A fever in a baby is a rectal temperature of 100.4°F (38°C) or higher. That single number is the standard threshold used by pediatricians regardless of whether your baby is two weeks old or ten months old, though what you should do about it depends heavily on your baby’s age.
The Temperature That Counts as a Fever
The specific number that defines a fever changes depending on how you take it. A rectal, ear, or forehead (temporal artery) reading of 100.4°F or higher is a fever. An oral reading of 100°F or higher qualifies. An armpit reading of 99°F or higher suggests a fever, though armpit readings are the least reliable.
For babies under 3 months, rectal temperature is the gold standard. Ear thermometers and forehead scanners are convenient for older babies, but for newborns and very young infants, a rectal reading gives the most accurate result. If your baby is under 3 months and you get a concerning reading from a forehead or armpit thermometer, follow up with a rectal check before making decisions.
Why Age Changes Everything
A fever of 100.4°F in a one-month-old and the same fever in a one-year-old are very different situations. Young infants have immature immune systems, which means infections can escalate quickly and are harder to detect by appearance alone. A baby can look relatively fine and still have a serious bacterial infection brewing. That’s why age-based guidelines exist:
- Under 3 months: Any rectal temperature of 100.4°F or higher needs prompt medical evaluation. Don’t wait, don’t give fever-reducing medicine first to “see if it helps.” This is the age group where doctors take the most aggressive approach to ruling out serious infections.
- 3 to 6 months: A temperature up to 101°F is worth calling your pediatrician about, especially if your baby seems unusually irritable, sluggish, or uncomfortable. A temperature above 101°F warrants a call regardless of how your baby is acting.
- 6 to 24 months: A temperature above 101°F that lasts longer than one day without other symptoms deserves medical attention. If your baby has additional symptoms like persistent vomiting, a rash, or difficulty breathing, call sooner.
The American Academy of Pediatrics has detailed clinical guidelines specifically for well-appearing infants between 8 and 60 days old with fevers. Doctors evaluating babies this young typically run blood tests, urine tests, and sometimes a spinal fluid analysis to check for bacterial infections. That sounds alarming, but it reflects how seriously the medical community treats fevers in this age group, not how likely a worst-case scenario is.
What Causes Fevers in Babies
The vast majority of fevers in babies come from viral infections. Common colds (caused by over 100 different viruses), RSV, influenza, rotavirus, and adenovirus are frequent culprits. Viral fevers tend to come with widespread symptoms: runny nose, cough, general fussiness, and low-grade temperatures.
Bacterial infections are less common but more likely to need treatment. Ear infections, urinary tract infections, and certain skin infections fall into this category. Bacterial fevers often produce more localized symptoms, like ear pain or a red, tender area on the skin, rather than the “sick all over” pattern of a virus.
Vaccines can also trigger a mild fever, usually within 24 to 48 hours of the shot. These fevers are typically low-grade and resolve on their own. However, if your baby is under 3 months and develops a fever after vaccination, the same rules apply: get it checked.
Why the Body Creates a Fever
Fever is not a malfunction. When your baby’s immune system detects an invader, immune cells release signaling molecules that travel to the brain’s temperature-control center. The brain then raises the body’s thermostat, creating a warmer environment that helps fight off infection. Fever plays an integral role in the immune response and, while uncomfortable, does not need to be treated in an otherwise healthy child solely to bring the number down. The goal of treating a fever is comfort, not hitting a specific temperature.
How to Take Your Baby’s Temperature
For babies under 3 months, use a digital rectal thermometer. Apply a small amount of petroleum jelly to the tip, insert it about half an inch, and hold it in place until it beeps. It takes less than a minute and gives the most accurate reading available.
For babies 3 months and older, a temporal artery (forehead) thermometer or a tympanic (ear) thermometer works well for everyday checks. Armpit readings are the easiest to take but consistently read lower than core body temperature, so a “normal” armpit reading doesn’t always mean no fever. If an armpit reading comes back at 99°F or above, confirm with a more accurate method.
Comfort Measures at Home
If your baby is over 3 months old and the fever isn’t in a range that requires immediate medical attention, you can focus on keeping them comfortable. Offer frequent breast milk or formula to prevent dehydration. Don’t give fruit juice. If your baby is vomiting, a pediatric electrolyte solution can help replace lost fluids.
Dress your baby in one layer of lightweight clothing. It’s tempting to bundle a shivering baby in blankets, but extra layers trap heat and can push the fever higher. Keep the room at a comfortable temperature, and use a fan if it’s stuffy. A lukewarm sponge bath can help bring mild relief, but avoid cold water, ice, or alcohol rubs, which trigger shivering and can actually raise core temperature.
For fever-reducing medication, acetaminophen (Tylenol) can be given to babies based on their weight rather than age. Ibuprofen (Motrin, Advil) should not be given to babies 6 months or younger. Acetaminophen can be given every 4 to 6 hours, with no more than 5 doses in 24 hours. Ibuprofen can be given every 6 to 8 hours, with no more than 4 doses in 24 hours, and it’s best given with food to reduce stomach upset. Always dose by your baby’s current weight, not age, and use the measuring device that comes with the medication rather than a kitchen spoon.
Signs of Dehydration to Watch For
Fevers increase fluid loss, and babies dehydrate faster than adults. Watch for fewer wet diapers than usual, or no wet diapers for three hours or more. A dry mouth, no tears when crying, sunken eyes, and a sunken soft spot on top of the head are all red flags. Skin that stays “tented” after a gentle pinch instead of flattening back immediately also signals dehydration. A baby who is unusually sleepy, limp, or difficult to wake needs prompt medical attention.
Febrile Seizures
Some children experience seizures triggered by fever, called febrile seizures. These are most common between 6 months and 5 years, with the highest risk between ages 1 and 3. A simple febrile seizure lasts a few seconds to 15 minutes and happens only once within a 24-hour period. A complex febrile seizure lasts longer than 15 minutes, occurs more than once in 24 hours, or affects only one side of the body.
Febrile seizures are frightening to witness, but simple febrile seizures do not cause brain damage or increase the risk of epilepsy. They can happen as a temperature is rising, sometimes before you even realize your child has a fever. If your child has a seizure, place them on their side on a safe surface, don’t put anything in their mouth, and time the episode. If it lasts longer than 5 minutes, call emergency services.