Evidence-based guide to infant fever. Temperature thresholds by age, when to call the pediatrician, when to go to the ER, and what actually helps at home.
Fever in a baby is one of the most common reasons parents panic — and often, it shouldn't be. Fever itself isn't an illness; it's a sign the immune system is doing its job. What matters is what's causing it and how the baby is behaving.
Here's the evidence-based framework pediatricians use.
Under 3 months: Rectal temperature is the gold standard — the most accurate by a wide margin. Digital rectal thermometer, tip lubricated with petroleum jelly, inserted 1.5 cm, held for ~10 seconds.
3–6 months: Rectal preferred, but axillary (underarm) acceptable if measured carefully.
Over 6 months: Temporal artery (forehead) or tympanic (ear) thermometers can be useful for home screening when used according to the device instructions.
Not reliable: Touching forehead by hand, pacifier thermometers, smartphone apps with no hardware.
| Measurement | Fever threshold |
|---|---|
| Rectal | ≥ 38.0°C (100.4°F) |
| Oral | ≥ 37.8°C (100°F) |
| Axillary | ≥ 37.2°C (99°F) |
| Temporal (forehead) | ≥ 38.0°C (100.4°F) |
Normal body temperature varies by time of day, recent activity, and clothing. A single slightly elevated reading after bundling isn't a fever.
This is the single most important framework for parents:
Under 3 months with rectal temperature ≥ 38.0°C (100.4°F) — call the pediatrician or go to the ER the same day. Newborn immune systems are not yet capable of fighting serious bacterial infections efficiently. Fever at this age requires evaluation regardless of how the baby looks.
Fever above 38.9°C (102°F), or any fever with concerning behavior (lethargy, poor feeding, unusual irritability). Below that, monitor at home if baby is behaving normally.
Temperature number matters less; behavior matters more. A baby with 39°C who's playing and drinking is less concerning than a baby with 38°C who is limp, refusing fluids, and hard to rouse.
Call the pediatrician if:
Focus on how the child is acting. If fever responds to medication and the child is otherwise acting normally, many cases can be watched at home, but call if it lasts more than 72 hours or behavior becomes concerning.
Call emergency services or go to the ER immediately if your child has:
A non-blanching rash (purple spots that don't fade when you press a glass against them) with fever is a medical emergency. Go to the ER. Don't wait.
Hydration — top priority. Offer breast milk, formula, or water (over 6 months) frequently. Small amounts often, not large amounts occasionally.
Appropriate medication:
Medication reduces discomfort, not illness duration. The goal is a comfortable baby, not a normal thermometer.
Light clothing. Overdressing traps heat. One layer is plenty. Remove blankets.
Lukewarm (not cold) sponge bath if fever is high and distressing. Never cold water or alcohol rubs — both are harmful.
Alternating paracetamol and ibuprofen is sometimes suggested for stubborn fevers. Current guidance is to use one medication, not combine — less risk of dosing errors. Only alternate under pediatrician guidance.
A slightly raised temperature (up to 38°C) may coincide with teething, but teething does not cause true fever. If a baby has temperature ≥ 38°C, treat it as illness until proven otherwise. Attributing real infection to teething delays treatment.
Recording temperature readings, medication times, and symptoms over 24 hours gives your pediatrician a clear picture. "Has been sick for a while" is less useful than "39.2°C rectally at 2 PM yesterday, paracetamol at 3 PM, dropped to 38.1°C by 6 PM, rose to 39.5°C overnight." A baby fever tracker — even a simple note — turns vague concern into useful data.
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