The most accurate way to measure a newborn’s temperature is with a digital thermometer placed in the rectum. For babies under 3 months old, a rectal reading of 100.4°F (38°C) or higher counts as a fever and requires an immediate call to your pediatrician. Knowing the right technique, the right tool, and what the numbers mean can save you a lot of worry and help you act quickly when it matters.
Why Rectal Is the Standard for Newborns
A rectal thermometer gives the closest reading to true core body temperature, which is why it’s the recommended method from birth through age 2. Armpit (axillary) readings are convenient but consistently read lower. In studies comparing the two methods, armpit temperatures averaged about 0.8°C (roughly 1.4°F) lower than rectal readings, and the gap between individual measurements varied widely enough that an armpit reading alone can miss a real fever or falsely suggest one.
That said, armpit readings do have a role. For babies under 3 months, axillary temperature catches about 90% of true fevers, making it a reasonable first check. If an armpit reading looks elevated, follow up with a rectal measurement to confirm. For babies older than 3 months, armpit sensitivity drops to around 76%, which is too unreliable to use as your primary method.
Temporal artery thermometers (the kind you swipe across the forehead) are gaining support. Recent research suggests they may provide accurate readings even in newborns, and some hospitals now use them. Ear (tympanic) thermometers, however, should wait until your baby is at least 6 months old. Newborn ear canals are too small and curved for the infrared sensor to get a reliable reading.
Choosing the Right Thermometer
A basic digital thermometer is all you need. These are inexpensive, widely available, and work for rectal, armpit, or oral readings as your child grows. If you buy one specifically for rectal use, label it clearly so it never gets mixed up with an oral thermometer later.
Glass mercury thermometers are no longer recommended. If they break, the released mercury vapor can be inhaled. Digital pacifier thermometers and stick-on fever strips are also unreliable and best avoided. If you want a secondary thermometer for quick forehead checks, a temporal artery model is a reasonable choice, but keep a standard digital thermometer on hand for rectal confirmation when accuracy matters most.
How to Take a Rectal Temperature Safely
The process sounds intimidating but is straightforward once you’ve done it a couple of times. Here’s what to do:
- Position your baby. Lay them stomach-down across your lap, or place them on their back with legs pulled gently toward their chest. Either position gives you a clear view and keeps the baby stable.
- Lubricate the thermometer tip. Apply a small dab of petroleum jelly to the silver tip and around the opening of the anus. This makes insertion comfortable and smooth.
- Insert gently, no more than half an inch. For babies under 6 months, push the tip in only until the silver end disappears, roughly half an inch. For older infants up to 12 months, no more than 1 inch. If you feel any resistance at all, stop immediately.
- Hold still until the beep. Keep one hand on your baby’s lower back or legs to prevent sudden movement. Most digital thermometers beep within 10 to 30 seconds.
- Remove and read. Slide the thermometer out gently, note the number, and clean the tip with rubbing alcohol or soap and water.
The key safety rule is never force the thermometer. The rectal wall in a newborn is thin and delicate. If your baby is clenching or fussy, try again in a few minutes or take an armpit reading first to decide if a rectal check is needed.
How to Take an Armpit Temperature
Place the thermometer tip in the center of your baby’s bare armpit, then hold the arm snugly against their body until the thermometer beeps. Make sure the tip touches only skin, not clothing. Armpit readings are best used as a screening tool. If the number reaches 99°F (37.2°C) or higher, take a rectal reading to confirm whether your baby actually has a fever.
What the Numbers Mean
Normal rectal temperature for a newborn ranges from about 97.9°F to 100.3°F (36.7°C to 37.9°C). Normal armpit temperature runs lower, from about 96.1°F to 99°F (35.6°C to 37.2°C). These ranges reflect genuine fluctuations throughout the day and in response to feeding, sleeping, and activity.
The critical threshold: a rectal temperature of 100.4°F (38°C) or higher in a baby under 3 months is treated as a medical emergency. At this age, a baby’s immune system is immature enough that even a modest fever can signal a serious infection. Don’t wait to see if it comes down on its own. Call your pediatrician or go to the emergency room.
Low temperature also matters. A reading below 97.7°F (36.5°C) is considered hypothermia in a newborn. Premature babies are especially vulnerable because they have less body fat and a larger surface area relative to their weight, which means they lose heat quickly. Skin-to-skin contact is one of the most effective ways to warm a cold newborn. If your baby’s temperature stays low despite warming efforts, contact your pediatrician.
When Bundling Mimics a Fever
Before concluding your baby has a fever, consider what they’re wearing. In a controlled study of healthy full-term newborns, babies wrapped in five blankets plus a hat in a warm room (about 80°F) saw their rectal temperature rise by an average of 1°F over two and a half hours, climbing at a steady rate of nearly half a degree Fahrenheit per hour with no sign of leveling off. Two of the twelve bundled babies hit 100.4°F, the clinical fever threshold, despite being perfectly healthy.
If your baby feels warm and you get an elevated reading, try removing a layer or two, wait 15 to 20 minutes, and recheck. A temperature driven by overbundling will come down once the extra layers are gone. One that stays elevated points to something else going on.
Signs That Need Immediate Attention
A fever in a baby under 3 months always warrants a call. But certain symptoms alongside a fever, or even without one, signal a true emergency:
- Inconsolable crying that nothing calms
- Difficult to wake or unusually limp
- Trouble breathing after clearing the nose
- Blue or grayish color on the lips, tongue, or nails
- A seizure
- A new rash or unexplained bruising
- Refusal to move an arm or leg
- A stiff neck or confused appearance
Any of these symptoms in a newborn calls for 911 or an immediate trip to the emergency room, regardless of what the thermometer says.