A fever in a baby is the immune system doing its job, and in most cases your role is to keep your baby comfortable while their body fights off the infection. The threshold that counts as a fever is a rectal temperature of 100.4°F (38°C) or higher. One critical exception: any baby younger than 3 months with a rectal temperature at or above 100.4°F needs immediate medical attention, no matter how well they seem.
Why Babies Get Fevers
Fever is not the illness itself. It’s a defense mechanism. When your baby’s immune system detects a pathogen, it triggers inflammation and raises body temperature to create an environment where immune cells work more effectively and invading germs struggle to reproduce. Artificially suppressing a fever, whether with medication or physical cooling, can interfere with that process. This doesn’t mean you should let your baby suffer, but it does mean the goal of treatment is comfort, not getting the number on the thermometer back to normal.
How to Take an Accurate Temperature
For babies, a rectal thermometer is the most reliable method. Temporal artery (forehead) thermometers are convenient but tend to read about 0.2°C lower than rectal readings, and their accuracy drops further when a child actually has a fever. One study of children under 3 found that forehead thermometers only detected about 62% of fevers confirmed by rectal reading. Armpit thermometers performed even worse, catching only about 12% of true fevers and reading nearly a full degree Celsius below rectal temperature.
If you’re using a forehead or armpit thermometer as a quick check, that’s fine for everyday monitoring. But when you need a definitive answer, especially for a young infant where the exact temperature determines whether you call the doctor, use a rectal thermometer.
Comfort Measures That Actually Help
Most of what your baby needs during a fever is simple: fluids, rest, and not being overdressed.
Hydration matters more than food in the short term. Offer breast milk or formula frequently. For older babies already on solids, an electrolyte drink designed for infants can help replace fluids lost through sweating or if there’s any vomiting or diarrhea. Your baby’s body burns through fluids faster during a fever, so don’t wait for them to seem thirsty.
Dress your baby in a single light layer. Bundling a feverish baby in blankets traps heat and can push the temperature higher. Keep the room at a comfortable temperature rather than cranking up the heat.
A lukewarm sponge bath (water between 90°F and 95°F) can bring some relief. Use it for 20 to 30 minutes, and stop immediately if your baby starts shivering. Shivering is the body’s way of generating more heat, which defeats the purpose. Never use cold water, ice, or rubbing alcohol. These cool the skin too fast and can cause a dangerous drop in body temperature.
When Fever-Reducing Medication Makes Sense
If your baby is clearly uncomfortable, fussy, or having trouble sleeping because of the fever, medication can help. The two options are acetaminophen (Tylenol) and ibuprofen (Advil, Motrin), but they have different age cutoffs.
Acetaminophen can be given to babies 8 weeks and older. Do not give it to any infant under 8 weeks. Babies that young with a fever of 100.4°F or higher need to be seen by a doctor, not treated at home.
Ibuprofen is not safe for babies under 6 months old unless a doctor specifically tells you otherwise.
Dosing for both medications is based on your baby’s weight, not age. The correct dose is printed on the packaging, but if your baby is very young or you’re unsure, call your pediatrician’s office for guidance. Never give aspirin to a child of any age.
Alternating Medications
You may have heard about alternating acetaminophen and ibuprofen to keep fever down more consistently. Research shows this approach does reduce fever more effectively than acetaminophen alone at the six-hour mark. However, the American Academy of Pediatrics flags two concerns: the risk of dosing errors goes up when you’re tracking two different medications on different schedules, and the safety picture is less clear when both drugs are used over multiple days. If you decide to alternate, keep a written log of what you gave and when. For most fevers, sticking with a single medication is simpler and safer.
Temperature Thresholds by Age
How urgently you need to respond depends on your baby’s age and how high the fever climbs.
- Under 3 months: A rectal temperature of 100.4°F or higher requires a call to your doctor right away, regardless of other symptoms. Young infants have immature immune systems, and fever at this age can signal a serious infection that needs evaluation.
- 3 to 6 months: Call your doctor if the temperature reaches 101°F (38.3°C) or higher, or if your baby seems unusually irritable, lethargic, or uncomfortable even at lower temperatures.
- 6 to 24 months: A temperature above 101°F that lasts longer than one day warrants a call, even if your baby doesn’t have other symptoms.
For children under 2, any fever lasting more than 24 hours should be evaluated. For children 2 and older, the threshold extends to 72 hours.
Signs That Need Immediate Attention
A fever by itself is rarely dangerous, but certain symptoms alongside a fever signal something more serious. Get medical care right away if your baby shows any of the following:
- Breathing trouble: Fast breathing, flaring nostrils, or skin pulling in between the ribs with each breath.
- Color changes: Skin or lips that look blue, purple, or gray.
- Extreme sleepiness: Sleeping far more than usual or being very difficult to wake up.
- Unusual behavior: Acting strangely, seeming withdrawn, or not making eye contact the way they normally do.
- Severe fussiness: Pain or irritability that keeps getting worse or won’t let up, even with comfort measures.
- Trouble swallowing: Excessive drooling or spitting that suggests they can’t swallow normally.
Watching for Dehydration
Dehydration is the most common complication of fever in babies, especially when vomiting or diarrhea are involved. Knowing the early signs lets you act before it becomes serious.
Mild to moderate dehydration looks like: fewer than six wet diapers in a day, a dry mouth, fewer tears when crying, less interest in playing, and a soft spot on the head that appears sunken. These signs mean you need to push fluids more aggressively and contact your pediatrician.
Severe dehydration is harder to miss. Your baby may urinate only once or twice a day, have sunken eyes, wrinkled skin, or cool and discolored hands and feet. They may be extremely fussy or, conversely, excessively sleepy. Severe dehydration needs medical treatment promptly.
The simplest way to track hydration is to count wet diapers. If you’re changing fewer diapers than usual, your baby needs more fluids. Offer small, frequent feedings rather than waiting for larger ones.