A fever in an infant 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 the temperature is taken rectally, in the ear, or with a temporal artery thermometer. Anything below 100.4°F is considered an elevated temperature but not a true fever.
Why 100.4°F Is the Cutoff
A healthy infant’s normal body temperature hovers around 98.6°F but fluctuates throughout the day. It tends to be lower in the morning and slightly higher in the late afternoon or after a warm bath, a feeding, or active crying. These normal swings can push a reading up to about 100°F without meaning anything is wrong.
The 100.4°F threshold exists because temperatures at or above that point reliably signal that the body’s immune system is mounting a response to something, whether a virus, bacterial infection, or recent vaccination. Below that line, the elevation is too small to distinguish from normal daily variation.
How to Get an Accurate Reading
For babies under 3 months old, a rectal thermometer is the most reliable method. Other approaches, like armpit (axillary) readings, can run about a degree lower than the true core temperature and may give you a falsely reassuring number at exactly the age when accuracy matters most. Digital rectal thermometers give a result in under a minute.
For older infants, ear (tympanic) and forehead (temporal artery) thermometers are reasonable alternatives. If you get a borderline reading with one of these, a rectal check can confirm it. Forehead strip thermometers and pacifier thermometers are not accurate enough to rely on.
Age Changes Everything
The number that defines a fever stays the same, but what it means depends heavily on your baby’s age.
- Under 3 months: Any rectal temperature of 100.4°F or higher needs immediate medical evaluation, even if the baby looks fine. At this age the immune system is still immature, and a fever can be the only visible sign of a serious bacterial infection. Pediatricians treat this as urgent.
- 3 to 6 months: A fever still warrants a call to your pediatrician promptly, though the urgency is somewhat less than in a newborn. Your doctor will likely want to know how the baby is acting, feeding, and whether other symptoms are present.
- Over 6 months: Fever is more common and often tied to routine viral illnesses. The baby’s behavior, hydration, and overall appearance matter more than the exact number on the thermometer.
How Long a Fever Should Last
For children under 2 years old, a fever lasting more than 24 hours is worth a call to the pediatrician, even if the child seems otherwise okay. For children 2 and older, that window extends to 72 hours. These timelines assume the child is drinking fluids, producing wet diapers, and not showing worrisome symptoms. A fever that keeps climbing above 104°F or returns repeatedly after going away deserves attention sooner.
Teething Does Not Cause a True Fever
This is one of the most common misconceptions. Research shows that teething can cause a slight uptick in body temperature, but it stays below the 100.4°F threshold. If your baby is cutting a tooth and the thermometer reads 100.4°F or higher, something else is going on. Teething and illness often overlap in timing simply because babies teethe frequently during the same months they start encountering more infections.
Febrile Seizures
Some children between 6 months and 5 years old experience seizures triggered by fever. These febrile seizures are frightening to watch but are typically brief and do not cause lasting harm. They can happen at any fever level at or above 100.4°F, and they tend to occur as the temperature rises rapidly rather than after it has been elevated for a while. A child who has one febrile seizure has a slightly higher chance of having another with future fevers, but febrile seizures do not mean a child has epilepsy.
Managing a Fever at Home
For babies older than 3 months, fever itself is not dangerous. It’s part of the body’s defense against infection. The goal of treatment is comfort, not hitting a specific number on the thermometer.
Acetaminophen (Tylenol) can be given to infants 3 months and older, dosed by weight. Ibuprofen (Advil, Motrin) is only safe for babies 6 months and older. You can give ibuprofen every 6 to 8 hours as needed. Always use the dosing syringe that comes with the product rather than a kitchen spoon, and go by weight rather than age when possible for the most accurate dose.
Lightweight clothing, a comfortable room temperature, and plenty of fluids (breast milk, formula, or an electrolyte solution for older babies) all help. Lukewarm sponge baths can offer temporary relief, but avoid cold water or rubbing alcohol, both of which can make things worse.
Signs That Need Immediate Attention
Beyond the thermometer reading, certain behaviors signal that a fever may be part of something serious:
- Unusual sleepiness or difficulty waking: A baby who is hard to rouse or won’t make eye contact.
- Refusing fluids: Especially if wet diapers drop noticeably, fewer than three in 24 hours for a young infant.
- A rash that doesn’t fade when you press on it, particularly small purple or red dots.
- Trouble breathing: Fast breathing, flaring nostrils, or skin pulling in between the ribs with each breath.
- Seizures: Any shaking or stiffening episode.
- A bulging soft spot on top of the head.
Any of these, with or without a high number on the thermometer, calls for prompt medical care. A baby who has a moderate fever but is alert, feeding, and making wet diapers is in a very different situation from one who is limp and unresponsive at the same temperature. Trust what you observe, not just the number.