The most accurate way to take a baby’s temperature is rectally, using a digital thermometer. For babies under 3 months old, a rectal reading is the only method reliable enough to trust. As your baby gets older, you gain more options, but the technique matters just as much as the tool you choose.
Which Method to Use at Each Age
For newborns and babies under 3 months, use a rectal thermometer. At this age, even a slight fever can signal a serious infection, so you need the most accurate reading possible. Armpit (axillary) readings can serve as a quick screening tool, but they’re less precise and can underestimate the actual temperature by nearly a full degree.
Starting at 3 months, forehead (temporal artery) thermometers become a reliable option. These are fast, non-invasive, and work well for fussy babies who won’t stay still. At 6 months, ear (tympanic) thermometers also become usable, though earwax or incorrect positioning can throw off the reading. Oral thermometers aren’t appropriate until a child is at least 4 years old.
How to Take a Rectal Temperature
It sounds intimidating, but rectal readings are straightforward and quick. You’ll need a digital thermometer (not glass, and never mercury) and a small amount of petroleum jelly or water-based lubricant.
- Position your baby. Lay them on their back and gently lift their thighs toward their chest. Alternatively, place them belly-down across your lap with one hand pressing lightly on their lower back to keep them steady.
- Lubricate the tip. Apply a thin layer of petroleum jelly to the thermometer’s sensor end.
- Insert gently. Slide the tip in half an inch to one inch (1.3 to 2.5 centimeters), no deeper. You’ll feel slight resistance, but don’t force it.
- Hold still. Keep the thermometer in place, holding it between two fingers with your palm resting against your baby’s bottom, until it beeps.
- Read and clean. Note the number, then clean the thermometer with rubbing alcohol or soap and water. Label it so it’s never accidentally used orally.
The whole process takes about 15 to 30 seconds with a modern digital thermometer. If your baby is squirming, the belly-down-on-your-lap position tends to keep them calmer.
How to Take an Armpit Temperature
Armpit readings are the easiest method for a quick check, especially if your baby seems warm but you’re not sure a rectal reading is warranted. Place the thermometer tip in the center of your baby’s bare armpit, then hold their arm snug against their body until the thermometer beeps.
The trade-off is accuracy. A large review of over 3,200 infants and children found the average difference between armpit and rectal temperatures was 0.85°C (roughly 1.5°F), but individual readings could swing anywhere from nearly identical to almost 2°C off. In newborns specifically, the gap is smaller, averaging about 0.17°C. Still, if an armpit reading concerns you, follow up with a rectal reading to get the real number.
Using Forehead and Ear Thermometers
Forehead thermometers scan across the temporal artery and give a reading in one to two seconds. For babies 3 months and older, they’re accurate enough for everyday use and especially practical when you want to check a sleeping baby without waking them. Follow the manufacturer’s instructions, as most require a slow swipe across the forehead from the center to the hairline.
Ear thermometers measure infrared heat from the eardrum. They work well for babies over 6 months, but the reading depends on correct placement. Pull the ear gently back and up to straighten the ear canal, then insert the probe snugly. If your baby has been lying on one ear, wait a few minutes before measuring that side, since the compressed ear will read higher. Earwax buildup can also block the sensor and produce a falsely low number.
What Counts as a Fever
The threshold depends on your baby’s age and how you measured:
- Rectal or forehead: 100.4°F (38°C) or higher is a fever.
- Armpit: 99°F (37.2°C) or higher in babies under 3 months. For older children, 103°F (39.4°C) or higher from an armpit reading warrants a call to your pediatrician.
A common instinct is to add a degree to an armpit reading to estimate the “real” temperature. This is unreliable. The actual difference between armpit and core temperature varies too widely from one reading to the next. If an armpit reading looks borderline, take a rectal temperature instead of guessing.
When a Fever Needs Immediate Attention
Your baby’s age changes everything about how seriously to treat a fever. For babies under 3 months, any rectal temperature of 100.4°F (38°C) or higher needs a call to your pediatrician right away, even if your baby seems fine otherwise. At this age, fevers can indicate infections that progress quickly, and pediatricians want to know immediately.
For babies 3 to 6 months old, call if the temperature reaches 100.4°F or if your baby seems unusually lethargic, irritable, or refuses to eat, even with a lower reading. Between 6 and 24 months, a fever above 100.4°F that persists for more than a day warrants a call.
Thermometer Safety
Always use a digital thermometer. If you have an old glass thermometer containing mercury in your home, don’t throw it in the trash. Mercury is toxic, and a broken thermometer can release it into your environment. Contact your local health department or waste disposal authority for safe disposal options.
If you use the same type of digital thermometer for both rectal and other readings, designate one thermometer for rectal use only and label it clearly. Clean it with rubbing alcohol before and after every use. Avoid using a rectal thermometer immediately after a bath, as your baby’s core temperature may be temporarily off from the warm water.