A rectal temperature of 100.4°F (38°C) or higher means your baby has a fever. What you do next depends mostly on your baby’s age. For any baby under 3 months old, a fever at or above that threshold needs immediate medical attention, even if your baby looks fine. For older babies, you can often manage the fever safely at home while watching for specific warning signs.
Why Age Changes Everything
A fever is the body fighting an infection, and in older children that’s usually a good sign. But very young infants have immature immune systems, which means infections can escalate quickly and don’t always show obvious symptoms. A newborn can have a serious bacterial infection and still look relatively normal. That’s why the medical response is so different depending on how old your baby is.
Under 3 months: Any rectal temperature of 100.4°F (38°C) or higher warrants a call to your pediatrician or a trip to the emergency room. Don’t give fever-reducing medication first and wait to see what happens. For babies in this age range, especially those under 28 days old, doctors will typically run blood and urine tests to rule out bacterial infections. Hospitalization for monitoring is standard for the youngest newborns because serious infections like meningitis, while uncommon, can be difficult to detect from appearance alone.
3 to 6 months: A temperature up to 101°F (38.3°C) doesn’t automatically require emergency care, but contact your pediatrician if your baby seems unusually irritable, lethargic, or uncomfortable. Any temperature above 101°F at this age also warrants a call.
6 to 24 months: A fever above 101°F (38.3°C) that lasts longer than one day without other symptoms is worth reporting to your doctor. If your baby is acting mostly normal, drinking well, and making eye contact, you have more room to manage the fever at home.
How to Take an Accurate Temperature
For babies under 3 months, a rectal thermometer is the gold standard. Forehead and ear thermometers are convenient, but they’re less reliable in very young infants, and the difference of even half a degree matters when you’re right at the 100.4°F threshold. To take a rectal temperature, apply a small amount of petroleum jelly to the tip, gently insert it about half an inch, and hold it in place until the thermometer beeps.
For babies older than 3 months, a temporal (forehead) thermometer is a reasonable option for everyday monitoring. If you get a borderline reading and need precision, follow up with a rectal reading. Armpit temperatures tend to run lower than the true core temperature, so they’re the least reliable method.
Comfort Measures That Help
External cooling methods like lukewarm baths or wrapping your baby’s legs in damp towels cool the skin’s surface but don’t actually reduce the internal fever. That said, they can still make your baby feel more comfortable, which is the real goal. A damp sponge or cloth dabbed on the skin, lighter clothing, and lighter bedding all help. The key is watching your baby’s cues: if they start shivering or seem uncomfortable from the cooling, stop. Shivering actually raises body temperature.
Don’t bundle a feverish baby in extra layers. A single light layer of clothing is enough. Keep the room at a comfortable temperature and skip heavy blankets.
Keeping Your Baby Hydrated
Fever increases fluid loss, so hydration is one of the most important things you can control at home. For babies under 6 months, that means offering breast milk or formula more frequently than usual. For babies over 6 months who are eating solids, small sips of water between feedings help too.
Watch for these signs of dehydration, which mean you need medical attention:
- Fewer wet diapers: No wet diaper for three hours or longer is a red flag.
- No tears when crying.
- Sunken soft spot on top of the head.
- Sunken eyes or cheeks.
- Dry mouth.
When Fever-Reducing Medication Is Appropriate
Acetaminophen (Tylenol) can be given to babies 3 months and older. Ibuprofen (Advil, Motrin) should not be given to any baby under 6 months old unless specifically directed by a doctor. Both medications are dosed by weight, not age, so always check the packaging or ask your pharmacist if you’re unsure about the correct amount.
The goal of medication isn’t to bring the temperature back to normal. It’s to make your baby comfortable enough to rest and drink fluids. A baby whose fever drops from 103°F to 101°F but who is now nursing well and sleeping is a good outcome. Never give aspirin to an infant or child, as it’s linked to a rare but serious condition affecting the liver and brain.
Warning Signs That Need Immediate Attention
Beyond the age-specific thresholds above, certain symptoms alongside a fever mean you should head to the emergency room rather than wait for a callback from your pediatrician:
- Difficulty breathing or breathing that looks labored.
- Blue, purple, or gray tint to the skin or lips.
- Extreme sleepiness: Your baby is unusually hard to wake up, seems floppy, or won’t make eye contact.
- Seizure (see below).
- A rash that appears suddenly, blisters, or looks infected.
- Persistent fussiness or pain that won’t ease with comfort measures.
- Trouble swallowing, especially with excessive drooling.
What to Know About Febrile Seizures
Febrile seizures are convulsions triggered by fever, most common in children between 6 months and 5 years old, with the peak risk between ages 1 and 3. They usually happen when a temperature spikes rapidly, not necessarily at the highest point of a fever. A simple febrile seizure lasts a few seconds to 15 minutes and occurs only once in a 24-hour period.
Watching your baby have a seizure is terrifying, but simple febrile seizures do not cause brain damage or long-term health problems. During a seizure, lay your baby on a flat surface on their side, don’t put anything in their mouth, and time the seizure if you can. If it lasts longer than five minutes, call 911. Even for shorter seizures, contact your pediatrician afterward so they can evaluate whether any follow-up is needed.
Prolonged febrile seizures lasting more than 30 minutes do carry a small increased risk of epilepsy later, particularly in children who already have developmental or neurological conditions. But for the vast majority of children, a single febrile seizure is a one-time event with no lasting consequences.