The most accurate way to check a newborn’s temperature is with a digital rectal thermometer. For babies under 3 months old, a rectal reading is the clinical standard because it reflects core body temperature more reliably than any other method. A normal rectal temperature for a newborn falls between about 97.9°F and 100.3°F (36.6°C to 37.9°C), and anything at or above 100.4°F (38.0°C) is considered a fever.
Why Rectal Is the Go-To for Newborns
Newborns have tiny ear canals and thin skin that make other thermometer types less reliable at this age. Ear (tympanic) thermometers are not considered accurate until a baby is at least 6 months old, and even then, earwax or the shape of the ear canal can throw off the reading. Forehead (temporal artery) thermometers can be used as a quick first pass for babies 3 months and older, but if the reading looks elevated, a rectal temperature is still needed to confirm.
Some newer research suggests temporal artery thermometers may provide accurate readings even in newborns, but in clinical practice, rectal remains the reference point. If your baby is under 3 months and you suspect a fever, a rectal reading is what your pediatrician will want to hear.
What You Need
Use a standard digital thermometer with a flexible tip. You can find these at any pharmacy for a few dollars. Avoid glass thermometers, which can break, and mercury thermometers, which pose a toxicity risk. It helps to label the thermometer so it’s only used rectally and not mixed up with an oral thermometer later.
You’ll also need petroleum jelly (like Vaseline) to lubricate the tip, and a clean surface to lay your baby on, such as a changing table or your lap.
How to Take a Rectal Temperature
Start by cleaning the thermometer with cool soapy water or wiping it down with an alcohol swab. Turn it on and make sure the screen is ready. Apply a small dab of petroleum jelly to the silver tip.
Lay your baby face-up on a flat, padded surface. You can also place them belly-down across your lap if that feels more secure. Gently hold their legs up with one hand, similar to a diaper change position. With your other hand, slowly insert the lubricated tip about half an inch to one inch into the rectum. Do not force it. If you feel resistance, stop.
Hold the thermometer in place with two fingers, keeping your palm flat against your baby’s bottom so the thermometer can’t slide in further. Most digital thermometers beep within 10 to 20 seconds when the reading is complete. Gently remove it and check the number on the display.
Cleaning the Thermometer
Clean the thermometer both before and after every use. Cool soapy water works well, or you can wipe it with an alcohol-soaked cotton ball. Never use hot water, bleach, or household cleaners, and don’t put it in the dishwasher. Storing a dirty thermometer can reintroduce germs the next time you use it.
When an Armpit Reading Is Enough
An armpit (axillary) reading is less accurate than rectal, but it can work as a quick screening method. Place the thermometer tip in the center of your baby’s dry armpit, hold their arm snugly against their body, and wait for the beep. Keep in mind that armpit readings tend to run about 1°F lower than rectal readings. If an armpit reading looks elevated, follow up with a rectal temperature to get the true number.
What Counts as a Fever
For newborns, the threshold is 100.4°F (38.0°C) measured rectally. This number is not a rough guideline. It is the specific cutoff used by pediatric hospitals, including Children’s Hospital of Philadelphia, and in the American Academy of Pediatrics clinical guidelines. A rectal temperature at or above 100.4°F in a baby younger than 2 months is treated as a medical emergency and warrants an immediate trip to the emergency department.
Fever in a newborn is taken far more seriously than in an older child because their immune systems are immature. For babies 8 to 21 days old, AAP guidelines call for a full hospital workup for any fever. Even for babies up to 2 months, a fever triggers urgent evaluation. This is why having an accurate reading matters so much at this age.
Signs to Watch Alongside Temperature
Temperature is one piece of the puzzle. Even if the number on the thermometer looks normal, pay attention to how your baby is acting. Contact your pediatrician if you notice any of the following:
- Feeding changes: missing two or more feedings in a row, or eating poorly
- Unusual sleepiness: harder than usual to wake up, or seeming floppy and limp
- Irritability: crying more than normal and being very difficult to calm
- Signs of dehydration: fewer wet diapers, dry mouth, crying without tears, or a sunken soft spot on the head
- Breathing difficulty: fast or labored breathing, or skin that looks bluish or gray
- Vomiting: throwing up after feedings or not keeping liquids down for eight hours
Seek emergency care immediately if your baby has a seizure, trouble breathing, skin that looks blue or purple, or is unresponsive and difficult to rouse. These signs warrant a 911 call regardless of what the thermometer reads.
Tips for Getting a Good Reading
Try to take the temperature when your baby is relatively calm. A screaming, squirming baby makes it harder to hold the thermometer steady and can slightly elevate body temperature from exertion. Right after a bath isn’t ideal either, since skin temperature can shift temporarily.
If the number surprises you, wait 15 minutes and take it again. A single slightly elevated reading after a warm bath or a long stretch of crying doesn’t necessarily mean fever. Two consistent readings give you a clearer picture. When you call your pediatrician, tell them the exact number, the method you used (rectal, armpit, or forehead), and the time you took it.