Quick answer: Method depends on age: rectal temperature (most accurate) for under 3 months; temporal artery (forehead scan) or tympanic (ear) from 3 months; axillary (armpit) is least accurate — add 0.5°C if used. Any temperature of 38°C (100.4°F) or above in a baby under 3 months is a medical emergency.
Rectal temperature in newborns: technique
Rectal temperature is the gold standard for babies under 3 months because it most accurately reflects core body temperature. The other methods have accuracy limitations in newborns — ear canals are too small and curved for reliable tympanic readings; forehead scanning has a wider margin of error in very young infants. Technique: use a flexible-tip digital rectal thermometer; apply a small amount of petroleum jelly to the tip; lay the baby on their back with legs held up as for a diaper change; gently insert the tip 1–2 cm (not further) into the rectum; hold firmly but gently until the thermometer beeps. Normal rectal temperature: 36.6–100.4°F (100.4°F (38°C)). At or above 100.4°F (100.4°F (38°C)) in a baby under 3 months: emergency doctor or the ER attendance.
Temporal artery (forehead) from 3 months
Temporal artery thermometers scan the temporal artery across the forehead and are fast (2–3 seconds), non-invasive, and reasonably accurate from 3 months. To use correctly: drag the thermometer firmly across the centre of the forehead from one temple to the other in a straight line (don’t tilt or lift during the scan); press the button throughout; capture the reading immediately. Common errors: not maintaining firm contact throughout; tilting the thermometer; taking a reading on a flushed or sweaty forehead (inaccurate — dry and cool the forehead first). Normal range: 36.4–37.5°C. Above 100.4°F (100.4°F (38°C)): fever.
Tympanic (ear) from 3 months
Ear thermometers are the most widely used clinical thermometers in doctor surgeries and hospitals in the UK. Accuracy depends on correct technique: pull the ear gently back and up to straighten the ear canal (down and back for babies under 1 year); insert the probe until it seals the canal opening; press the button and hold until the reading is taken (1–2 seconds). Ear infections can produce falsely elevated readings from the affected ear — take from both ears if an ear infection is suspected and use the higher reading. Normal range: 35.8–37.5°C (ear readings run slightly lower than rectal).
Axillary (armpit) — and why it’s less reliable
Armpit temperature runs 0.5°C lower than core temperature. If you use axillary measurement, add 0.5°C to determine approximate core temperature — a baby whose axillary temperature is 37.5°C has an approximate core temperature of 38.0°C (fever). Axillary temperature is the least accurate method and should not be the primary method for a baby under 3 months. Use it only as a quick screening check if no better method is available, and follow up with a more accurate method if concerned.
Frequently Asked Questions
What temperature is a fever in a baby?
38°C (100.4°F) or above, regardless of measurement method (assuming correct technique and appropriate conversion for axillary). In a baby under 3 months, any temperature of 100.4°F (38°C)+ is a medical emergency — call 911 or go to the ER. Don’t give paracetamol and wait to see if it resolves at this age.
My baby’s hands are cold but their forehead feels hot — do they have a fever?
Take their temperature with a thermometer to get a reliable reading. Hand temperature and forehead touch are both unreliable subjective assessments. Only a calibrated thermometer gives a clinically useful reading.
Can I use a forehead strip thermometer?
Forehead strip thermometers measure skin surface temperature, not core temperature, and are consistently less accurate than digital thermometers in clinical testing. They’re adequate for casual monitoring but not for clinical decision-making — don’t use them as the basis for whether to seek medical attention.
Related Reading
- Baby fever: temperature chart, when to go to ER
- Best baby thermometers 2025: fast, accurate and parent-approved
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