The most accurate way to take a newborn’s temperature is with a digital rectal thermometer. For babies under 3 months old, a rectal reading is the gold standard because it reflects core body temperature more reliably than any other method. The process is straightforward once you know the technique, and most digital thermometers give a reading in under 30 seconds.
Why Rectal Is the Go-To for Newborns
Ear thermometers need to be carefully positioned inside a small ear canal to get an accurate reading, and newborn ear canals are too tiny for a reliable measurement. The AAP considers ear thermometers appropriate only for babies 6 months and older. Oral thermometers are off the table until around age 4. Forehead (temporal artery) thermometers can technically be used at any age and cause less discomfort, but rectal readings remain the most precise for the youngest babies, which matters most when even a slight fever in a newborn is medically significant.
If your baby seems warm and you only have a forehead thermometer, it’s reasonable to use it as a first check. But if the reading looks elevated, follow up with a rectal thermometer to confirm.
Choosing the Right Thermometer
Any basic digital thermometer works for rectal readings, but thermometers designed specifically for rectal use have a helpful feature: a small ridge near the tip that acts as a built-in depth guide so you don’t insert it too far. These are inexpensive and widely available at pharmacies. If you also have an oral thermometer in the house, label each one clearly. You never want to mix them up.
Step-by-Step Rectal Temperature
Before you start, clean the thermometer tip with rubbing alcohol and let it dry, or wash it with soap and cool water. Apply a small amount of petroleum jelly to the tip for lubrication.
Position your baby in one of two ways. The first option is belly-down across your lap, with your palm resting on their lower back to keep them steady. The second is face-up on a firm surface with their legs bent toward their chest, your hand gently holding the back of their thighs. Either position works well. Many parents find the belly-down position easier for keeping a squirmy baby still.
Gently insert the lubricated tip into the anal opening about ½ inch to 1 inch (1.25 to 2.5 centimeters), just until the tip is fully inside the rectum. Don’t force it. Hold the thermometer in place between two fingers with your palm flat against your baby’s bottom so it can’t slip in further if they move. Wait for the beep.
After the reading, remove the thermometer, note the number, and clean it again with rubbing alcohol before storing it.
What Counts as Normal
A normal rectal temperature for a newborn is in the range of 97.9°F to 100.4°F (36.6°C to 38°C). A rectal reading of 100.4°F (38°C) or higher is considered a fever. For babies under 3 months old, any fever warrants a call to your pediatrician, even if your baby otherwise seems fine. In very young infants, fever can be the only visible sign of a serious infection, so doctors treat it with urgency at this age.
If you’re using an underarm (axillary) reading instead, the threshold is lower: 99°F (37.2°C) or above is considered a fever for infants. Keep in mind that underarm readings run consistently lower than rectal ones, and the gap between them varies too much from baby to baby to reliably convert one to the other. Adding a degree to an underarm reading is a common suggestion, but research shows this isn’t dependable. If an underarm reading concerns you, confirm it rectally.
Taking an Underarm Temperature
Underarm readings are less accurate, but they work as a quick screening method, especially if your baby is sleeping and you don’t want to disturb them with a rectal thermometer. Place the tip of a digital thermometer in the center of your baby’s armpit, making sure it’s touching skin and not clothing. Fold their arm down against their body and hold it gently in place until the thermometer beeps. Typical healthy underarm temperature for an infant is around 98.2°F (36.8°C).
Tips for a Smooth Reading
Timing matters. Avoid taking your baby’s temperature right after a bath or when they’ve been tightly swaddled, since both can temporarily skew the number. A good time is when they’ve been at rest for at least 15 to 20 minutes.
If your baby is fussy and won’t stay still, try the belly-down lap position and talk or sing to them. Having a second adult hold the baby’s attention with a toy or gentle voice can help. The actual measurement takes only 10 to 30 seconds with most modern digital thermometers, so it’s over quickly.
Keep your thermometer somewhere easy to find. Late-night temperature checks with a sick baby are stressful enough without digging through drawers. A bedside drawer or the top of your changing station works well.
What a Fever Means in a Newborn
In older children, a low-grade fever is often something you can manage at home and monitor. In a baby under 3 months, the calculus is different. Their immune system is still developing, and fever can signal infections that escalate quickly. A rectal temperature at or above 100.4°F in this age group is always worth a same-day call to your pediatrician, regardless of whether other symptoms are present. If your baby also seems unusually sleepy, refuses to eat, has a rash, or is difficult to wake, seek care immediately rather than waiting for a callback.