A baby has a fever when their rectal temperature reaches 100.4°F (38°C) or higher. That single number is the standard used by pediatricians, and it applies from the newborn stage onward. But knowing whether your baby has a fever involves more than just the number on the thermometer. How you take the temperature, what your baby looks and acts like, and how old they are all shape what that reading means.
The Temperature That Counts as a Fever
The threshold depends on where you measure. A rectal, ear, or forehead (temporal artery) reading of 100.4°F or higher is a fever. An armpit reading of 99°F or higher suggests a fever, though armpit readings are less reliable. These cutoffs are the same whether your baby is two weeks old or ten months old.
Babies’ body temperatures naturally fluctuate throughout the day, running slightly higher in the late afternoon and evening. A reading of 99°F rectally in the evening is not a fever. It’s normal variation. What matters is crossing that 100.4°F line.
Why Rectal Temperatures Are the Most Reliable
For babies under three months, a rectal thermometer is the only method accurate enough to trust. Forehead (temporal artery) thermometers tend to read about 0.2°C lower than rectal, and armpit thermometers run a full 0.9°C lower on average. In one emergency department study, armpit thermometers detected only 11.5% of fevers that rectal readings confirmed, and forehead thermometers caught just 61.5%. That gap matters when even a small difference in temperature changes the medical response.
For babies older than three to six months, a forehead or ear thermometer can work as a screening tool. But if you get a borderline reading and want certainty, rectal is still the gold standard.
How to Take a Rectal Temperature
It sounds intimidating, but the process is straightforward. Use a digital thermometer labeled for rectal use. Apply a small dab of petroleum jelly (Vaseline) to the tip. Lay your baby on their back with their knees gently bent toward their chest. Insert the thermometer tip about one inch (2.5 cm) into the rectum and hold it in place with your fingers. A digital thermometer will beep when the reading is ready, usually within 10 to 30 seconds.
Never force the thermometer or let go of it while it’s inserted. Clean it with soap and cool water afterward, and keep it separate from any thermometer you use orally for older family members.
Signs of Fever Without a Thermometer
Sometimes you notice something is off before you reach for the thermometer. Babies with a fever often feel warmer than usual on their forehead, back, or belly. Their skin may look flushed or feel clammy. Beyond warmth, look for behavioral changes: unusual fussiness, being harder to console, feeding less than normal, or seeming sleepier and less alert than usual. Shivering or sweating can also signal that their body is fighting to regulate its temperature.
These signs are useful clues, but they’re not substitutes for an actual reading. A baby can feel warm from being overbundled, from crying, or simply from a warm room. The only way to confirm a fever is with a thermometer.
Overbundling Can Fool You
If your baby feels warm and you’ve had them wrapped in blankets or dressed in heavy layers, don’t take a temperature right away. Bundling raises skin temperature significantly, as much as 2.67°C per hour in one study. The good news: bundling does not meaningfully raise rectal temperature in a healthy baby kept at normal room temperature (around 72 to 75°F). So if you’re using a rectal thermometer, overbundling is unlikely to produce a false fever reading. But if you’re checking with a forehead or armpit thermometer, remove a layer, wait 10 to 15 minutes, and then recheck.
What Your Baby’s Age Means for a Fever
Age changes the urgency dramatically. Here’s how pediatric guidelines break it down:
- Under 3 months: Any rectal temperature of 100.4°F or higher needs immediate medical attention. At this age, a baby’s immune system is still immature, and a fever can be the only sign of a serious infection. The American Academy of Pediatrics has specific evaluation protocols for babies as young as 8 days old because the risks are real. Don’t wait to see if it resolves on its own.
- 3 to 6 months: A temperature up to 101°F may not require an urgent visit if your baby is acting normally, eating, and staying hydrated. But if the fever reaches 102°F or higher, or your baby seems unusually irritable, sluggish, or uncomfortable even at a lower temperature, that warrants a call to your pediatrician.
- 6 to 24 months: A fever above 101°F that lasts longer than one day without other obvious symptoms (like a runny nose or mild cold) is worth a call. Fevers with clear cold symptoms in an otherwise alert, drinking baby are generally less concerning, but trust your instincts.
Watch for Dehydration
Fever increases fluid loss, and babies dehydrate faster than adults. The clearest sign to track is wet diapers. Fewer than usual is a yellow flag. No wet diaper for three hours or more is a red flag. Other signs of dehydration include crying without producing tears, a dry mouth and tongue, and in young babies, a sunken soft spot on the top of the head. A baby who is still alert, playful, and feeding is unlikely to be significantly dehydrated, even with a fever.
Signs That Need Emergency Attention
Most fevers in babies are caused by common viral infections and resolve within a few days. But certain signs alongside a fever signal something more serious. Difficulty breathing, including fast breathing, grunting, or skin pulling in between the ribs with each breath, is an emergency. Bluish or gray lips, tongue, or gums mean your baby isn’t getting enough oxygen. A seizure, even a brief one, means calling 911. Extreme lethargy where your baby is difficult to wake or doesn’t respond to you is also cause for immediate care.
Severe vomiting or diarrhea combined with signs of dehydration can escalate quickly in a small baby. If your baby won’t take fluids at all, don’t wait for the next feeding cycle to see if things improve.