A baby fever is a rectal temperature of 100.4°F or higher. Learn what causes infant fevers, when to worry by age, home care tips, fever medications, and when to call the doctor or go to the ER.
As pediatricians, we see worried parents call about fevers every day. It is one of the most common reasons families reach out to us, and we want you to know exactly what to watch for and when to seek care.
A baby fever is a rectal temperature of 100.4°F (38.0°C) or higher. Most infant fevers are caused by common viral infections and resolve on their own within a few days. However, any fever in a baby under 3 months old requires immediate medical attention. Knowing when a fever is a normal immune response and when it signals something serious can help you stay calm and act quickly. This guide covers what causes baby fevers, age-based temperature thresholds from the American Academy of Pediatrics (AAP), home care tips, fever-reducing medications, and when to call the doctor or head to the ER. This guide focuses on babies under 12 months. If your toddler has a fever with no other symptoms, see our Toddler Fever With No Other Symptoms guide.
A fever in babies is a rectal temperature of 100.4°F (38.0°C) or higher — it means your baby's immune system is fighting an infection. A fever is not an illness itself. The AAP defines a fever in infants as a rectal temperature of 100.4°F (38.0°C) or higher.
Normal body temperature in babies ranges from 97.0 to 99.5°F (36.1 to 37.5°C). Temperature naturally goes up and down throughout the day. It tends to be lowest in the early morning and highest in the late afternoon.
A slight rise in temperature does not always mean your baby is sick. Overdressing, a warm bath, or active play can all raise body temperature temporarily.
The AAP recommends rectal thermometry as the gold standard for babies under 3 months old. A rectal reading gives the most accurate picture of your baby's core body temperature.
Here is how to take a rectal temperature:
Other types of thermometers are less reliable for young babies:
For babies under 3 months, only rectal temperature should be used to make decisions about fever care.
How serious an infant fever is depends mostly on your baby's age — any fever in a newborn under 3 months requires emergency care.
The younger your baby, the more seriously you should take a fever.
Viral infections are the most common cause of fever in babies. These infections are usually mild and clear up on their own.
Viral infections: The common cold, influenza, RSV, roseola, hand-foot-and-mouth disease, and stomach bugs can all cause fever. Viral fevers typically run 101 to 104°F and last 2 to 4 days. This is a normal immune response and does not by itself mean your baby is seriously ill.
Bacterial infections: Less common but potentially more serious. Urinary tract infections (UTIs) are the most common serious bacterial infection in febrile infants and may present with fever as the only symptom. Ear infections, pneumonia, and in rare cases, meningitis can also cause fever.
Post-immunization fever: Very common and expected. Fever after routine vaccinations usually appears within 24 hours and resolves within 48 hours. It is typically mild, under 102°F, and is a normal sign that the vaccine is working. Acetaminophen can be given for comfort if your baby is 3 months or older. The AAP does not recommend giving fever medicine before vaccines as a preventive measure, as prophylactic acetaminophen may reduce the immune response to certain vaccines. However, it is safe and appropriate to treat fever with acetaminophen after it develops.
Teething: Research shows that teething may cause a slight temperature rise but does not cause a true fever of 100.4°F or higher. If your teething baby has a true fever, look for another cause. For more on teething and temperature, see our guide Can Teething Cause a Fever?
Overdressing: Bundling your baby in too many layers or blankets can raise body temperature. If you suspect overdressing, remove a layer and recheck the temperature in 15 to 20 minutes. For tips on what your baby should wear when sick, check out our article How to Dress Baby With Fever at Night.
Some fever symptoms need emergency care right away. For a newborn fever when to call the doctor, the answer is always immediately. Call 911 or go to the emergency room if your baby has any of these:
Worried about your baby's fever? Start a visit with a Blueberry pediatrician — available 24/7 from home.
Medication can help your baby feel more comfortable, but it will not make the infection go away faster. The goal is comfort, not bringing the temperature down to normal.
Acetaminophen (Tylenol): Safe for babies 3 months and older. Give 10 to 15 mg per kg of body weight every 4 to 6 hours as needed. Do not exceed 5 doses in 24 hours. Always dose by weight, not age. Never give acetaminophen to a baby under 3 months without your doctor's approval.
Ibuprofen (Motrin/Advil): Safe for babies 6 months and older. Give 5 to 10 mg per kg of body weight every 6 to 8 hours as needed. Do not exceed 4 doses in 24 hours. Ibuprofen also reduces inflammation, which acetaminophen does not. (Remember, for babies 3 to 5 months old, acetaminophen is the only safe option regardless.)
Alternating acetaminophen and ibuprofen: Some doctors recommend alternating the two medications. However, the AAP advises caution with this approach because it increases the risk of dosing errors and accidental overdose. If your doctor recommends alternating, follow their specific instructions carefully. For detailed dosing information, see our Baby Tylenol Dosing Guide.
Never give aspirin to a baby or child. Aspirin in children is linked to Reye's syndrome, a rare but life-threatening condition that causes swelling in the liver and brain.
The best home care for a baby with fever is staying hydrated, wearing light clothing, and resting.
Keep your baby hydrated: Offer frequent breastfeeds or formula feeds. For babies 6 months and older, small sips of water or an oral rehydration solution like Pedialyte can also help. Watch for signs of dehydration like fewer wet diapers, dry mouth, or no tears when crying.
Dress your baby lightly: A single layer of clothing and a light blanket is usually enough. Overdressing can trap heat and raise body temperature. For tips on what to put your baby in at night, read our guide How to Dress Baby With Fever.
Keep the room comfortable: A room temperature of 68 to 72°F is ideal. Use a fan for air circulation if needed, but do not point it directly at your baby.
Let your baby rest: Sleep helps the immune system work. Do not wake your baby to give fever medicine unless your doctor tells you to.
Febrile seizures are convulsions triggered by fever. They affect 2 to 5 percent of children between 6 months and 5 years old. Seeing your baby have a seizure is terrifying, but febrile seizures are almost always harmless.
There are two types:
Simple febrile seizures last less than 15 minutes and do not recur within 24 hours. These make up the majority of cases.
Complex febrile seizures last longer than 15 minutes, happen more than once in 24 hours, or involve only one side of the body. These need further medical evaluation.
The good news: Simple febrile seizures do not cause brain damage, epilepsy, or developmental problems. The AAP states that children who experience simple febrile seizures have the same cognitive outcomes as children who do not. About one-third of children who have a febrile seizure will have another one, usually within the next 12 months. Fever-reducing medication does not prevent febrile seizures.
Most common fears about baby fevers are not supported by evidence — fever does not cause brain damage, and medicine cannot prevent febrile seizures.
Myth: High fevers cause brain damage.
Fact: Fevers caused by infection do not cause brain damage. Body temperature would need to reach 107.6°F (42°C) or higher to risk brain injury — this does not happen from infections alone. Only external causes like heatstroke can push temperatures that high.
Myth: You need to "break" the fever.
Fact: Fever is your baby's immune system at work. The goal of fever medicine is comfort, not eliminating the fever. A baby who has a fever but is playing, drinking, and alert is often fine without medication.
Myth: Teething causes high fevers.
Fact: Research by Massignan et al. (2016) found that teething may cause mild temperature elevation but does not cause true fever above 100.4°F. A teething baby with a real fever likely has another cause.
Myth: A higher number always means a sicker baby.
Fact: How your baby looks and acts is more important than the exact temperature. A baby with a 103°F fever who is drinking well and making eye contact may be less concerning than a baby with a 101°F fever who is limp, pale, and refusing to feed. Always look at the whole picture.
Myth: Fever medicine will prevent febrile seizures.
Fact: Studies show that giving acetaminophen or ibuprofen during a fever does not prevent febrile seizures. The exact mechanism is not fully understood, but seizures appear to involve multiple factors including genetic susceptibility and the rate of temperature change.
If you feel panicked when your baby gets a fever, you are not alone. Research shows that fever anxiety remains widespread among parents, but most fever fears are not supported by evidence. The most important thing you can do is watch how your baby acts, not obsess over the number on the thermometer. When in doubt, trust your instincts and call your pediatrician.
Most baby fevers are caused by common viral infections and resolve on their own within 2 to 4 days. Your baby's fever is likely not serious if your baby is over 3 months old with a temperature under 102°F, is still drinking fluids and having wet diapers, is alert and responsive between episodes of fussiness, and has a known cause for the fever such as a recent vaccination or a cold.
If your baby looks and acts mostly like themselves between bouts of fussiness, the fever is probably doing its job — helping fight off an infection.
You do not have to wait until office hours to get answers about your baby's fever. Blueberry pediatricians are available 24/7 for virtual visits from the comfort of your home. Start a visit if:
A virtual visit takes just minutes to start and can save you an unnecessary trip to urgent care or the ER. Start a Blueberry visit now.
A rectal temperature of 100.4°F (38.0°C) or higher is considered a fever in babies. The AAP recommends rectal thermometry as the most accurate method for babies under 3 months old.
Forehead thermometers can be convenient but give inconsistent results in young infants. For babies under 3 months, always use a rectal thermometer to get an accurate reading.
Any fever of 100.4°F or higher in a baby under 3 months old requires immediate emergency evaluation. For older babies, go to the ER if your baby has difficulty breathing, is unresponsive, has a stiff neck, or has a seizure.
The AAP advises caution when alternating acetaminophen and ibuprofen due to the risk of dosing errors. If your doctor recommends it, keep a written log of each dose, the time, and which medication you gave.
No. Fevers caused by infection do not cause brain damage. Body temperature would need to reach 107.6°F or higher to risk brain injury, and only external causes like heatstroke can push temperatures that high.
Generally, no. Sleep helps the immune system work. Do not wake your baby to give fever medicine unless your doctor specifically tells you to. If your baby wakes on their own and seems uncomfortable, you can give the appropriate dose at that time.
Medical Disclaimer: This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your pediatrician or a qualified healthcare provider with any questions about your baby's health.
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