A fever in a baby is a rectal temperature of 100.4°F (38°C) or higher. This is the standard threshold used by the American Academy of Pediatrics and major children’s hospitals, regardless of the baby’s age. How urgently you need to act, though, depends heavily on how old your baby is.
The 100.4°F Threshold
Normal body temperature for infants falls between 97.7°F and 99.5°F (36.5°C to 37.5°C). Anything at or above 100.4°F (38°C), taken rectally, is considered a fever. This number isn’t arbitrary. It’s the cutoff that clinical guidelines use to determine whether a young infant needs medical evaluation, and it applies from birth onward.
A temperature slightly below that line, like 99.8°F or 100.1°F, is not technically a fever, but it may still signal that your baby’s body is fighting something. If your baby seems uncomfortable or unusually fussy at those readings, it’s reasonable to keep monitoring closely and recheck in 30 to 60 minutes.
Why Your Baby’s Age Changes Everything
A fever of 100.4°F in a two-week-old is treated very differently than the same fever in a nine-month-old. That’s because young infants have immature immune systems and fewer ways to show you they’re seriously ill. A fever can be the only visible sign of a dangerous bacterial infection.
For babies under 8 weeks old (roughly two months), any rectal temperature of 100.4°F or higher within the past 24 hours typically warrants immediate medical evaluation. The AAP breaks this down further into age windows of 8 to 21 days, 22 to 28 days, and 29 to 60 days, each with slightly different evaluation steps. But the bottom line for parents is the same: a fever in a baby younger than two months old is urgent. Don’t wait to see if it resolves on its own.
Babies who were born premature (before 37 weeks), spent time in the NICU, or have complex medical histories may carry additional risk even beyond the two-month mark. For babies older than three months, a fever is still worth monitoring, but you generally have more time to observe how your baby is acting before deciding on next steps.
How to Take an Accurate Temperature
A rectal thermometer gives the most reliable reading for babies under three months old, and it’s the method pediatricians trust most. Use a standard digital thermometer with a small amount of petroleum jelly on the tip, inserted about half an inch. The reading takes only a few seconds with most modern thermometers.
Forehead (temporal artery) thermometers are easy to use and work reasonably well for babies three months and older. For infants younger than three months who seem sick, a forehead reading can serve as a first check, but a rectal temperature is the one your pediatrician will want to know.
Ear thermometers are not recommended for babies under six months. A baby’s ear canal is too small and curved to produce consistent readings at that age. After six months, ear thermometers become more reliable.
Armpit readings are the least accurate method. Research comparing armpit and rectal temperatures in infants found the two can differ by as much as 1.0°C (1.8°F), and armpit readings caught only about 65% of true fevers detected rectally. If you take an armpit temperature and the number seems borderline or your baby looks unwell, follow up with a rectal reading to be sure.
What Counts as a Fever by Measurement Method
- Rectal: 100.4°F (38°C) or higher
- Forehead: 100.4°F (38°C) or higher
- Ear: 100.4°F (38°C) or higher (use only after 6 months)
- Armpit: 99°F (37.2°C) or higher may suggest a fever, but confirm with a rectal reading
Fever Reducers and Age Restrictions
Acetaminophen (Tylenol) can be given to babies who are at least two months old and weigh enough to receive a dose. Always go by your baby’s weight rather than age when measuring the amount, and use the syringe that comes with the product rather than a kitchen spoon.
Ibuprofen (Advil, Motrin) is not safe for babies under six months old. It has not been studied enough in that age group, and the FDA has not approved its use before six months. Once your baby is old enough, ibuprofen can be given every six to eight hours as needed. Avoid combination medications (products with more than one active ingredient) for children under six years old.
For babies younger than two months, do not give any fever-reducing medication before calling your pediatrician. The fever itself is the signal your doctor needs, and masking it can delay important evaluation.
Febrile Seizures
Some babies and toddlers have seizures triggered by fever, called febrile seizures. These occur most often between 6 months and 5 years of age, with the highest risk between 12 and 18 months. They’re frightening to witness, but the reassuring reality is that simple febrile seizures do not cause brain damage, intellectual disability, or learning problems. They also don’t mean your child has epilepsy. Most children who have one never have lasting effects from it.
Signs That Matter More Than the Number
Temperature alone doesn’t tell the whole story. A baby with a 101°F fever who is feeding well, making eye contact, and wetting diapers normally is in a very different situation than one with the same fever who is limp, refusing to eat, or difficult to wake. Pay attention to how your baby looks and behaves, not just the number on the thermometer.
Specific warning signs that call for prompt medical attention in any infant with a fever include unusual drowsiness or difficulty waking, a weak or high-pitched cry, refusing multiple feedings in a row, fewer wet diapers than usual, a rash that doesn’t fade when you press on it, and trouble breathing. A fever that persists beyond 24 hours in a baby under three months, or beyond two to three days in an older infant, also warrants a call to your pediatrician even if your baby seems otherwise okay.