A fever in a newborn is a rectal temperature of 100.4°F (38°C) or higher. This threshold applies to all infants under 3 months old, and unlike a fever in an older child or adult, it is always treated as a medical emergency. Even if your baby looks perfectly fine, a temperature at or above 100.4°F in the first few months of life requires immediate medical evaluation.
Why the Threshold Is 100.4°F
The 100.4°F cutoff isn’t arbitrary. It’s the temperature the American Academy of Pediatrics and most pediatric emergency guidelines use to trigger a specific evaluation process for infants between 8 and 60 days old. Below that number, a baby’s temperature can fluctuate normally based on clothing, room temperature, or recent feeding. At or above it, the concern shifts to infection.
Newborns have immature immune systems that can’t fight off bacteria the way older children can. A fever may be the only visible sign of a serious bacterial infection. Roughly 10 percent of high-risk febrile infants turn out to have one. That’s a significant number, which is why pediatricians don’t take a wait-and-see approach with this age group.
How to Take Your Newborn’s Temperature
Rectal temperature is the only reliable method for babies under 3 months. Forehead scanners, armpit readings, and ear thermometers can all read lower than the actual core body temperature, which means they could miss a real fever or give you false reassurance. A digital rectal thermometer is inexpensive and gives results in about 10 seconds.
To use one, apply a small amount of petroleum jelly to the tip, lay your baby on their back with knees bent toward their chest, and gently insert the thermometer about half an inch into the rectum. Hold it in place until it beeps. If the reading is 100.4°F or above, don’t retake it hoping for a lower number. Call your pediatrician or head to the emergency department right away.
What Happens at the Hospital
When you bring a febrile newborn in for evaluation, the medical team will run a series of tests to look for the source of infection. For babies under 3 weeks old, this typically includes a blood culture, a urine sample, testing for herpes simplex virus, and a lumbar puncture (spinal tap) to check for meningitis. That combination of tests sounds aggressive, but it exists because newborns can have serious infections with almost no outward symptoms beyond the fever itself.
For babies between 3 and 4 weeks old, the workup is similar but may be adjusted based on certain blood markers that help doctors gauge inflammation levels. Babies closer to 2 months old may undergo a slightly less intensive evaluation depending on how they look clinically, but the initial testing still involves blood and urine cultures at minimum. Many of these infants are admitted to the hospital for observation and started on antibiotics while the cultures are pending, which usually takes 24 to 48 hours to come back.
Signs That Signal Greater Urgency
Any fever in a newborn warrants a call to your pediatrician, but certain symptoms alongside a fever mean you should go directly to an emergency room:
- Lethargy or difficulty waking. A baby who is unusually floppy, unresponsive, or hard to rouse is showing signs of a potentially serious infection.
- Poor feeding. Refusing the breast or bottle, or eating less than half of normal amounts over several feedings.
- Skin color changes. A pale, mottled, or bluish tint to the skin, especially around the lips.
- Seizures. Any shaking, jerking, or stiffening episode requires emergency care immediately.
- A bulging soft spot. The fontanelle on top of your baby’s head should be flat or slightly concave. If it’s noticeably raised or tense, that can indicate increased pressure inside the skull.
Fever After Vaccinations
Babies receive their first round of routine vaccinations at 2 months old, and a mild fever afterward is common and expected. The CDC notes that low-grade fevers, mild fussiness, and soreness at the injection site are normal reactions that resolve on their own within a day or two. However, if your baby is still under 3 months old at the time of vaccination and develops a temperature of 100.4°F or higher, you should still call your pediatrician. They can help determine whether the fever is a vaccine response or something that needs further evaluation.
Why You Shouldn’t Give Medication Without Guidance
Your instinct may be to bring the fever down, but fever-reducing medications have age restrictions for infants. Ibuprofen is not approved for use in babies under 6 months old because it hasn’t been established as safe for that age group. Acetaminophen (the active ingredient in infant Tylenol) is generally considered safer for younger babies, but dosing in newborns is weight-based and should only be given with a pediatrician’s specific instructions. Never give a newborn any fever reducer before calling your doctor first. In this age group, lowering the fever before the baby is evaluated can actually mask the very symptom that tells the medical team something is wrong.
In the meantime, you can keep your baby comfortable by dressing them in a single light layer and keeping the room at a normal temperature. Avoid cold baths or rubbing alcohol, both of which can cause dangerous drops in body temperature.
When a Fever Is Less Worrisome
Once your baby passes the 3-month mark, the rules relax somewhat. Older infants have more mature immune systems, and a fever in a 4- or 5-month-old is more commonly caused by a routine viral infection. At that age, your pediatrician is more likely to advise monitoring at home and may recommend acetaminophen for comfort. The 100.4°F threshold still matters, but the urgency and the likelihood of an invasive workup decrease as babies get older.
For babies under 3 months, though, there is no “mild” fever. The number is the number, and it always calls for professional evaluation. Trust the thermometer, and trust your instincts if something about your baby seems off, even if the temperature reads normal.