A fever in an infant is a rectal temperature of 100.4°F (38°C) or higher. What you should do depends almost entirely on your baby’s age. For any baby younger than 3 months, a temperature of 100.4°F or above needs immediate medical evaluation, even if your baby seems fine otherwise. For older infants, you typically have more room to manage the fever at home while watching for warning signs.
Age Determines How Urgently You Should Act
The younger the baby, the more seriously a fever needs to be taken. Here’s how the thresholds break down:
- 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. Newborns don’t have a mature immune system, so even a mild fever can signal a serious infection. Don’t give fever-reducing medication first and wait to see what happens. Get the baby evaluated.
- 3 to 6 months: A temperature up to 101°F (38.3°C) doesn’t automatically require a visit, but call your doctor if your baby seems unusually irritable, sluggish, or uncomfortable. If the temperature goes above 101°F, contact your doctor regardless of how your baby is acting.
- 6 to 24 months: A temperature above 101°F that lasts longer than one day without other symptoms is worth a call. If your baby has additional symptoms like vomiting, diarrhea, or a rash, call sooner.
How to Take an Accurate Temperature
For babies under 3 months, a rectal thermometer is the standard. It’s the most reliable method for young infants, and it’s the reading your pediatrician will ask about. Temporal artery (forehead) and ear thermometers are convenient for older babies but can be less precise in very young infants. If you use a forehead thermometer and get a borderline reading on a baby under 3 months, follow up with a rectal check to be sure.
To take a rectal temperature, apply a small amount of petroleum jelly to the tip of a digital thermometer and insert it about half an inch. Hold it in place until the thermometer beeps. Clean the thermometer afterward and label it so it’s only used rectally.
Keeping Your Baby Comfortable at Home
Fever itself is not dangerous. It’s your baby’s immune system working. The goal of home care isn’t to eliminate the fever completely but to help your baby rest and stay hydrated.
Dress your baby for the room, not for the illness. It’s tempting to bundle up a sick baby, but extra layers can trap heat and push the temperature higher. If your room is between 61°F and 68°F (16–20°C), a single lightweight layer or sleep bag is enough. In warmer rooms, a short-sleeve bodysuit or even just a diaper is fine. The key is to avoid overdressing.
A lukewarm (not cold) sponge bath can bring mild relief, but if your baby seems distressed by it, stop. Never use cold water or rubbing alcohol, both of which can cause shivering and actually raise core body temperature.
Fever-Reducing Medication by Age
Acetaminophen (Tylenol) can be given to infants 8 weeks and older. You can dose it every 4 to 6 hours as needed, but no more than 5 doses in a 24-hour period. Always go by your baby’s weight, not age, when measuring the dose, and use the syringe that comes with the product rather than a kitchen spoon.
Ibuprofen (Advil, Motrin) is not recommended for babies under 6 months. Once your baby is old enough, it can be given every 6 to 8 hours, with a maximum of 4 doses in 24 hours. Again, dose by weight.
Never give aspirin to an infant or child. It’s linked to a rare but serious condition called Reye’s syndrome. If you’re unsure about dosing for either acetaminophen or ibuprofen, your pediatrician’s office can walk you through it over the phone.
Hydration Matters More Than Food
Fever increases fluid loss, so keeping your baby hydrated is the single most important thing you can do at home. If you’re breastfeeding or formula feeding, offer feeds more frequently than usual. Babies older than 6 months who are eating solids can also have small sips of water between feedings.
Track wet diapers to gauge hydration. A newborn up to 4 months old should produce at least 6 wet diapers in a day. Babies 4 months and older should have at least 3. Fewer wet diapers than that, a dry mouth, crying without tears, or a sunken soft spot on the head are all signs of dehydration that need medical attention.
Fevers After Vaccination
A low-grade fever in the days following a vaccine is common and generally not a cause for concern. Most post-vaccination fevers appear within a week of the shot, though the exact timing varies depending on the vaccine. These fevers are a sign that your baby’s immune system is responding to the vaccine, and they typically resolve on their own within a day or two. You can use acetaminophen (if your baby is old enough) for comfort, but there’s no need to give it preventively before a vaccine appointment.
When a Fever Triggers a Seizure
Febrile seizures happen in some children between 6 months and 5 years old, with the highest risk between ages 1 and 3. During a simple febrile seizure, your child may stiffen, twitch, or lose consciousness for a few seconds up to 15 minutes. It’s terrifying to watch, but simple febrile seizures do not cause brain damage or long-term health problems, and having one does not mean your child has epilepsy.
If it happens, lay your baby on a flat surface on their side, clear the area of anything hard or sharp, and don’t put anything in their mouth. Time the seizure. If it lasts longer than 5 minutes, call 911. After a first febrile seizure, contact your pediatrician even if your baby recovers quickly, so they can evaluate what caused the fever.
Signs That Need Emergency Care
Beyond the age-specific thresholds above, certain symptoms alongside a fever mean you should seek care right away:
- Breathing trouble: fast, labored, or noisy breathing, or pauses in breathing
- Color changes: skin, lips, or nail beds that look blue, purple, or gray
- Altered consciousness: your baby is limp, hard to wake, or strangely unresponsive
- Rash: especially one that appears suddenly, blisters, or doesn’t fade when you press on it
- Persistent pain or fussiness: inconsolable crying that doesn’t improve with comfort measures
- Dehydration signs: sunken soft spot, no tears when crying, significantly fewer wet diapers
Trust your instincts. If something about your baby’s behavior feels off in a way you can’t quite name, that’s a valid reason to call your pediatrician or go to urgent care. You know your baby’s normal, and a change from that baseline matters more than a number on a thermometer.