A fever in a baby is a rectal temperature of 100.4°F (38°C) or higher. That single number applies across all infant ages, from newborns through toddlerhood, and it’s the threshold used by the American Academy of Pediatrics and most pediatric guidelines. What changes with age isn’t the definition of a fever but how urgently it needs to be evaluated.
How to Get an Accurate Reading
For babies under three months, a rectal thermometer is the most reliable way to check. Armpit readings are the least accurate method and can underestimate a true fever. If you get a borderline armpit reading, follow up with a rectal measurement to confirm. Digital ear thermometers work in older babies but can be thrown off by earwax or a small ear canal.
The fever thresholds vary slightly by method:
- Rectal, ear, or forehead (temporal artery): 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
For babies, rectal is the standard. When you call your pediatrician about a fever, they’ll ask for a rectal temperature specifically, so it’s worth having a dedicated rectal thermometer at home and being comfortable using it before your baby gets sick.
Why Your Baby’s Age Matters More Than the Number
A 100.4°F fever in a two-week-old is a completely different situation than the same temperature in a nine-month-old. That’s because very young babies have immature immune systems, and a fever can be the only visible sign of a serious bacterial infection. The younger the baby, the more aggressively the fever needs to be investigated.
For babies under 28 days old, any fever at or above 100.4°F typically means a trip to the emergency department. At this age, doctors will run a full workup, including blood tests, urine tests, and often a spinal tap, because infections like meningitis can progress rapidly and present with no symptoms other than fever.
For babies one to three months old, the evaluation is still thorough but somewhat less aggressive. Your pediatrician will likely want to see the baby promptly. Blood and urine tests are standard, though additional testing depends on how the baby looks and acts.
For babies three months and older, a fever alone is less alarming. At this age, doctors focus more on identifying a source (ear infection, viral illness, urinary tract infection) and on how your baby is behaving. A baby over three months who is feeding well, making eye contact, and acting relatively normal with a moderate fever is in a very different category than one who is limp and unresponsive.
Signs That Matter More Than the Thermometer
Parents often fixate on the exact number, but a baby’s behavior is a more telling indicator of how sick they are. A baby with a 101°F fever who is alert, feeding, and making wet diapers is generally in better shape than a baby with 100.5°F who won’t wake up or refuses to eat.
Watch for these red flags regardless of temperature:
- Unusual sleepiness: Sleeping far more than normal or being very difficult to wake up
- Floppiness or limpness: A noticeable loss of muscle tone when you pick them up
- Dehydration signs: Fewer wet diapers, crying without tears, dry mouth, or a sunken soft spot on the head
- Breathing trouble: Rapid breathing, grunting, or visible effort with each breath
- Color changes: Skin or lips that look blue, purple, or gray
- Inconsolable crying: Pain or fussiness that keeps getting worse or won’t respond to comfort
Any of these symptoms warrants immediate medical attention, whether or not a thermometer confirms a fever.
Febrile Seizures
Febrile seizures happen in about 2% to 5% of children and are one of the most frightening things a parent can witness. They’re triggered by a rapid rise in body temperature, not necessarily a high peak, and they most commonly occur between six months and five years of age.
Most febrile seizures are “simple,” meaning they last under 15 minutes, and the vast majority end on their own within a few minutes. During a seizure, place your baby on their side, keep the area around them clear, and do not put anything in their mouth or try to restrain them. Once it stops, your baby will likely be very sleepy, which is normal. If a seizure lasts longer than five minutes, call 911, because longer seizures are unlikely to stop without medical intervention.
Simple febrile seizures, while terrifying, do not cause brain damage and do not mean your child has epilepsy. They are a relatively common response to fever in young children.
Treating a Baby’s Fever Safely
Fever itself isn’t dangerous in most cases. It’s the body’s way of fighting infection. The goal of treating a fever isn’t to eliminate it entirely but to keep your baby comfortable enough to sleep, eat, and stay hydrated.
Acetaminophen (Tylenol) can be given to babies two months and older. Ibuprofen (Motrin, Advil) is only safe for babies six months and older. Both are dosed by weight, not age, so check the packaging carefully or ask your pediatrician for the correct dose. Never give aspirin to a baby or child.
Beyond medication, keep your baby in light clothing, offer frequent feedings to prevent dehydration, and keep the room at a comfortable temperature. Lukewarm baths can help but avoid cold water or alcohol rubs, which can cause shivering and actually raise core body temperature. If your baby is under three months old, do not give fever-reducing medication without calling your pediatrician first, because the priority at that age is identifying the cause of the fever, not masking it.