A baby has a fever when their temperature reaches 100.4°F (38°C) or higher, measured rectally. But since you can’t always grab a thermometer in the moment, there are reliable physical and behavioral cues that signal your baby is running hot. Here’s how to check, what the numbers mean, and what to watch for at every age.
The Temperature That Counts as a Fever
For babies of all ages, the medical threshold is 100.4°F (38°C) taken rectally. That single number is the line. But what you do about it depends on your baby’s age:
- Under 3 months: Any rectal temperature of 100.4°F or higher is treated as urgent, regardless of how your baby looks or acts. This age group needs immediate medical evaluation because young infants can’t fight infections the way older babies can.
- 3 to 6 months: A temperature up to 101°F (38.3°C) is concerning if your baby also seems unusually irritable, sluggish, or uncomfortable. Anything above 101°F warrants a call to your pediatrician.
- 6 to 24 months: A temperature above 101°F that lasts longer than one day, even without other symptoms, should be evaluated.
How to Take Your Baby’s Temperature
A rectal thermometer is the most accurate way to measure a baby’s core body temperature. It’s not the most pleasant option, but it gives the reading doctors trust most, especially for newborns. Use a digital thermometer with a flexible tip, apply a small amount of petroleum jelly, and insert it about half an inch. You’ll get a reading in under a minute.
A forehead (temporal artery) thermometer is a good alternative. It’s nearly as accurate as a rectal reading and far less disruptive, which makes it practical for a sleeping or fussy baby. For infants under 3 months who seem sick, a forehead reading works as an initial check, but your pediatrician may want a rectal confirmation.
Ear thermometers are fast (they take about one second) but aren’t reliable for young babies. Ear canals in infants are too small for the sensor to get an accurate reading. Most guidelines recommend waiting until at least 6 to 7 months before using one. Even then, earwax can throw off the result.
Skip the old-school method of touching your baby’s forehead with the back of your hand. It can tell you something feels off, but it can’t distinguish between 99°F and 101°F, and that difference matters.
Signs of Fever You Can Spot Without a Thermometer
Sometimes you notice something is wrong before you reach for the thermometer. Babies with a fever often feel noticeably warm to the touch on their chest, back, or belly (not just hands and feet, which can run cool normally). Their skin may look flushed or blotchy, and their cheeks may turn pink or red.
Behavior changes are often the first clue. A baby with a fever may become unusually fussy or irritable for no clear reason, or swing the other direction and seem excessively sleepy and hard to wake. You might notice they’re less interested in feeding, pulling away from the breast or bottle after just a few minutes. Some babies breathe faster than usual or seem to be working harder to breathe.
None of these signs alone confirms a fever, but together they paint a clear picture. If your baby feels warm and is acting differently than normal, take their temperature.
Red Flags That Need Immediate Attention
Most fevers in babies are caused by common viral infections and resolve on their own. But certain signs alongside a fever point to something more serious. Get emergency care if your baby has any of the following with a fever:
- A dark or purple rash that looks like tiny bruises and doesn’t fade when you press on it. This type of rash (called petechial) can signal a dangerous infection.
- Difficulty breathing or breathing that’s noticeably faster or more labored than usual.
- Extreme sleepiness or poor responsiveness. If your baby is hard to wake up or seems limp and uninterested in their surroundings, that’s different from normal tiredness.
- Fewer wet diapers than usual, which suggests dehydration. This is especially concerning if your baby is also refusing fluids.
- A seizure. Some babies experience febrile seizures during a fever, which typically involve shaking or jerking of the arms and legs, body stiffness, and a brief loss of consciousness. Most last under 15 minutes and stop on their own, but any seizure in a baby should be evaluated right away.
What a Febrile Seizure Looks Like
Febrile seizures can happen when a baby’s temperature rises quickly. They’re more common than most parents expect. During one, your baby may shake all over, go stiff, jerk their arms and legs, or briefly lose consciousness. It’s terrifying to watch, but the vast majority of simple febrile seizures end within 15 minutes and don’t cause lasting harm.
A seizure that lasts longer than 15 minutes, happens more than once in 24 hours, or affects only one side of the body is considered complex and needs prompt medical evaluation. During any seizure, place your baby on a safe surface away from hard objects, turn them gently onto their side, and note how long it lasts. Don’t put anything in their mouth.
Tracking Hydration During a Fever
Fever increases fluid loss, and babies dehydrate faster than adults. The simplest way to monitor hydration is by counting wet diapers. If your baby is producing noticeably fewer wet diapers than usual, they may not be getting enough fluid. Other dehydration signals include a dry mouth, no tears when crying, and a sunken soft spot on top of the head.
Offer frequent, smaller feedings rather than waiting for scheduled ones. Breastfed babies may want to nurse more often, which is fine and helpful. Formula-fed babies should continue their regular formula. For babies over 6 months, small sips of water between feedings can help.
How to Monitor a Fever at Home
Once you’ve confirmed a fever, recheck the temperature every few hours rather than constantly. Write down the readings and the times so you can share them with your pediatrician if needed. Note any behavioral changes alongside the temperature: when your baby last ate, how many wet diapers they’ve had, and whether they seem to be improving or getting worse.
Dress your baby in light, breathable clothing. Bundling them up can trap heat and push the temperature higher. A single layer with a light blanket is enough. Keep the room at a comfortable temperature, not cold. A lukewarm (not cold) sponge bath can provide some comfort, but it won’t treat the fever itself.
For babies over 3 months, infant-appropriate fever reducers can help bring the temperature down and ease discomfort. Your pediatrician can guide you on the right product and dose based on your baby’s weight. For babies under 3 months, don’t give any medication before calling your doctor first, since that age group needs evaluation before treatment.