Baby Fever: What Temperature Means Call the Doctor
Fever in babies starts at 100.4°F. The AAP's call-the-doctor numbers by age (under 3 months, 3 to 6 months, older), how to take it, and what helps at night.
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A fever in a baby is a temperature of 100.4°F (38°C) or higher. What that number means depends on her age. The AAP says to call the pediatrician right away for any baby younger than 3 months (12 weeks) with a temperature of 100.4°F or higher, even if she seems fine. From 3 to 6 months the AAP’s number to call about is 101°F (38.3°C), and after 6 months it is 103°F (39.4°C). At any age, call right away for a fever that rises above 104°F (40°C) repeatedly, and call if a fever lasts more than 24 hours in a child under 2. How she looks and acts matters as much as the reading.
The numbers, in one table
I went looking for this table the first time my daughter felt hot at 2 a.m., and found parenting blogs that disagreed on the middle row. So here are the AAP’s own numbers, from its HealthyChildren pages, which I loaded and read on November 11, 2026.
| Her age | Temperature | What the AAP says |
|---|---|---|
| Younger than 3 months (12 weeks) | 100.4°F (38°C) or higher | Call your pediatrician right away, “even if your baby shows no other signs or symptoms of being ill” |
| 3 to 6 months | 101°F (38.3°C) or higher | You “may need to notify the doctor” |
| Older than 6 months | 103°F (39.4°C) or higher | You “may need to notify the doctor” |
| Any age | Above 104°F (40°C) repeatedly | Call right away |
| Under 2 years | Any fever lasting more than 24 hours | Call |
One wrinkle, so it doesn’t throw you. The AAP’s own pages word the newborn row slightly differently. One says “younger than 3 months (12 weeks)”, one says “3 months of age or younger”, and its thermometer page says “2 months old or younger.” The cutoff number is the same on all three: 100.4°F. If your baby is anywhere near that age line and has a fever, call. Nobody at the office will mind.
The AAP’s reason for the strict newborn rule is that a doctor needs to examine a very young baby to rule out a serious infection. It is not that the fever itself is dangerous at 100.4°F.
Why the fever itself is usually not the enemy
The AAP describes fever as “usually a sign that their body is fighting an illness or infection” and says fevers “are generally harmless.” It lists colds, ear infections, urinary tract infections and respiratory illnesses like RSV and flu among the common causes in babies.
That changed how I watched her. I stopped staring at the number and started watching her. The AAP calls it “a good sign if your child plays and interacts with you” once she is more comfortable. A baby at 102°F who feeds and looks around is a different situation from a baby at 100.8°F who is limp and won’t wake properly.
How to take a baby’s temperature
Feeling her forehead doesn’t count. The AAP says touch “is not accurate, especially if your baby is having chills,” and that fever strips are not reliable either.
- Rectal is the most accurate, the AAP says, “especially for infants under 3 months of age.” A basic digital one with a short tip, like the Frida digital rectal thermometer, is the type built for this.
- Forehead (temporal artery) is next most accurate and can be used at any age. The Exergen TAT-2000C+ is the kind you slide across the forehead.
- Ear thermometers wait until 6 months. The AAP says to avoid them before then because a young baby’s ear canals are too small for an accurate reading.
- Armpit is the least accurate. It works as a quick screen, not as the number you report.
For a rectal reading, the AAP’s steps are: a little petroleum jelly on the tip, baby belly-down across your lap or on her back with legs pulled up, slide it in half an inch for a baby under 6 months, and hold still until it beeps. If you meet resistance, stop. Label that thermometer so it never gets used anywhere else.
Tell the office which method you used. They need that as well as the number.
What helps on a fever night
Fluids first. The AAP says the most important thing is making sure she drinks enough. For a baby, that means breast milk or formula, offered more often. Signs of dehydration on the AAP’s list are crying without tears, a dry mouth and fewer wet diapers.
One light layer. “Do not overdress your child. A single layer of clothing is good,” the AAP says. Bundling a hot baby works against you.
Skip the old tricks. Alcohol baths, ice packs and sponging “are no longer recommended,” according to the AAP.
Medicine is a phone-call question. The AAP says that for a child under two, you should contact your pediatrician or pharmacist for the correct dose. Seattle Children’s adds that a baby under 3 months shouldn’t get fever medicine unless a doctor says so. I’m not printing doses here, because the right one depends on her weight and her doctor.
Safe sleep stays the same. On her back, on a flat, firm surface, with nothing else in it. If a stuffy nose is part of the picture, baby sleep with a cold covers what’s safe overnight. Betteroo also has a plain rundown of what to expect from naps and nights while she’s unwell, including when to wake a sleeping sick baby to feed.
When to call the pediatrician right away
The AAP’s list, for a baby with a fever at any age. Call right away if she:
- is younger than 3 months with a temperature of 100.4°F (38°C) or higher
- has a fever above 104°F (40°C) repeatedly
- looks very ill, is unusually drowsy, or is very fussy
- has been in a very hot place, such as an overheated car
- has a stiff neck, breathing difficulty, an unexplained rash, or repeated vomiting or diarrhea
- has signs of dehydration: dry mouth, a sunken soft spot, or far fewer wet diapers and she can’t take fluids
- has an immune system or heart problem
- has had a seizure
Also call if she still “acts sick” once the fever comes down, if she seems to be getting worse, or if the fever goes past 24 hours and she’s under 2.
If she is having trouble breathing, call 911. And if you think she has overheated in a car or a hot room, the AAP treats that as heatstroke, not fever: call 911 or go to the emergency department.
Well again, sleep still scrambled?
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FAQ: fever in babies
Is 99.5°F a fever in a baby?
No. The AAP’s cutoff is 100.4°F (38°C) or higher. It also notes that body temperature runs highest in the late afternoon and early evening, so a slightly warmer evening reading is expected.
Does teething cause a fever?
Not a true one. The AAP says teething can nudge temperature up within the normal range, but research shows it “does not cause a true fever.” If she has a real fever, look for another cause and call.
Can a fever cause a seizure?
It can, mostly between 6 months and 5 years, and the AAP says febrile seizures are “almost always” harmless. A seizure is still on the call-right-away list. Our roseola guide walks through what to do during one.
Should I wake my baby to check her temperature?
For a baby under 3 months with a fever, you’ll already be on the phone with her doctor, so ask them. For an older baby, checking her breathing, color and how easily she stirs tells you a lot without a full wake-up. If she’s very hard to wake, call.
Sources
- AAP (HealthyChildren.org): Fever: When to Call the Pediatrician
- AAP (HealthyChildren.org): Fever and Your Baby
- AAP (HealthyChildren.org): How to Take Your Child’s Temperature
- AAP (HealthyChildren.org): Fever Without Fear
- Seattle Children’s: Fever
More Is This Normal? for the next long night.
