A fever in an infant is defined as a rectal temperature of 100.4°F (38°C) or higher. This is the standard threshold used by pediatricians, emergency departments, and the American Academy of Pediatrics. Because infants can’t hold an oral thermometer in place, a rectal reading is the gold standard for babies, and the number that matters most when deciding whether your child needs medical attention.
Fever Thresholds by Measurement Method
The number that qualifies as a fever depends on where you take the temperature. A rectal, ear, or temporal artery (forehead) reading of 100.4°F or higher is a fever. An oral temperature hits fever at 100°F (37.8°C), and an armpit reading qualifies at 99°F (37.2°C).
Armpit temperatures are the least accurate of the three and can read lower than the actual core body temperature. If you get an armpit reading that seems borderline, confirm it with a rectal thermometer before making decisions. For infants under 3 months old, a rectal temperature is especially important because even small differences in accuracy can change what happens next.
Why Age Changes Everything
A fever of 100.4°F means very different things depending on how old your baby is. In a 10-month-old, it usually signals a routine viral infection. In a newborn, the same number can indicate a serious bacterial infection that needs immediate evaluation.
If your baby is younger than 2 months and has a rectal temperature of 100.4°F or higher, go to an emergency department immediately. Newborns have immature immune systems, so their bodies can’t fight infections the way older children can. A fever at this age sometimes points to a urinary tract infection, which affects more than 10% of febrile infants. More dangerous infections like meningitis are far rarer (fewer than 0.05% of cases), but the consequences are severe enough that hospitals evaluate young infants aggressively.
The AAP breaks its clinical guidelines into three age windows: 8 to 21 days, 22 to 28 days, and 29 to 60 days. Each group has its own evaluation steps because the risk of serious infection drops as infants get older. A 3-week-old with a fever will typically receive more testing than a 7-week-old with the same temperature.
How to Take a Rectal Temperature
Use a digital thermometer labeled for rectal use. Apply a small amount of petroleum jelly to the tip. Lay your baby face-down on your lap or face-up on a firm surface, then gently insert the tip about half an inch into the rectum. Hold it in place until the thermometer beeps, which usually takes 10 to 20 seconds. Clean the thermometer with rubbing alcohol or soap and water afterward, and label it so it doesn’t get mixed up with oral thermometers.
The reading you get is your baby’s core body temperature, which is why it’s more reliable than a forehead swipe or armpit check. If your baby is squirming too much to get a rectal reading safely, a temporal artery thermometer on the forehead is a reasonable backup for babies over 3 months.
What a Fever Actually Does
Fever is not an illness. It’s a response. When your baby’s immune system detects an invader like a virus or bacteria, it sends chemical signals to the hypothalamus, a small region deep in the brain that acts as the body’s thermostat. The hypothalamus raises the set point for body temperature, which is why your baby may feel cold or shiver even though their temperature is climbing. The higher temperature makes it harder for some pathogens to reproduce and helps immune cells work more efficiently.
This means a mild fever in an older infant is often the body doing exactly what it should. The number on the thermometer matters less than how your baby is acting. A baby with a 101°F fever who is alert, feeding, and making wet diapers is in a very different situation than a baby with a 100.5°F fever who is limp and unresponsive.
Warning Signs Beyond the Number
Certain symptoms alongside a fever signal that something more serious may be happening. Watch for:
- Unusual sleepiness or floppiness: your baby is harder to wake than normal, or feels limp when you pick them up
- Dehydration: fewer wet diapers, crying without tears, dry mouth, or a sunken soft spot on the head
- Skin color changes: skin or lips that look blue, purple, or gray
- Breathing difficulty: rapid breathing, grunting, or visible effort with each breath
- A rash: especially one that appears suddenly, blisters, or looks infected
- Persistent distress: pain or fussiness that keeps getting worse or won’t respond to comfort
Any of these symptoms alongside a fever warrants immediate medical attention regardless of your baby’s age.
Fever-Reducing Medication by Age
Acetaminophen (Tylenol) is the only fever reducer approved for infants under 6 months. Ibuprofen (Advil, Motrin) is not considered safe for babies younger than 6 months and is not FDA-approved for that age group. Once your baby is 6 months or older, either medication can be used.
Dosing for both medications is based on your baby’s weight, not age. If you don’t know your baby’s current weight, age can serve as a rough guide, but weight is more accurate. Ibuprofen can be given every 6 to 8 hours. Keep in mind that the goal of fever-reducing medication isn’t to bring the temperature back to normal. It’s to make your baby more comfortable so they can rest and feed. A baby who is sleeping comfortably with a low fever doesn’t necessarily need medication at all.
For any infant under 2 months with a rectal temperature at or above 100.4°F, do not give medication and wait to see what happens. Head to an emergency department first. The fever itself is the signal that needs evaluation at that age.