A regular body temperature for a baby is between 97.7°F and 99.5°F (36.5°C to 37.5°C) when measured rectally. This range holds for newborns through the first year of life, though readings will shift slightly depending on where and how you take the measurement. A temperature of 100.4°F (38°C) or higher is considered a fever in infants.
Normal Range by Measurement Method
Rectal temperature is considered the most accurate way to measure a baby’s core body temperature, and it’s the method most pediatricians recommend for infants. The standard normal range of 36.5°C to 37.5°C (97.7°F to 99.5°F) is based on rectal readings. Research from the World Health Organization and confirmed by clinical studies shows that healthy newborns consistently settle into this range within the first 24 hours of life.
Armpit (axillary) readings run lower than rectal, often by half a degree or more. That means an armpit reading of 97.2°F might be perfectly normal even though it looks low. Forehead (temporal artery) thermometers fall somewhere in between. Ear thermometers are generally not recommended for babies under six months because the ear canal is too small for an accurate reading.
If you’re using anything other than a rectal thermometer, keep in mind that the number you see is an estimate. For babies under three months, a rectal reading gives you the most reliable picture.
What Counts as a Fever
A rectal, ear, or forehead temperature of 100.4°F (38°C) or higher is the clinical definition of a fever in babies. This threshold is consistent across guidelines from the Mayo Clinic, the American Academy of Pediatrics, and major children’s hospitals. It applies regardless of your baby’s age.
What changes with age is how seriously that fever needs to be taken. For babies under three months old, a rectal temperature of 100.4°F or higher is treated as a medical urgency. Young infants have immature immune systems, and it’s difficult to tell from outward symptoms alone whether a fever comes from a harmless virus or an early serious bacterial infection like meningitis or a bloodstream infection. The AAP recommends that all febrile infants 8 to 21 days old with a temperature at or above 100.4°F receive immediate medical evaluation and treatment. For babies 22 to 60 days old, guidelines become slightly more flexible based on how well the baby appears, but the fever still warrants prompt medical attention.
For babies 3 months to 3 years, the threshold for concern is a bit higher. A temperature of 102°F (38.9°C) or above, measured rectally or by forehead or ear, is the point where most guidelines recommend contacting your pediatrician.
Why the Number on the Thermometer Can Mislead You
A baby’s temperature is surprisingly sensitive to external conditions. In one controlled study, researchers wrapped healthy newborns in five blankets with a hat in a warm room (about 80°F). Within two and a half hours, rectal temperatures rose by an average of 0.56°C (about 1°F), climbing at a steady rate of roughly half a degree Fahrenheit per hour with no sign of leveling off. Two of the twelve bundled babies actually hit the 100.4°F fever threshold, despite being completely healthy.
This means that if your baby feels warm and registers a borderline temperature, it’s worth considering whether they’ve been overdressed, swaddled in heavy blankets, or sitting in a warm car seat. Try removing a layer, waiting 15 to 20 minutes, and retaking the temperature before drawing conclusions.
Normal body temperature also fluctuates throughout the day. It tends to be lowest in the early morning and highest in the late afternoon. A reading of 99.3°F at 5 p.m. is not the same thing as 99.3°F at 6 a.m. Both can be normal, but the evening reading is more expected.
How to Take a Rectal Temperature
Lay your baby face-down across your lap or face-up on a firm surface. Apply a small amount of petroleum jelly to the tip of a digital rectal thermometer. Gently insert the tip about half an inch to one inch into the rectum. Hold it in place until the thermometer beeps, which usually takes about 15 to 30 seconds with modern digital models. Label this thermometer so it’s only used rectally.
If you’re uncomfortable with rectal readings, a forehead thermometer is a reasonable alternative for screening. Just know that if the forehead reading looks elevated, confirming with a rectal measurement gives you the most accurate number to share with your pediatrician.
Signs That Matter More Than the Number
Temperature is one data point. How your baby looks and acts often matters more than the exact reading on the thermometer. A baby with a temperature of 100.2°F who is limp, won’t wake up, or refuses to eat is more concerning than a baby at 101°F who is fussy but alert and feeding.
Symptoms that signal something serious include difficulty breathing, a stiff neck, skin that looks mottled or has purple spots, inconsolable crying (especially when touched or moved), or being unusually difficult to wake. Shaking chills lasting more than 30 minutes, signs of dehydration (no wet diaper in over eight hours, no tears when crying, very dry mouth), or a baby who simply looks “out of it” also warrant immediate medical evaluation.
For a baby under three months, trust the thermometer over your instincts in one specific way: even if the baby looks fine, a rectal temperature of 100.4°F or higher needs a call to your pediatrician right away. Young infants can have serious infections without showing obvious signs of illness, which is why the number alone triggers action at this age.