If your baby has a fever of 100.4°F (38°C) or higher measured rectally, your first steps depend on age. A baby under 3 months old with any fever at or above that threshold needs medical evaluation right away, even if they seem fine otherwise. For older babies, most fevers can be managed safely at home with fluids, light clothing, and close monitoring.
How to Take an Accurate Temperature
A rectal temperature is the gold standard for babies. It’s the most accurate method, and it’s what your pediatrician will ask about when you call. Use a regular digital thermometer with a small amount of petroleum jelly on the tip, inserted about half an inch. If the reading is 100.4°F or higher, that’s a fever.
Temporal artery thermometers (the ones you swipe across the forehead) can also provide accurate readings, even in newborns. Ear thermometers aren’t recommended until at least 6 months of age. Armpit readings work in a pinch for babies over 3 months, but they tend to run lower than the actual core temperature. If you use any method other than rectal and the number seems off, follow up with a rectal reading to be sure.
Why Age Changes Everything
For babies under 3 months, a fever of 100.4°F or higher is treated as a medical urgency. Young infants have immature immune systems, and a fever can be the only visible sign of a serious bacterial infection. The American Academy of Pediatrics has specific clinical guidelines for evaluating febrile infants as young as 8 days old, and the workup can include blood tests, urine cultures, and sometimes a spinal fluid check. This isn’t meant to frighten you. It’s standard practice because infections at this age can escalate quickly, and early treatment makes a significant difference. Don’t wait to see if the fever resolves on its own. Call your pediatrician immediately or go to the emergency room.
For babies 3 to 6 months old, a fever of 102°F or higher, or a lower fever that lasts more than a day, warrants a call to your doctor. At this age, many fevers are caused by common viral infections, but your pediatrician may want to examine your baby to rule out anything more serious.
For babies older than 6 months, fever is usually less concerning on its own. What matters more is how your baby is acting. A baby who is still making eye contact, taking fluids, producing wet diapers, and responding to you is generally fighting off a routine infection. A baby who is limp, difficult to wake, inconsolable, or refusing all fluids needs medical attention regardless of the number on the thermometer.
Keeping Your Baby Comfortable at Home
Fever is your baby’s immune system doing its job, so the goal isn’t necessarily to eliminate the fever entirely. It’s to keep your baby comfortable and hydrated while their body fights the infection.
Dress your baby in a single layer of lightweight, breathable cotton, like a onesie or light pajamas. If it’s bedtime, pair that with a low-TOG sleep sack (0.2 to 0.5 TOG, depending on room temperature). Skip extra blankets, socks, and layers. Bundling a feverish baby traps heat and can actually push the temperature higher. If the room is above 77°F, a diaper and thin cotton onesie may be all they need. For younger babies, always use a sleep sack instead of loose blankets.
Offer frequent feedings. Breast milk, formula, or (for babies over 6 months) small sips of water all help replace the extra fluid lost through sweating and faster breathing. Watch for signs of dehydration: fewer than four wet diapers in 24 hours, no tears when crying, a dry mouth, or a sunken soft spot on top of the head.
A lukewarm sponge bath can provide some temporary relief. Never use cold water or rubbing alcohol, both of which can cause shivering (which raises core temperature) or be directly harmful.
When Fever Reducers Are Appropriate
Acetaminophen (Tylenol) can be given to babies 8 weeks and older. Ibuprofen (Motrin, Advil) is not safe for babies under 6 months. Both are dosed by weight, not age, so check the packaging carefully or ask your pharmacist. Never give aspirin to any child or teenager, as it’s linked to a rare but serious condition called Reye’s syndrome.
You don’t need to give a fever reducer every time the thermometer reads above 100.4°F. If your baby is sleeping comfortably or feeding normally, letting the fever run its course is perfectly reasonable. Medication is most helpful when the fever is making your baby visibly miserable, unable to sleep, or unwilling to drink.
Post-Vaccination Fevers
A mild fever after routine vaccinations is common and generally not a cause for concern. Depending on the vaccine, it can show up within hours or, in the case of the chickenpox vaccine, as late as two to four weeks afterward. Most post-vaccination fevers last about one day and resolve without treatment. If the fever is above 102°F, persists beyond 48 hours, or your baby seems unusually unwell, call your pediatrician to be safe.
Teething Does Not Cause True Fevers
Teething can nudge your baby’s temperature slightly above normal, but it won’t push it to 100.4°F or higher. That threshold defines a true fever, and a true fever signals an infection, not a tooth breaking through. If your teething-age baby has a genuine fever, look for another cause rather than attributing it to their gums. This is one of the most common misconceptions in infant care, and it can lead parents to delay seeking help for an actual illness.
Febrile Seizures
About 3 to 4 out of every 100 children will experience a febrile seizure, a convulsion triggered by a rapid rise in body temperature. These are terrifying to witness but are almost always harmless and don’t cause lasting damage. They tend to occur between 6 months and 5 years of age.
If your baby has a seizure, place them on the floor or bed away from hard or sharp objects. Turn their head to one side so saliva or vomit can drain. Do not put anything in their mouth; they will not swallow their tongue. Time the seizure. If it stops within five minutes, call your pediatrician. If it lasts longer than five minutes, call 911.
Signs That Need Immediate Medical Attention
- Any fever in a baby under 3 months old (100.4°F or higher rectally)
- Difficulty breathing, including flared nostrils, grunting, or ribs visibly pulling in with each breath
- A rash that doesn’t fade when you press on it, especially small purple or red dots
- Extreme sleepiness or difficulty waking, beyond normal tiredness from being sick
- Refusing all fluids for more than a few hours, or signs of dehydration
- A bulging or sunken soft spot on the top of the head
- Inconsolable crying that doesn’t improve with holding, feeding, or comfort measures
- A seizure lasting more than 5 minutes
Trust your instincts. If something about your baby feels wrong to you, even if you can’t pinpoint exactly what it is, that’s reason enough to call your pediatrician or head to the emergency room. Parents notice subtle changes in their baby’s behavior that no checklist can fully capture.