A fever in an infant is a rectal temperature of 100.4°F (38°C) or higher. This threshold applies from birth through the first few months of life and is the standard used by the American Academy of Pediatrics. But a fever in a newborn carries very different risks than a fever in an older baby, so understanding the age-specific context matters.
The 100.4°F Threshold
Pediatricians define an infant fever as a rectal temperature at or above 100.4°F (38°C). This number isn’t a rough guideline. It’s the specific cutoff that triggers different levels of medical evaluation depending on your baby’s age. A reading of 100.3°F is technically not a fever; 100.4°F is. That single tenth of a degree matters because it changes how a doctor will respond.
Normal body temperature in infants fluctuates throughout the day, typically ranging from about 97°F to 100.3°F. Factors like bundling in warm clothing, a recent bath, or time of day can push a reading slightly higher. If your baby feels warm but registers below 100.4°F rectally, that’s not considered a true fever.
Why Age Changes Everything
A fever of 100.4°F in a two-week-old baby is treated as a medical emergency. The same temperature in a six-month-old is usually far less urgent. The reason comes down to immune system development: very young infants don’t have the same ability to fight off infections, and a fever can be the only visible sign of a serious bacterial infection.
For babies under 28 days old, any fever at or above 100.4°F typically leads to an immediate and thorough workup. Doctors will draw blood, collect a urine sample, and often perform a lumbar puncture (spinal tap) to check for bacterial infections, including meningitis. They’ll also test for herpes simplex virus, which can be dangerous in newborns. This may sound aggressive, but newborns can deteriorate quickly, and these infections are treatable when caught early. Babies this young are almost always admitted to the hospital while results come back.
For babies between about one and two months old, the evaluation is similar but may be slightly less extensive. Doctors use blood markers to help gauge infection risk and decide whether a spinal tap is necessary. Premature babies, those with complex medical histories, or those who spent time in a neonatal intensive care unit are considered higher risk regardless of age.
After three months, a fever becomes less automatically alarming. Most fevers in this age group are caused by common viral infections and resolve on their own. Your baby’s behavior, feeding patterns, and overall appearance matter more than the number on the thermometer at this stage.
Why Rectal Temperature Is the Standard
For infants, a rectal thermometer is the most accurate way to measure core body temperature. Armpit (axillary) readings, forehead strips, and ear thermometers are convenient, but they can be unreliable in babies. Research comparing rectal and armpit readings in newborns found that while the average difference was small (about 0.1°C), individual readings could vary by as much as 0.7°C in either direction. That’s more than a full degree Fahrenheit of potential error, which is enough to miss a real fever or flag a false one.
The discrepancy is even larger in babies whose body temperature is already abnormal. In those cases, armpit readings diverged from rectal readings by up to 1°C. For a baby near the 100.4°F cutoff, that kind of inaccuracy can change whether or not they get the evaluation they need. If you take an armpit or forehead reading and it seems elevated, follow up with a rectal reading before calling your pediatrician, so you can report the most reliable number.
How to Take a Rectal Temperature
Lubricate the tip of a digital rectal thermometer with petroleum jelly. Lay your baby face-down on your lap or face-up on a firm surface. Gently insert the tip about half an inch to one inch into the rectum and hold it in place until the thermometer beeps. The whole process takes about 15 to 30 seconds with a modern digital thermometer. Label this thermometer so it’s only used rectally, and clean it with rubbing alcohol or soap and water after each use.
Fever Reducers for Infants
Acetaminophen (Tylenol) is the only fever-reducing medication recommended for babies under six months old. Ibuprofen (Advil, Motrin) should not be given to infants younger than six months unless specifically directed by a doctor. Dosing for both medications is based on your baby’s weight, not their age, so always check the weight-based dosing chart on the package or ask your pediatrician for the correct amount.
For babies under three months, do not give any fever-reducing medication before speaking with a doctor. The fever itself is the signal that prompts evaluation, and reducing it before a medical assessment can mask important information. The goal at that age is not to treat the fever at home. It’s to get the baby seen.
Signs That Need Immediate Attention
Beyond the thermometer reading, certain symptoms in a feverish infant signal a need for urgent care. Watch for difficulty breathing, skin or lips that appear blue, purple, or gray, and any change in consciousness such as unusual drowsiness or difficulty waking your baby. Persistent vomiting, refusal to feed, a bulging soft spot on the head, and inconsolable crying that doesn’t improve with comfort are also red flags.
A fever that lasts more than two to three days in an older infant, or one that keeps climbing despite fever reducers, warrants a call to your pediatrician even if your baby otherwise seems okay. Most viral fevers peak within the first 48 hours and gradually improve, so a fever that persists beyond that window or returns after seeming to resolve may point to a secondary infection.
What a Fever Actually Does
Fever is not an illness. It’s the body’s immune response to infection. When your baby’s immune system detects a virus or bacteria, it raises the body’s internal thermostat to create an environment that’s less hospitable to the invader. In older infants, a fever on its own is generally not dangerous and is actually a sign that the immune system is working. The concern with very young babies isn’t the fever itself but rather what might be causing it, since their immune systems aren’t yet mature enough to reliably contain serious infections.
The height of the fever doesn’t reliably predict how serious the underlying illness is. A baby with a 104°F fever from a common virus can be more alert and comfortable than a baby with a 101°F fever from a bacterial infection. How your baby looks and acts is a better gauge of severity than the number alone, especially after the first few months of life.