A baby has a fever when their rectal temperature reaches 100.4°F (38°C) or higher. If your baby is under 3 months old and hits that number, that alone is reason to call your pediatrician right away, regardless of how your baby looks or acts. For older babies, most fevers are caused by common viral infections, resolve within two to three days, and can be safely managed at home with fluids, comfort measures, and careful monitoring.
Age Matters More Than the Number
The single most important factor in deciding what to do is your baby’s age. A fever that’s perfectly manageable in a 6-month-old can signal a serious infection in a newborn, because very young immune systems handle bacteria differently.
For babies under 12 weeks old, a temperature of 100.4°F or above needs immediate medical attention. Your baby will need to be evaluated in a healthcare setting to determine what testing is necessary. The American Academy of Pediatrics has specific guidelines for infants 8 to 60 days old, breaking them into even narrower age windows (8 to 21 days, 22 to 28 days, and 29 to 60 days) because risk levels shift quickly in those early weeks. More than 10% of febrile infants in this age range turn out to have a urinary tract infection. The chance of something more serious, like meningitis, is much lower (under 0.05%), but doctors take every fever in this age group seriously because the stakes are high.
For babies 3 months and older who are otherwise acting like themselves, eating, and staying hydrated, you have more room to manage things at home while keeping a close eye on symptoms.
How to Take an Accurate Temperature
Rectal temperature is the gold standard for babies. Forehead and underarm readings can be off by a degree or more, which matters when you’re making decisions about a young infant. Here’s how to do it safely:
- Use a digital thermometer. If you also use a digital thermometer for oral readings on older family members, keep a separate one labeled for rectal use only.
- Lubricate the tip with petroleum jelly.
- Position your baby on their back with thighs lifted, or belly-down across your lap.
- Insert gently, about half an inch to one inch (1.3 to 2.5 cm) into the rectum.
- Hold the thermometer in place until it beeps. Never walk away while the thermometer is inserted.
- Clean the tip before and after every use according to the manufacturer’s instructions.
Keeping Your Baby Comfortable
Fever itself is your baby’s immune system fighting an infection, not a disease on its own. The goal at home isn’t to eliminate the fever entirely. It’s to keep your baby comfortable, hydrated, and resting.
Dress your baby in light, comfortable clothing or pajamas. Avoid bundling them in extra blankets or layers, which can trap heat and push their temperature higher. If your baby is shivering, a single light layer is fine until the shivering passes.
Hydration is critical. Babies lose fluid faster when they’re feverish, and dehydration can develop quickly. If you’re breastfeeding, offer the breast more often than usual in shorter sessions. If your baby is on formula, continue with normal-strength formula (never water it down) and offer smaller, more frequent feedings. Babies on solid foods can also have small sips of water between feedings. Avoid fruit juice or fizzy drinks, which can make things worse if your baby also has diarrhea or vomiting.
Spotting Dehydration Early
Watch for these warning signs that your baby isn’t getting enough fluid:
- Fewer wet diapers than usual
- A sunken soft spot on top of the head
- Sunken eyes
- Few or no tears when crying
- Unusual drowsiness or irritability
If you notice any of these, contact your pediatrician. A pharmacist can also recommend an oral rehydration solution designed for infants, given in small, frequent sips.
Fever-Reducing Medication
Acetaminophen (Tylenol) can be given to bring a fever down and ease discomfort, but it should not be given to babies under 2 months old without a doctor’s guidance. For babies under 12 weeks, remember that any fever warrants a call to your doctor before you reach for medication.
Ibuprofen (Advil, Motrin) is not safe for babies under 6 months old. The FDA has not approved its use in that age group.
For either medication, dosing is based on your baby’s weight, not their age. If you don’t know your baby’s current weight, age can be used as a rough guide, but weight is more accurate. Acetaminophen can be given every four hours, up to five doses in 24 hours. Ibuprofen (for babies 6 months and older) can be given every six to eight hours. The liquid form of acetaminophen comes in a standard concentration of 160 mg per 5 mL, so double-check that you’re using the measuring tool that came with the product, not a kitchen spoon.
When a Fever Needs Medical Attention
Beyond the under-12-weeks rule, certain patterns and symptoms mean it’s time to call your doctor:
- Fever without other symptoms (no cough, runny nose, or diarrhea) lasting more than 48 hours
- Fever with other symptoms lasting more than 3 days (72 hours)
- Babies 6 to 12 months old with a fever persisting beyond 48 hours
- Temperature above 104°F (40°C) at any age
- Signs of dehydration that aren’t improving with extra fluids
- A baby who looks or acts very sick, is difficult to wake, or has a weak cry
Most fevers from viral illnesses land between 101°F and 104°F and resolve within two to three days. A fever that breaks this pattern, either by climbing unusually high, lasting too long, or coming with worrisome behavior changes, deserves a professional evaluation.
What to Do if Your Baby Has a Seizure
Febrile seizures happen in a small percentage of children with fevers. They look alarming: your baby may stiffen, twitch, or shake, and their eyes may roll. Most febrile seizures stop on their own within a few minutes and do not cause lasting harm. Here’s what to do in the moment:
- Stay calm. The seizure will likely end on its own.
- Place your baby gently on the floor or another flat surface, away from furniture or sharp objects.
- Turn them on their side to prevent choking.
- Loosen any clothing around their head and neck.
- Watch their breathing. Look for a bluish color around the face, which signals a breathing problem.
- Time the seizure. Note when it started and when it stops.
Do not put anything in your baby’s mouth, try to hold them down, or place them in water. If the seizure lasts longer than five minutes, or your baby has trouble breathing afterward, call emergency services. Even if the seizure is brief and resolves quickly, let your pediatrician know it happened so they can evaluate your baby and discuss next steps.