A newborn has a fever when their rectal temperature reaches 100.4°F (38°C) or higher. In babies under 3 months old, even a slight fever at this threshold is considered serious and needs prompt medical attention. Unlike older children, newborns can’t reliably fight infections on their own, so a fever at this age is never something to wait out or treat at home first.
Why Rectal Temperature Is the Only Reliable Method
For newborns, a rectal thermometer is the standard. Forehead strips, ear thermometers, and armpit readings are convenient for older kids, but they aren’t accurate enough to trust in a baby this young. Research comparing methods found that armpit (axillary) readings detected only 73% of fevers that a rectal thermometer caught. The average difference between the two was about 1.3°F (0.7°C), but in some cases it varied by as much as 5.4°F (3°C). That gap is large enough to miss a genuine fever entirely.
A normal rectal temperature for a newborn falls between 97.7°F and 100.3°F (36.5°C to 37.9°C). Anything at or above 100.4°F is a fever. If you only have an armpit reading and it’s above 99.0°F (37.2°C), take a rectal reading to confirm.
How to Take a Rectal Temperature Safely
You’ll need a digital thermometer labeled for rectal use and a small amount of petroleum jelly. Turn the thermometer on and apply a thin layer of lubricant to the tip. Lay your baby on their back and gently lift their thighs toward their chest, or place them belly-down across your lap with one hand on their lower back to keep them steady.
Insert the tip about half an inch to one inch (1.3 to 2.5 centimeters) into the rectum. Never force it past any resistance. Hold the thermometer in place until it beeps, then gently remove it. The whole process takes about 15 to 30 seconds with a digital thermometer. Clean the tip with rubbing alcohol or soap and water afterward, and label this thermometer so it’s only used rectally.
Behavioral Signs That Something Is Wrong
Temperature alone doesn’t tell the whole story. Only about half of newborns with an infection actually develop a fever. Some, especially premature babies, may have a lower-than-normal temperature when they’re sick. That makes it important to watch for other changes in how your baby looks and acts.
Signs that your newborn may be ill include:
- Feeding changes: refusing two or more feedings in a row or eating much less than usual
- Unusual sleepiness: sleeping far more than normal or being difficult to wake
- Floppiness or limpness: less muscle tone than usual when you pick them up
- Persistent crying: crying more than normal or being impossible to soothe
- Breathing difficulty: fast breathing, grunting, or visible effort with each breath
- Color changes: skin or lips that look blue, purple, or gray
- Withdrawal: seeming less alert, not making eye contact, or acting “off” in a way that concerns you
If your baby shows any of these signs, whether or not you’ve measured a fever, that warrants a call to your pediatrician right away.
Overheating vs. a True Fever
Newborns can’t regulate their body temperature well, so bundling them in too many layers or keeping the room too warm can push their temperature up without any illness involved. An overheated baby typically looks flushed, has hot skin (especially on the face), and may seem restless or irritable.
If you suspect overheating, remove a layer of clothing and wait 15 to 20 minutes before retaking the temperature. A baby who was simply too warm will cool down quickly once the extra layers come off. If the temperature stays at or above 100.4°F after you’ve adjusted their clothing and environment, treat it as a real fever.
Why Newborn Fevers Are Treated Urgently
The American Academy of Pediatrics has specific guidelines for evaluating babies between 8 and 60 days old who develop a fever of 100.4°F or higher. These guidelines break infants into three age groups (8 to 21 days, 22 to 28 days, and 29 to 60 days) because the risks and recommended evaluations differ at each stage. Younger newborns face higher risk from bacterial infections, so doctors take an aggressive approach to testing.
When you bring a febrile newborn in, expect the medical team to check blood and urine at minimum. Depending on your baby’s age and how they look clinically, additional testing may be needed. For the youngest newborns, a brief hospital stay for monitoring is common even when initial results look reassuring. This can feel alarming as a parent, but the careful approach exists because serious infections in newborns can progress quickly and don’t always produce obvious symptoms early on.
What Not to Give a Newborn for Fever
Do not give any fever-reducing medication to a newborn without direction from a doctor. Ibuprofen is not approved for babies under six months. Acetaminophen (the active ingredient in infant Tylenol) is generally not recommended for babies under 3 months unless a healthcare provider specifically instructs you on dosing. In newborns, the priority is identifying the cause of the fever, not masking it with medication. Bringing the temperature down at home can actually make it harder for doctors to assess what’s going on.
If your newborn’s rectal temperature hits 100.4°F or higher, skip the home remedies and call your pediatrician immediately, or head to the emergency room if it’s after hours. For babies under 28 days old, this is always treated as an emergency regardless of how well your baby appears to be acting.