A rectal temperature is the most accurate way to check a newborn’s temperature, and it’s the method pediatricians recommend for any baby under 3 months old. A reading of 100.4°F (38°C) or higher taken rectally counts as a fever in a newborn. While the idea of taking a rectal temperature can feel intimidating for new parents, the process is straightforward and takes less than a minute.
Why Rectal Is the Go-To for Newborns
Rectal readings reflect your baby’s core body temperature more reliably than any other method at this age. Armpit (axillary) readings are less accurate because they measure skin surface temperature, which can be thrown off by clothing, swaddling, or room temperature. Ear thermometers aren’t recommended until a baby is at least 6 months old because a newborn’s ear canals are too small to get a reliable reading.
Newer temporal artery thermometers (the kind you swipe across the forehead) are showing promise for newborns, and some research suggests they can provide accurate readings even in the first few months. But rectal remains the standard your pediatrician will ask about if you call with concerns about a fever, so it’s worth knowing how to do it.
What You Need
Use a basic digital thermometer. These are inexpensive, widely available, and beep when the reading is complete. Mercury thermometers should never be used. The American Academy of Pediatrics advises against keeping them in the house at all because the liquid mercury inside is hazardous if the glass breaks.
You’ll also need petroleum jelly or another water-based lubricant to coat the tip of the thermometer before insertion. Keep these supplies together in one spot so you’re not scrambling when your baby feels warm.
Label the thermometer you use rectally or designate it with a piece of tape so it doesn’t accidentally get used in someone’s mouth later.
Step-by-Step: Taking a Rectal Temperature
Lay your baby on their back on a flat, stable surface like a changing table or bed. Lift their legs gently, the same way you would for a diaper change. Some parents find it easier to lay the baby belly-down across their lap, but on the back with legs raised gives you the best visibility and control.
Turn on the digital thermometer and apply a small dab of petroleum jelly to the silver tip. Gently insert the tip into the rectum about half an inch to one inch, no deeper. You should feel very little resistance. If you do, stop. Never force the thermometer.
Hold the thermometer in place between two fingers, with your other hand steadying the baby’s legs or bottom. Keep the baby still so the thermometer doesn’t shift. Most digital thermometers will beep within 10 to 20 seconds when the reading is complete. Wait for the beep before removing it.
Slide the thermometer out gently, read the display, and note the number. If you’re planning to call your pediatrician, write down the exact reading and the time you took it.
Cleaning the Thermometer
After each use, clean the probe end with rubbing alcohol or soap and water, then rinse it under cool water. Don’t use hot water, which can damage or crack a digital thermometer. Let it air dry or pat it with a clean cloth before storing it. Cleaning both before and after use is good practice, especially if the thermometer has been sitting in a drawer for a while.
Armpit Temperature as a Backup
If you’re away from home or simply can’t manage a rectal reading, an armpit temperature can serve as a rough screening tool. Place the thermometer tip in the center of your baby’s armpit and hold their arm snugly against their body until the thermometer beeps. Keep in mind that armpit readings typically run about 1°F lower than rectal readings, so what looks normal under the arm could actually be a low-grade fever. If an armpit reading seems borderline or your baby seems off, follow up with a rectal temperature to get the real number.
What the Numbers Mean
A normal rectal temperature for a newborn falls between roughly 97.5°F and 100.3°F (36.4°C to 37.9°C). Slight variations throughout the day are completely normal, with body temperature tending to run a bit higher in the late afternoon.
A rectal temperature of 100.4°F (38°C) or above is a fever. In a baby under 3 months old, any fever at this threshold warrants an immediate call to your pediatrician, even if your baby seems fine otherwise. Young infants don’t always show obvious signs of illness the way older children do, and a fever at this age can signal an infection that needs prompt evaluation.
For babies between 3 and 6 months, call if the temperature reaches 100.4°F and your baby seems unusually fussy, lethargic, or is eating poorly. After 6 months, a fever above 100.4°F that lasts more than a day is the general threshold for calling.
Signs to Watch Alongside a Fever
Temperature is just one piece of the picture. Pay attention to how your baby is acting. Contact your pediatrician if your newborn has a fever along with any of these:
- Refusing to eat. Missing two or more feedings in a row, or eating noticeably less than usual.
- Unusual sleepiness. Sleeping far more than normal, being difficult to wake, or seeming floppy when you pick them up.
- Increased crying. Crying more than usual or being very hard to calm down.
- Rash. Especially one that appears quickly, blisters, or looks infected.
- Other symptoms. Persistent cough, diarrhea, or vomiting alongside a fever may prompt an earlier call depending on severity.
Tips for Getting an Accurate Reading
Timing matters. Avoid taking your baby’s temperature right after a bath, since warm water can temporarily raise skin temperature and affect the reading. Wait about 15 to 20 minutes. Similarly, if your baby has been tightly swaddled or bundled in heavy clothing, unwrap them for a few minutes first. Overdressing alone can make a baby feel warm to the touch without an actual fever being present.
If the number seems surprisingly high and your baby doesn’t seem sick, wait 15 minutes, make sure your baby isn’t overdressed, and take the temperature again. A single elevated reading after a warm bath or a long nap in heavy pajamas doesn’t necessarily mean fever. A consistent reading of 100.4°F or above on a repeat check is more meaningful.