A fever in a baby is a rectal temperature of 100.4°F (38.0°C) or higher. What you should do depends almost entirely on your baby’s age. If your baby is younger than 3 months and has a rectal temperature of 100.4°F or above, that’s a situation requiring immediate medical evaluation, even if your baby looks fine. For older babies, most fevers are caused by common viral infections and can be managed safely at home.
Why Age Matters So Much
A fever in a newborn or very young infant is treated far more seriously than the same fever in an older baby. Babies under 3 months have immature immune systems, and a fever can be one of the only signs of a serious bacterial infection. The UK’s National Institute for Health and Care Excellence classifies any baby from birth to 3 months with a temperature of 100.4°F or higher as high-risk for serious illness. Babies aged 3 to 6 months with a temperature of 102.2°F (39°C) or higher fall into an intermediate-risk category.
For babies under 3 months, hospitals typically run blood and urine tests and may need to monitor the baby for 24 to 48 hours while waiting for results. This can feel alarming, but it’s standard protocol because infections in very young infants can escalate quickly. Don’t give fever-reducing medication and wait it out. Go to the emergency room or call your pediatrician immediately.
How to Take an Accurate Temperature
For babies from birth to 3 months, a regular digital thermometer used rectally is the most accurate method. The American Academy of Pediatrics recommends rectal thermometers for all children under 3 years old. Armpit (axillary) readings run about 1.3°F (0.7°C) lower than rectal readings, which means they can miss a real fever or underestimate its severity.
To take a rectal temperature, apply a small amount of petroleum jelly to the tip and insert it about half an inch. Hold it in place until it beeps. If you get an armpit reading that concerns you, follow up with a rectal reading to confirm. Temporal artery (forehead) thermometers can also provide accurate readings, even in newborns. Digital ear thermometers are not recommended until a baby is at least 6 months old.
Fever-Reducing Medication by Age
Acetaminophen (Tylenol) can be given to babies older than 2 months. Ibuprofen (Motrin, Advil) is not safe until 6 months of age. Both are dosed by weight, not age, so always check the packaging or ask your pediatrician for the correct amount.
For acetaminophen infant liquid (160 mg per 5 mL):
- 6 to 11 lbs (0 to 3 months): 1.25 mL. Only give this if your doctor specifically tells you to.
- 12 to 17 lbs (4 to 11 months): 2.5 mL
- 18 to 23 lbs (12 to 23 months): 3.75 mL
For ibuprofen concentrated infant drops (50 mg per 1.25 mL):
- 12 to 17 lbs (6 to 11 months): 1.25 mL
- 18 to 23 lbs (12 to 23 months): 1.875 mL
Never give aspirin to a baby, child, or teenager. Aspirin use during a viral illness is linked to Reye’s syndrome, a rare but potentially fatal condition that causes swelling in the liver and brain. Acetaminophen and ibuprofen are the only safe options.
Home Comfort Measures
Medication isn’t always necessary. A fever is your baby’s immune system fighting off infection, and bringing the number down doesn’t make the illness go away faster. The real goal is keeping your baby comfortable. If your baby is sleeping peacefully, eating reasonably well, and doesn’t seem distressed, you may not need to give anything at all.
When your baby does seem uncomfortable, these steps help:
- Keep fluids going. Offer breast milk, formula, or an electrolyte solution like Pedialyte frequently. Fever increases fluid loss, and dehydration is one of the real risks to watch for.
- Dress lightly. One layer of clothing is enough. Bundling a feverish baby in blankets can trap heat and push the temperature higher.
- Use a cool washcloth. A damp cloth on the forehead can provide relief. Do not give an ice bath or use cold water, which causes shivering that can actually raise the body’s core temperature.
Fever After Vaccines
A low-grade fever in the day or two after routine vaccinations is common and not a cause for concern. You can place a cool, wet washcloth on the injection site if it looks sore or swollen. Acetaminophen is fine for babies over 2 months if they seem uncomfortable. These post-vaccine fevers are typically mild and resolve on their own within a day or two.
How Long Fevers Typically Last
Most viral fevers in babies run their course within 2 to 3 days. The temperature may spike, come down, and spike again before resolving completely. This pattern is normal. If a fever persists beyond three days, it’s worth calling your pediatrician even if your baby otherwise seems okay, because longer fevers sometimes point to a bacterial infection or another issue that needs attention.
Signs That Need Immediate Attention
Beyond the under-3-months rule, certain symptoms alongside a fever signal a trip to the emergency room regardless of your baby’s age:
- Unresponsiveness or extreme sleepiness: Your baby is difficult to wake up, or once awake, does not become more alert or comfortable when the fever drops.
- Signs of dehydration: No tears when crying, no wet diapers for 8 hours or longer, or a sunken soft spot on the head.
- Color changes: Blue or pale lips, tongue, or fingernails.
- Unexplained rash or bruising: A new rash that appears alongside a fever, especially one with purple or red spots that don’t fade when you press on them.
Call 911 if your baby cannot be awakened at all or has blue lips, tongue, or nails. These are signs of a medical emergency that requires immediate help.