A fever in an infant is defined as a rectal temperature of 100.4°F (38°C) or higher. This is the threshold used by the American Academy of Pediatrics and virtually every pediatric emergency department. It applies regardless of whether your baby is a week old or eleven months old, though what you should do about it changes significantly depending on age.
Why the Threshold Is 100.4°F
Normal body temperature in infants fluctuates throughout the day, generally ranging from about 97°F to 100.3°F. The 100.4°F cutoff isn’t arbitrary. It’s the point at which clinical evidence shows the reading reliably reflects a true immune response rather than normal variation caused by bundling, a warm room, or recent feeding. Below that number, even if your baby feels warm, they don’t technically have a fever.
That said, the number alone doesn’t tell the full story. A baby at 100.5°F who is feeding well and alert is in a very different situation than a baby at 100.5°F who is limp and refusing to eat. Temperature is one signal among several.
How to Measure Accurately
For infants, a rectal thermometer is the gold standard. It gives the most accurate core body temperature, and it’s the method pediatricians rely on when making clinical decisions. Forehead (temporal artery) and ear thermometers can be convenient for older babies, but they’re less precise in young infants. Armpit readings are the least accurate of all and can underestimate a true fever by a degree or more.
If you take an armpit temperature and get 99°F or higher, that’s worth following up with a rectal reading to confirm. Different methods have slightly different fever thresholds:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
To take a rectal temperature, apply a small amount of petroleum jelly to the tip of a digital thermometer, insert it about half an inch, and hold it in place until it beeps. It’s uncomfortable for everyone involved, but it takes under 30 seconds and gives you the reading your pediatrician will ask for.
Age Changes Everything
The same 100.4°F fever carries very different levels of urgency depending on how old your baby is. This is the single most important thing to understand about infant fevers.
Under 3 months: Call your pediatrician for any fever at or above 100.4°F, even if your baby looks fine. Young infants have immature immune systems, and serious bacterial infections can be present without obvious symptoms beyond the fever itself. In many cases, your doctor will want to see the baby promptly or send you to the emergency department. Don’t give fever-reducing medication and wait to see what happens at this age. Call first.
3 to 6 months: Call your pediatrician if the temperature reaches 100.4°F or higher, or if it’s below that threshold but your baby seems unusually fussy, lethargic, or is feeding poorly.
6 to 24 months: A fever above 100.4°F that lasts more than one day warrants a call. If the fever is high (above 102°F) or your baby is clearly uncomfortable, you don’t need to wait a full day before reaching out.
Common Causes of Infant Fevers
Viral infections, including colds and flu, are by far the most frequent cause. With many viral illnesses, fever may be the only symptom for the first 24 hours before other signs like congestion, coughing, or loose stools appear. These fevers typically resolve on their own within three to five days.
Vaccines are another common trigger. Fever after immunization usually begins within 12 hours of the shot and lasts two to three days. It’s generally low-grade and resolves without treatment, though you can use fever reducers if your baby seems uncomfortable (provided they’re old enough).
Less commonly, fevers can signal bacterial infections like urinary tract infections, ear infections, or, rarely, more serious conditions like meningitis. This is precisely why age matters so much: the younger the infant, the harder it is to distinguish a harmless virus from something dangerous based on symptoms alone.
Managing a Fever at Home
Fever itself isn’t dangerous in most cases. It’s part of the body’s defense against infection. The goal of treatment isn’t to eliminate the fever entirely but to keep your baby comfortable enough to sleep and feed.
Acetaminophen (Tylenol) can be given to infants as young as newborns, but always confirm the dose with your pediatrician for babies under 3 months. Dosing is based on weight, not age. For a baby weighing 12 to 17 pounds, the typical dose of children’s liquid suspension is half a teaspoon (2.5 ml), given every four hours as needed.
Ibuprofen (Motrin) is not recommended for babies under 6 months old. Once your baby is past that age, it can be given every six hours. Again, dosing is weight-based, and the packaging includes a chart, but your pediatrician can confirm the right amount.
Never give aspirin to an infant or child. Keep your baby in light clothing, offer frequent feedings to prevent dehydration, and skip the old advice about alcohol baths or ice packs, which can cause shivering and actually raise core temperature.
Signs That Need Immediate Attention
Beyond the temperature number, watch for changes in how your baby acts. A fever paired with any of the following warrants a prompt call or visit:
- Lethargy or difficulty waking: Your baby is unusually limp, unresponsive, or hard to rouse from sleep.
- Refusal to feed: Skipping one feeding isn’t alarming, but repeated refusals suggest something more is going on.
- Dehydration signs: Fewer than three wet diapers in 24 hours, no tears when crying, or a sunken soft spot on the head.
- Skin changes: A rash that doesn’t fade when you press on it, or skin that looks mottled, pale, or bluish.
- Persistent crying: Inconsolable crying that doesn’t respond to holding, feeding, or comfort measures.
Trust your instincts here. If your baby has a fever and something feels off to you, even if you can’t pinpoint exactly what, that’s reason enough to call your pediatrician. You know your baby’s normal behavior better than anyone.