The most accurate way to check a baby’s temperature is with a digital thermometer used rectally, especially for infants under 3 months old. A reading of 100.4°F (38°C) or higher counts as a fever. The method you use matters because different spots on the body give different readings, and accuracy is critical when your baby is very young.
Which Method to Use at Each Age
For babies from birth to 3 months, a rectal temperature is the gold standard. It gives the most reliable core body temperature, and at this age, even a small fever can signal something serious. Temporal artery thermometers (the ones you swipe across the forehead) may also provide accurate readings in newborns, though rectal remains the preferred method for this age group.
From 3 months to 4 years, you have more options. Rectal is still the most accurate, but you can also use an armpit (axillary) reading or a temporal artery thermometer. Ear thermometers become an option once your baby reaches 6 months old. Before that age, a baby’s ear canals are too small to get a reliable reading.
How to Take a Rectal Temperature
This sounds intimidating, but it’s straightforward once you’ve done it. Start by cleaning the tip of a digital thermometer with rubbing alcohol or soap and water. Apply a small amount of petroleum jelly to the tip for lubrication.
Lay your baby face-down on your lap or on a firm surface, or place them on their back with their legs held gently toward their chest. Gently insert the thermometer tip about half an inch to one inch into the rectum. Don’t force it. Hold the thermometer in place with two fingers, keeping your hand cupped around your baby’s bottom so it can’t slide in further. Wait for the beep, then remove it and read the number.
Label this thermometer so it’s only used rectally. Keep a separate one for oral or armpit readings as your child grows.
Armpit Readings: Quick but Less Reliable
Armpit temperatures are the easiest to take. Just place the thermometer tip in the center of your baby’s armpit, fold their arm snugly against their body, and wait for the beep. The problem is accuracy. In a study published in the Annals of Emergency Medicine, armpit readings detected only about 12% of fevers confirmed by rectal thermometer. Even when researchers lowered the fever cutoff to 99.9°F, the detection rate climbed to just 23%.
That means an armpit reading can tell you your baby doesn’t have a fever if the number is clearly normal, but a normal-looking armpit reading doesn’t rule a fever out. If your baby feels warm and the armpit reading seems borderline, follow up with a rectal temperature. Armpit readings typically run about 1°F lower than rectal readings, though this gap isn’t consistent enough to rely on a simple conversion.
Forehead (Temporal Artery) Thermometers
These thermometers use an infrared sensor that you swipe across your baby’s forehead. They’re fast, non-invasive, and reasonably accurate. Temporal artery readings typically fall within 0.4°F of a rectal reading, and they correlate more closely with rectal temperatures than armpit readings do.
At the standard fever threshold of 100.4°F, temporal artery thermometers catch about 62% of true fevers. With a slightly lower cutoff of 99.9°F, that jumps to roughly 81%. So they’re a solid screening tool, especially for babies over 3 months. For newborns under 3 months, though, don’t rely on a forehead thermometer alone if the reading seems off and your baby is acting unwell. Confirm with a rectal reading.
Ear Thermometers After 6 Months
Ear (tympanic) thermometers measure infrared heat from the eardrum, which shares a blood supply with the brain’s temperature-regulation center. To get an accurate reading, you need a clear path to the eardrum. For babies between 6 months and 1 year, gently pull the ear straight back to open the canal. For children over 1 year, pull the ear back and slightly upward. Insert the thermometer snugly into the ear canal and press the button.
Earwax buildup, ear infections, or a squirming baby can all throw off the reading. If the number seems surprisingly high or low, try the other ear or switch to a different method.
Getting an Accurate Reading
Timing matters. A bath, heavy crying, or being bundled in blankets can temporarily raise your baby’s skin temperature. Wait about 15 to 20 minutes after a bath or after removing heavy clothing before taking a reading. If your baby has been nursing or drinking a bottle, that won’t affect a rectal or forehead reading, but it’s still good practice to wait a few minutes for them to settle.
Avoid mercury thermometers entirely. They’ve been phased out due to the toxicity risk if the glass breaks. Digital thermometers are inexpensive, fast, and safe. Keep one in your diaper bag and one at home so you’re never caught without one.
What Temperature Counts as a Fever
The threshold depends on how you measured. A rectal temperature of 100.4°F (38°C) or higher is a fever. For armpit readings, 99°F (37.2°C) is generally considered elevated, but because armpit readings are less reliable, a rectal confirmation is worthwhile before acting on the number.
In babies under 3 months, a rectal temperature of 100.4°F or higher requires immediate medical attention, even if your baby seems fine otherwise. At this age, a fever can be the only visible sign of a serious infection. For older babies, fever alone is less alarming, but certain symptoms alongside it signal an emergency.
Fever Signs That Need Immediate Attention
Call your pediatrician or head to the emergency room if your baby has a fever along with any of the following:
- A widespread red or purple rash that doesn’t fade when you press on it
- Stiff neck or severe headache (in older infants who can express discomfort)
- Difficulty breathing or noticeably fast breathing
- Extreme sleepiness or limpness, where your baby is hard to wake or unusually unresponsive
- Signs of dehydration: dry diapers for several hours, very dark urine, or no tears when crying
- Seizures
A combination of high fever with cracked red lips, a red tongue, red eyes, swollen hands and feet, or enlarged lymph nodes also warrants an immediate call. These can point to conditions that need prompt treatment.
Keeping Track of Readings
When your baby is sick, take their temperature at consistent times (morning and evening, for example) and write down the number, the time, and which method you used. This log is genuinely useful when you call your pediatrician because “the fever was 101.2 rectally at 6 a.m. and 100.8 at noon” gives them a much clearer picture than “they felt warm all day.” Many baby-tracking apps have a temperature log built in, or a note on your phone works fine.