A baby has a fever when their rectal temperature reaches 100.4°F (38°C) or higher. That single number is the standard threshold pediatricians use for infants of all ages, and a rectal reading is the most reliable way to confirm it. But before you even reach for a thermometer, your baby’s behavior often gives the first clue: fussiness that seems different from normal, poor feeding, disrupted sleep, or skin that feels noticeably warm to the touch.
What Temperature Counts as a Fever
The number that matters depends on where you measure. A rectal, ear, or forehead temperature of 100.4°F or higher is a fever. An armpit reading uses a lower cutoff of 99°F (37.2°C) because the skin surface runs cooler than the body’s core. If you take an oral temperature on an older baby, the threshold is 100°F (37.8°C).
These thresholds don’t change based on your baby’s age, but what you should do about the reading does. For babies under 3 months, any rectal temperature at or above 100.4°F warrants an immediate call to your pediatrician or a trip to the emergency room, even if your baby seems fine otherwise. At that age, a fever can signal a serious infection that young immune systems can’t fight well on their own. The American Academy of Pediatrics has specific evaluation guidelines for infants 8 to 60 days old with fever, which can include blood work and urine testing to rule out bacterial infections.
For babies 3 to 6 months old, a temperature up to 101°F (38.3°C) is less alarming on its own, but contact your pediatrician if your baby seems unusually irritable, sluggish, or uncomfortable, or if the temperature goes higher than 101°F. For babies 6 to 24 months, a temperature above 101°F that lasts more than a day needs medical attention, even without other symptoms.
Behavioral Signs Before You Use a Thermometer
Most parents notice something is off before they ever check a number. Babies with a fever are usually more irritable than normal, and the higher the temperature climbs, the more pronounced the fussiness becomes. You might notice your baby refusing to eat or nursing for shorter periods. Sleep patterns often shift too, with more frequent waking or difficulty settling down.
Touch is a reasonable first screen. If your baby’s forehead, back, or belly feels warmer than usual, that’s worth following up with a thermometer. Flushed cheeks or skin that looks redder than normal can also be a visual cue. Keep in mind that these signs aren’t definitive. A baby who just woke up from a nap in a warm room or was bundled in heavy clothing can feel warm without having a true fever. If your baby feels hot, remove a layer and wait 15 to 20 minutes before taking a temperature to rule out overheating from the environment.
One thing to watch carefully: a baby who becomes listless, drowsy, or unresponsive goes beyond typical fever fussiness. That level of lethargy is a warning sign regardless of the thermometer reading.
Which Thermometer to Use
For babies under 6 months, a digital rectal thermometer gives the most accurate result. Clinical evidence shows that rectal electronic thermometers stay within 0.9°F (0.5°C) of true core body temperature 95% of the time. No other consumer thermometer type meets that accuracy standard.
Forehead (temporal artery) thermometers are popular because they’re fast and non-invasive, but they can be off by as much as 2°F in either direction. Ear (tympanic) thermometers have an even wider margin of error, averaging about 1°F higher than core temperature and sometimes swinging nearly 4°F off. Armpit readings tend to run low and are similarly unreliable. These tools are fine for a quick check on an older child, but for an infant where a single degree changes the medical response, rectal is the way to go.
To take a rectal temperature: apply a small amount of petroleum jelly to the tip of the thermometer, lay your baby on their back with knees bent toward the chest, and gently insert the tip about half an inch. Hold it in place until the thermometer beeps. The whole process takes under a minute.
Red Flags That Need Immediate Attention
Some symptoms alongside a fever point to something more serious than a common virus. Get emergency care if your baby shows any of the following:
- Purple or blood-colored spots or dots on the skin, which can indicate a dangerous bloodstream infection
- A stiff neck or resistance to moving the head normally
- Nonstop crying, especially crying that worsens when you touch or move your baby
- Extreme drowsiness or appearing “out of it” when awake
- Difficulty breathing or breathing that looks unusually fast
In younger babies who can’t point to what hurts, persistent crying with a fever often means pain. Hidden sources include ear infections, urinary tract infections, and sore throats. Meningitis, a bacterial infection of the membranes covering the brain and spinal cord, can cause extreme irritability or lethargy in babies who are too young to show the classic stiff-neck symptom that older children display.
Watching for Dehydration
Fever increases fluid loss, and babies dehydrate faster than adults. Monitor your baby’s wet diapers closely. Fewer wet diapers than usual, or none for three hours, is an early sign of dehydration. Other signals include a dry mouth, no tears when crying, sunken eyes, and a sunken soft spot on top of the head. If you gently pinch the skin on your baby’s hand or belly and it doesn’t flatten back right away, that suggests more significant fluid loss.
Offer frequent feedings, whether breast milk, formula, or an electrolyte solution recommended by your pediatrician for older babies. Small, frequent amounts are easier for a feverish baby to tolerate than large feedings spaced far apart.
Safe Ways to Bring the Fever Down
Not every fever needs medication. Fever is the body’s natural response to infection and helps fight off illness. For mild fevers where your baby is still drinking and relatively comfortable, you can simply keep them lightly dressed and offer extra fluids.
When a fever is making your baby miserable, acetaminophen is an option for babies 8 weeks and older. Dosing is based on weight, not age. Do not give acetaminophen to any infant under 8 weeks old. Ibuprofen is not recommended until 6 months of age unless your pediatrician specifically approves it.
Never give aspirin to a baby or child. Lukewarm sponge baths can offer temporary comfort, but avoid cold water or alcohol rubs, which can cause shivering and actually raise core temperature. Keep the room at a comfortable temperature and dress your baby in a single light layer. Bundling a feverish baby in blankets traps heat and can push the temperature higher.