What Counts as a Fever, and When to Call
100.4°F at any age. What changes is the urgency — and under three months, that number means call now even if the baby seems perfectly well.
A fever is 100.4°F (38°C) or higher. That is the number, and it is the same number at every age.
What changes with age is not the definition — it is how urgently you need to act. And for one group, babies under three months, the urgency is immediate regardless of how well the baby seems.
Here is the threshold that actually matters at each age, how to take the temperature so the number means something, and the signs that matter more than the number does.
The number that changes everything: under three months
If your baby is three months or younger and has a rectal temperature of 100.4°F (38°C) or higher, call your pediatrician straight away.
The American Academy of Pediatrics is unusually firm about this, and about one detail in particular: do this even if the baby seems completely fine. No other symptoms, feeding normally, settled — still call.
The reason is that a very young baby’s immune system does not localize infection the way an older child’s does, and a serious infection can be present without the baby looking unwell. The fever may be the only sign there is. This is not a case where waiting to see how the night goes is reasonable.
The thresholds by age
| Age | Call the doctor when |
|---|---|
| 0–3 months | 100.4°F rectal or above — immediately, even if the baby seems well |
| 3–6 months | 101°F or above |
| Over 6 months | 103°F or above — or lower, if they seem unwell |
Notice what this table is not saying. It is not saying a 102°F fever in a nine-month-old is fine. It is saying the number alone is not the trigger at that age — how the child is doing is.
How you take it changes the answer
A temperature reading is only as good as the method, and the methods are not interchangeable.
- Rectal is the reference standard, and for babies under three months it is the one that counts. The 100.4°F threshold is a rectal threshold.
- Forehead and ear thermometers are convenient and less reliable, particularly in small babies. Useful for tracking a trend, not for making the under-three-months decision.
- Armpit readings run low and are the least reliable of the common methods.
- Oral is reasonable in an older child who can hold it under the tongue properly — and not right after a hot or cold drink.
If you are calling a doctor, say which method you used. It changes how they read the number.
What matters more than the number
Parents are often told not to “treat the number,” which is true and unhelpful on its own. Here is the useful version: past the newborn stage, a fever is a sign the immune system is working, and how a child looks and behaves tells you more than the thermometer does.
Call regardless of the reading if a child:
- Is unusually difficult to wake, or floppy
- Is breathing fast, noisily, or with visible effort
- Has a rash that does not fade when you press a glass against it
- Has a stiff neck, or persistent severe headache
- Is not passing urine — noticeably fewer wet diapers, or none in eight hours
- Cannot keep fluids down
- Has had a fever for more than three days
- Has a seizure
- Simply seems very unwell to you, in a way you cannot name
That last one is not filler. Parents are often right before they can explain why, and no clinician minds a call that turns out to be nothing.
On bringing the temperature down
Medication is for comfort, not for the number. A child who is miserable, sore or unable to sleep may well feel better for it. A child with 102°F who is playing does not necessarily need anything.
Two things not to do: cold baths and rubbing alcohol. Both can cause shivering, which raises core temperature, and alcohol can be absorbed through the skin.
And check with your doctor or pharmacist about dosing by weight rather than age, particularly for babies under six months, where advice differs by product and by age in weeks.
The short version
100.4°F is a fever at any age. Under three months, that number alone means call now. Over six months, how they seem matters more than what the thermometer says. And if something feels wrong to you and you cannot say why, that is reason enough to ask.
Source: the American Academy of Pediatrics. Always follow your own pediatrician over anything you read here.