A fever in an infant is a rectal temperature of 100.4°F (38°C) or higher. That single number applies across infancy, but what you should do about it depends heavily on your baby’s age. A fever that warrants a phone call in a 4-month-old is an automatic emergency in a newborn.
Fever Thresholds by Age
For babies under 3 months old, any rectal temperature at or above 100.4°F (38°C) is considered a fever and requires an immediate call to your pediatrician, even if your baby looks fine otherwise. At this age, a young immune system can’t reliably fight off serious infections, and a fever may be the only visible sign of one. The American Academy of Pediatrics has specific clinical guidelines for evaluating otherwise well-appearing infants between 8 and 60 days old who develop a fever at this threshold, breaking the protocols into three age windows: 8 to 21 days, 22 to 28 days, and 29 to 60 days. That level of granularity tells you how seriously the medical community takes early infant fevers.
Between 3 and 6 months, the threshold for concern shifts slightly. A temperature up to 101°F (38.3°C) may not need urgent attention on its own, but it does if your baby seems unusually irritable, sluggish, or uncomfortable. Any temperature above 101°F at this age warrants a call to your doctor regardless of how your baby is acting.
From 6 to 12 months, a fever above 101°F (38.3°C) that lasts longer than one day without other symptoms is the general trigger to call. If the fever comes with clear signs of illness, like difficulty breathing or a rash, don’t wait the full day.
Why Rectal Temperature Is the Standard
For infants under 3 months, a rectal reading is the only measurement method considered reliable enough for clinical decisions. Forehead and underarm thermometers are convenient, but they can read lower than your baby’s true core temperature. When the difference between 100.2°F and 100.4°F determines whether your newborn needs immediate evaluation, that margin of error matters. If you get a concerning reading from a forehead or underarm thermometer, follow up with a rectal one to confirm.
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 it beeps. The process takes under a minute and gives the most accurate picture of what’s happening inside your baby’s body.
Overbundling Can Mimic a Fever
Before you panic about a borderline reading, consider what your baby is wearing. A Johns Hopkins study found that newborns wrapped in heavy blankets in a warm room saw their rectal temperatures rise by an average of 0.56°C (about 1°F) over two and a half hours, climbing at a steady rate of roughly half a degree Fahrenheit per hour with no sign of leveling off. Two of the twelve bundled newborns in the study actually reached 100.4°F, the clinical fever threshold, purely from being overbundled.
If your baby’s temperature is borderline, try removing a layer of clothing or blankets, wait 15 to 20 minutes, and retake the temperature. If the reading drops back to normal, the “fever” was likely environmental. If it stays elevated, treat it as a real fever and follow the age-based guidelines above.
Red Flag Symptoms Alongside a Fever
The number on the thermometer is only part of the picture. How your baby looks and acts matters just as much, especially after the newborn period. Watch for these warning signs:
- Unusual sleepiness or floppiness. A baby who is hard to wake, difficult to calm, or seems limp needs prompt attention.
- Breathing changes. Fast or labored breathing, flaring nostrils, or any visible struggle to get air.
- Dehydration. Fewer wet diapers than usual, crying with few or no tears, a dry mouth, or a sunken soft spot on the top of the head.
- Rash. Any rash that appears quickly, blisters, or looks infected, particularly when paired with a fever.
- Persistent vomiting. Vomiting after feedings or an inability to keep liquids down for eight hours.
Certain combinations require emergency care, not just a phone call. If your baby has trouble breathing, skin or lips that look blue, purple, or gray, seems unresponsive or withdrawn, or has pain or fussiness that keeps worsening, go to the emergency room.
Keeping a Febrile Baby Hydrated
Fever increases fluid loss, and babies dehydrate faster than adults. If you’re breastfeeding, nurse more frequently in shorter sessions. For formula-fed babies, offer smaller, more frequent bottles. Babies who have started solid foods can also have small sips of water between feedings. Don’t water down formula, and avoid fruit juice or sugary drinks, which can worsen diarrhea or vomiting.
The earliest signs of dehydration in an infant are subtle: sunken eyes, a sunken soft spot on the head, fewer wet diapers, and drowsiness or increased irritability. If you notice any of these alongside a fever, contact your pediatrician. A pharmacist can recommend an oral rehydration solution designed for infants if your doctor advises one.
Fever Medication by Age
Acetaminophen (Tylenol) is generally the first option for infants and can be given starting at around 2 months of age, dosed by your baby’s weight. Always check with your pediatrician before giving it to a baby under 3 months, since at that age the fever itself needs medical evaluation before you mask it with medication.
Ibuprofen (Advil, Motrin) is not safe for babies under 6 months old. The FDA has not approved its use in that age group, and it has not been studied enough to confirm safety. After 6 months, ibuprofen can be given every 6 to 8 hours as needed, again dosed by weight. If you don’t know your baby’s current weight, use the age-based guidelines on the packaging, but weight-based dosing is more accurate.
Febrile Seizures
Some children experience seizures triggered by fever, most commonly between 6 months and 5 years of age, with peak risk between 12 and 18 months. These febrile seizures are frightening to witness but are typically not harmful. A simple febrile seizure lasts a few seconds to 15 minutes and affects the whole body. A complex febrile seizure lasts longer than 15 minutes, happens more than once in 24 hours, or affects only one side of the body.
If your child has a seizure, place them on their side on a safe surface and time it. Most will stop on their own. Call an ambulance if the seizure lasts longer than five minutes or is accompanied by vomiting, a stiff neck, breathing problems, or extreme sleepiness afterward. Having one febrile seizure does not mean your child has epilepsy, though it does slightly increase the chance of another febrile seizure with future fevers.