A baby has a fever when their rectal temperature reaches 100.4°F (38°C) or higher. That single number is the universal threshold used by pediatricians, but what counts as urgent depends heavily on your baby’s age. A 100.4°F reading in a two-week-old newborn is a medical emergency, while the same temperature in a nine-month-old is usually manageable at home.
The 100.4°F Threshold
Regardless of your baby’s age, a rectal temperature of 100.4°F (38°C) is the line between normal body heat and a true fever. Readings taken from other spots on the body use slightly different cutoffs: 99°F (37.2°C) from the armpit and 100.4°F from the forehead. These thresholds exist because normal body temperature fluctuates throughout the day, and anything below 100.4°F rectally falls within that normal range.
One common source of confusion is teething. Research has shown that while teething can cause a slight uptick in body temperature, it doesn’t push a baby’s temperature to 100.4°F or above. If your teething baby hits that number, something else is going on.
Why Your Baby’s Age Changes Everything
For babies under 3 months old, any fever of 100.4°F or higher needs immediate medical attention. A newborn’s immune system is immature, and fever at this age can signal a serious bacterial infection that’s hard to detect from symptoms alone. Hospitals follow strict protocols for febrile infants under 90 days, and the youngest babies (under 21 days old) are typically admitted for observation even when they look perfectly fine. Don’t give fever-reducing medicine and wait it out at this age. Call your pediatrician or go to the emergency room right away.
For babies 3 months to 3 years old, the concern level shifts. Johns Hopkins Medicine notes that a rectal, forehead, or ear temperature of 102°F (38.9°C) or higher in this age group warrants a call to your pediatrician. A temperature between 100.4°F and 102°F in an otherwise happy, feeding baby can often be monitored at home, though you should still contact your doctor if it lasts more than a day or two, or if your baby seems unusually fussy or lethargic.
How to Get an Accurate Reading
A rectal thermometer is the gold standard for babies, especially those younger than 3 months. It gives the most accurate core body temperature. To take a rectal reading, apply a small amount of petroleum jelly to the tip, gently insert it about half an inch, and hold it in place until it beeps. It’s uncomfortable for both of you, but the accuracy matters when you’re deciding whether to head to the ER at 2 a.m.
Forehead (temporal) thermometers are nearly as accurate as rectal ones and much easier to use on a squirmy baby. They’re a reasonable option for babies over 3 months. Ear thermometers aren’t reliable until a baby is at least 6 months old because the ear canal is too small for an accurate reading before that. Armpit readings are the least accurate of all methods, so treat them as a rough screening. If an armpit reading looks elevated, confirm it rectally.
When you call your pediatrician, report the exact number on the thermometer and how you took it. Saying “101.2 rectally” gives your doctor much more useful information than “she felt warm.”
Bringing a Fever Down at Home
For babies 3 months and older, acetaminophen (Tylenol) is the standard fever reducer. Dosing is based on your baby’s weight, not their age, so check the package carefully or ask your pediatrician for the right amount. Ibuprofen (Motrin, Advil) is not safe for babies under 6 months old.
Beyond medication, keep your baby lightly dressed and offer extra fluids. Breast milk, formula, or small sips of water (if your baby is old enough for water) help replace what they’re losing through sweat. Avoid bundling them in heavy blankets, which can trap heat and push their temperature higher. A single light layer is enough, even if they feel chilly to you. Don’t use cold packs on babies or young children since they can’t tell you when something is too cold against their skin.
Signs of Dehydration to Watch For
Fever speeds up fluid loss, and babies dehydrate faster than adults. The most practical thing to track is wet diapers. A baby should produce at least six wet diapers a day. If you’re seeing fewer than that, your baby may be getting dehydrated. Severe dehydration looks like only one or two wet diapers in a full day, a sunken soft spot on the top of the head, and no tears when crying. Any of these signs in a baby with a fever means they need medical attention promptly.
Febrile Seizures
Some children have seizures triggered by fever, called febrile seizures. They’re most common between 6 months and 5 years old, with the highest risk between ages 1 and 3. During a febrile seizure, a child may lose consciousness, shake or stiffen in their arms and legs, roll their eyes back, or lose bladder control. It’s terrifying to witness, but most febrile seizures are brief and don’t cause lasting harm.
Febrile seizures can happen as a fever is rising rapidly, sometimes before you even realize your child has a fever. They’re not caused by a specific temperature. If your baby has a seizure, lay them on their side on a safe surface, don’t put anything in their mouth, and time the seizure if you can. Call your pediatrician afterward, and call 911 if the seizure lasts longer than five minutes.
Quick Reference by Age
- Under 3 months: 100.4°F (38°C) rectally or higher. Seek immediate medical care, no matter how well the baby looks.
- 3 to 6 months: 100.4°F or higher. Call your pediatrician. Use acetaminophen only (no ibuprofen yet). Head to the ER for 102°F or higher, or if the baby is unusually sleepy or irritable.
- 6 to 12 months: 100.4°F or higher. Call your pediatrician for temperatures of 102°F or above, or if the fever persists beyond a day. Acetaminophen or ibuprofen are both options at this age.
- 12 months and older: A fever under 102°F in an active, feeding toddler can often be managed at home with fluids and fever reducers. Call your doctor if it reaches 102°F or lasts more than two to three days.