For a baby under 3 months, a digital rectal thermometer gives the most accurate reading, and any temperature at or above 100.4°F counts as a fever.
A glass mercury thermometer has no place in a nursery, and the armpit reading you grabbed in a panic is the least dependable number you can get. When the question is whether a newborn has a fever, accuracy decides whether you call the pediatrician at midnight or go back to bed. The digital rectal method takes about a minute and returns a number you can trust.
Why Rectal Readings Beat Every Other Method In Infants
Rectal temperature is the most accurate way to measure a baby’s body temperature, which is why it is the standard method from birth to 3 months. The core temperature inside the rectum tracks what the body is actually doing, while surface readings drift with room conditions.
The alternatives each have limits worth knowing before you reach for one:
- Armpit (axillary): usable from 3 months to 4 years, but typically the least accurate of the three body-site readings. Fine for a screening check, not for a number you act on.
- Oral: not appropriate for infants, who cannot hold a thermometer steady under the tongue long enough for a reliable result.
- Temporal artery (forehead): an option for babies 3 months and older, convenient but generally less precise than a rectal reading.
The takeaway is simple: age drives the method. Newborns get the rectal reading. Older babies have choices.
| Age | Recommended Method | Accuracy Notes |
|---|---|---|
| Birth to 3 months | Digital rectal | Most accurate; standard for newborns |
| 3 months to 4 years | Digital rectal or armpit | Rectal still reads most reliably |
| 3 months and up | Temporal artery | Convenient, less precise |
| Under 5 years (armpit) | Digital axillary | Fast and safe, but least accurate |
| Any age (oral) | Not for infants | Requires steady cooperation |
| Never | Mercury glass thermometer | Do not use — breakage and mercury risk |
How To Take A Baby’s Temperature With A Digital Thermometer
A digital thermometer is the only instrument you need — one device handles rectal, oral, and axillary readings. Work through these steps in order, and keep the whole process calm and unhurried.
- Turn on the digital thermometer and check that the display is working.
- Put a small amount of lubricant, such as petroleum jelly, on the tip.
- Lay the baby on the back and lift the thighs, or place the baby belly-down on a firm surface and steady the lower back.
- Insert the thermometer gently into the rectum, only 1/2 to 1 inch (1.3 to 2.5 cm). Never force it.
- Hold it in place until the thermometer beeps or otherwise signals it is done.
- Remove it and read the temperature on the display.
- Clean the thermometer after use.
When you have finished, the thermometer beeps and the reading appears on the display — that beep is your cue the measurement is complete. If your thermometer shows no beep, follow whatever completion signal its instructions describe.
One practical note before you buy anything: not every digital thermometer is shaped for this job, and a flexible tip with a short, rounded probe makes the process easier on both of you. Our roundup of the best digital thermometer for baby covers the models worth considering.
Whichever device you use, follow the manufacturer’s own instructions for your specific model, since probe depth guidance and display behavior vary slightly between brands. The NHS guide to taking your baby’s temperature walks through the same technique with additional safety notes.
What The Number Actually Means
A reading of 100.4°F or higher is considered a possible fever in a child, and 38°C (roughly 100.4°F) or above is generally treated as a high temperature in babies and children. A single elevated reading is information, not a verdict — how your baby looks and behaves matters just as much as the digits.
For an infant under 3 months, a rectal temperature at or above 100.4°F is a reason to call your pediatrician promptly rather than wait and recheck in the morning. Young babies have immature immune systems, and medical guidance consistently treats fever in this age group as something to have evaluated quickly.
Strip out the variables that inflate a reading. A baby bundled in layers, fresh out of a warm bath, or held against your body for a long stretch can run warm without being sick. If the first number surprises you, settle the baby in a normal environment, wait a short while, and take one more reading the same way before you decide anything.
Mistakes That Skew The Reading
Most unreliable readings trace back to four habits. Fix these and your numbers get steadier right away.
- Reaching for an armpit check on a newborn. It is the least accurate site, and it is not the method recommended for this age group.
- Using a mercury glass thermometer. Mercury glass thermometers should not be used at all on a baby.
- Going oral too early. Oral readings wait on a child who can cooperate; for an infant, that is not happening. If you ever do take an oral reading on an older child, wait 30 minutes after eating or drinking.
- Skipping the lubricant or rushing the insertion. Gentle, shallow placement protects the baby and produces a cleaner measurement.
FAQs
Can I use a digital thermometer in the armpit instead of the rectum?
You can, and it is acceptable from about 3 months onward when you need a quick screen. Just know that an armpit reading is typically the least accurate of the three body sites, and it is not the method recommended for babies under 3 months, where a rectal reading is the reliable choice.
How far should the thermometer go in?
Only 1/2 to 1 inch, which is 1.3 to 2.5 cm. Deeper is not better and never force it. Insert gently, hold until the thermometer signals completion, then remove and read the result.
What temperature counts as a fever in a baby?
A reading of 100.4°F or above is considered a possible fever in a child, and 38°C or higher is generally treated as a high temperature in babies. For any infant under 3 months, contact your pediatrician promptly rather than waiting to see what happens.
References & Sources
- NHS. “How to take your baby’s temperature.” Covers the recommended technique for taking a baby’s temperature.
- Mayo Clinic. “Thermometer basics: How to take your child’s temperature.” Details age-based method guidance and accuracy of each site.
- Johns Hopkins Medicine. “Measuring a Baby’s Temperature.” Supports digital thermometer use and correct measurement practice.