A fever in a newborn is a rectal temperature of 100.4°F (38°C) or higher. This threshold applies to all babies under 3 months old, and even a reading that barely crosses it is treated seriously. Unlike older children, where a low-grade fever can often be managed at home, any fever at or above 100.4°F in a newborn warrants immediate medical evaluation.
Why 100.4°F Is the Critical Number
In older kids and adults, fevers are common and usually harmless. Newborns are different. Their immune systems are still immature, which means they have limited ability to fight off infections on their own. A temperature of 100.4°F or above can be the only visible sign that a serious infection is developing, including bloodstream infections or meningitis. Babies this young don’t always look obviously sick, even when something significant is going on internally.
This is why the medical threshold for concern is lower than what most people expect. You might think 101°F or 102°F sounds like a “real” fever based on your own experience, but for a newborn, 100.4°F is the line. Don’t wait for a higher number.
How to Take an Accurate Temperature
A rectal thermometer is the gold standard for babies under 3 months. Forehead (temporal artery) and ear thermometers can also register 100.4°F as a fever, but rectal readings are considered the most reliable in this age group. Armpit (axillary) readings tend to run lower and can miss a true fever, so they’re not ideal for newborns when accuracy matters most.
To take a rectal temperature, use a digital thermometer with a flexible tip. Apply a small amount of petroleum jelly to the end, gently insert it about half an inch, and hold it in place until it beeps. If the reading is 100.4°F or higher, that’s a fever. It’s worth keeping a dedicated rectal thermometer on hand during the first few months so you’re not scrambling to find one at 2 a.m.
One thing to keep in mind: overdressing a baby or taking a temperature right after a warm bath can temporarily inflate the reading. If you get a borderline result and suspect overheating, remove a layer of clothing, wait 15 to 20 minutes, and check again.
What Happens at the Hospital
If your newborn has a fever, you’ll likely be directed to the emergency room rather than a regular office visit. What happens next depends partly on your baby’s age, broken into rough windows: the first three weeks of life, roughly three to four weeks, and one to two months.
For the youngest babies (under about 21 days), doctors typically run a full workup. This means collecting blood, urine, and spinal fluid to test for bacterial infections. That last one, a spinal tap, sounds frightening, but it’s a routine procedure in this situation and the fastest way to rule out meningitis. Babies in this age range are almost always admitted to the hospital and started on antibiotics while the team waits for culture results, which can take 24 to 48 hours.
For babies closer to one or two months old, the approach is somewhat more flexible. Blood and urine samples are still standard. A spinal fluid sample may or may not be needed depending on how the initial test results look. If all the early markers come back normal and the baby appears well, some of these older infants can be monitored closely without a full hospital stay, though the decision is made case by case.
The process can feel overwhelming, especially for new parents. It helps to know that this aggressive approach exists precisely because newborns can deteriorate quickly, and early treatment makes a significant difference in outcomes.
Signs That Something May Be Wrong
A fever is sometimes the only red flag, but other behavioral changes can accompany it or even appear before a temperature rises. Knowing what to watch for helps you act quickly.
- Lethargy or unusual sleepiness. A baby who is harder than usual to wake for feedings, or who seems limp and unresponsive to sounds and visual cues, may be seriously ill. This is different from normal newborn sleepiness.
- Poor feeding. Refusing the breast or bottle, having a weak suck, or showing no interest in eating can signal infection or other illness. Occasional spit-up is normal, but forceful or projectile vomiting after most feedings is not.
- Persistent crying or irritability. A baby who is continuously fretful and cannot be comforted, or who cries in a tone that sounds different from their usual cry, may be in pain or distress.
- Jitteriness or trembling. Shaking that doesn’t stop when you gently hold the baby’s limb can be a sign of illness.
- Discolored vomit. Green-tinged vomit specifically can indicate an intestinal blockage and needs immediate attention regardless of whether a fever is present.
A change in behavior is often one of the earliest signs of illness in a newborn, sometimes appearing before a measurable fever does. Trust your instincts if something feels off, even without a specific temperature reading to point to.
Fever-Reducing Medicine and Newborns
You should not give a newborn any fever-reducing medication without explicit direction from a doctor. Ibuprofen is not approved or considered safe for babies under 6 months old. Acetaminophen (Tylenol) is generally not recommended in the first 3 months either, and more importantly, reducing the fever at home is not the goal. The fever itself is the signal that your baby needs medical evaluation. Masking it with medication can delay diagnosis and make it harder for doctors to assess what’s happening.
If your baby is over 3 months old and develops a fever, the rules change considerably and over-the-counter options become available. But in those first 12 weeks, the priority is always evaluation first.
Normal Temperature Range for Newborns
A healthy newborn’s rectal temperature typically falls between 97.5°F and 99.5°F. Slight fluctuations throughout the day are normal. A reading below 97°F (hypothermia) can also be a concern in newborns and is worth reporting to your pediatrician, as very young babies can sometimes respond to infection by dropping their temperature rather than raising it.
Keeping your baby in one more layer of clothing than you’d wear in the same environment is a reasonable baseline. If you’re checking a temperature out of concern and getting readings in the 99°F to 100.3°F range, it’s worth monitoring closely and rechecking in 15 to 30 minutes, especially if the baby seems warm to the touch or fussy.