 20 seconds with a modern digital thermometer.
If your baby tenses up or you feel resistance when inserting the tip, stop. Gently reposition and try again, or wait a few minutes. Forcing the thermometer can cause discomfort or injury. A little patience and a light touch make all the difference.