A baby has a fever when their rectal temperature reaches 100.4°F (38.0°C) or higher. This is the standard threshold used by pediatricians and children’s hospitals regardless of the baby’s age, but what you should do about that fever depends heavily on how old your baby is.
The 100.4°F Threshold
For babies and young children, a fever is defined as a rectal, ear, or forehead temperature of 100.4°F (38.0°C) or higher. Normal body temperature fluctuates throughout the day, so a reading slightly above 98.6°F doesn’t necessarily mean your baby is sick. The 100.4°F mark is the point where the body is mounting a meaningful immune response, typically to fight off an infection.
It’s worth noting that teething, despite what many parents assume, does not typically push a baby’s temperature above 101°F (38.3°C). If your teething baby has a temperature higher than that, something else is likely going on.
Why the Thermometer You Use Matters
A rectal thermometer is the most accurate way to measure a baby’s temperature, and it’s the recommended method for any baby under 3 months old. This matters because at that age, even a small difference in reading could change whether your baby needs emergency care.
Armpit (axillary) readings are the least reliable. They can be useful as a quick first check if your child seems warm, but they tend to read lower than the true core temperature. Ear thermometers aren’t considered accurate for babies under 6 months because the ear canal is too small to get a reliable reading. After 6 months, ear thermometers become a reasonable option.
If you get a concerning armpit reading, follow up with a rectal measurement before making decisions about what to do next.
Age Changes Everything
The same 100.4°F fever that might be manageable in a 6-month-old can be a medical emergency in a newborn. A young baby’s immune system is still immature, which means infections can escalate quickly and are harder to pinpoint without testing.
If your baby is younger than 2 months and has a rectal temperature above 100.4°F, go to an emergency department immediately. Don’t wait to see if the fever comes down on its own, and don’t give fever-reducing medicine first. At this age, doctors need to evaluate the baby to rule out serious bacterial infections.
If your baby is between 2 and 3 months old with a temperature above 100.4°F, call your pediatrician right away. They’ll likely want to see the baby the same day or direct you to an emergency department depending on other symptoms.
The American Academy of Pediatrics published clinical guidelines that break infant fever evaluation into specific age windows: 8 to 21 days, 22 to 28 days, and 29 to 60 days. Each window has a different approach, with the youngest babies receiving the most thorough workup. For babies older than about 2 months, doctors can often take a less invasive approach if the baby looks well otherwise.
What Causes Fevers in Babies
The vast majority of fevers in infants come from infections, and most of those are viral. The common culprits include respiratory viruses (colds and flu), stomach bugs, ear infections, and urinary tract infections. Bacterial infections like pneumonia or skin infections are less common but more serious.
Vaccinations can also trigger a low-grade fever, usually within hours to a day after the shot. Some vaccines, particularly the measles vaccine, can cause a fever up to one or two weeks later. Vaccine fevers are generally mild and short-lived.
Rarely, fever signals something more serious like meningitis (infection of the membranes around the brain), Kawasaki disease (which causes inflammation in blood vessels), or multisystem inflammatory syndrome. These conditions have other distinctive symptoms alongside fever, which is why doctors pay close attention to the full picture, not just the number on the thermometer.
Warning Signs That Need Immediate Attention
A fever number alone doesn’t tell the whole story. How your baby looks and acts matters just as much. Seek emergency care if your baby has a fever along with any of these:
- Extreme sleepiness or unresponsiveness. If your baby is unusually hard to wake up or doesn’t respond to your voice or touch, that’s a red flag.
- Difficulty breathing. Watch for fast, labored, or shallow breaths. If the skin between the ribs pulls inward with each breath, or if the lips or face turn blue, call 911.
- Signs of dehydration. Dry mouth, cracked lips, fewer than six wet diapers in 24 hours, no tears when crying, or a sunken soft spot on the head.
- A rash that doesn’t fade when pressed, or purple spots on the skin. These can indicate a serious bacterial infection like meningitis.
- A bulging soft spot. This may signal increased pressure inside the skull.
- Seizures. Fevers can trigger febrile seizures, which involve uncontrollable shaking or stiffening. Lay your baby on their side and call your doctor.
- High-pitched, unusual crying that can’t be soothed. This may signal significant pain or distress.
- A stiff neck. If your baby resists moving their neck or can’t bend it forward, this could point to meningitis.
Treating a Fever at Home
For babies 3 months and older who are otherwise acting normally (feeding, making eye contact, having wet diapers), you can often manage a fever at home. Acetaminophen is safe for babies 3 months and older, dosed by weight rather than age. Ibuprofen is not approved for babies under 6 months old, so don’t use it before then unless specifically directed by your pediatrician. Either medication can be given every 6 to 8 hours as needed.
Keep your baby lightly dressed and offer frequent feedings to prevent dehydration. Breast milk, formula, or an electrolyte solution for older babies all help maintain fluid intake. You don’t need to bundle a feverish baby in extra blankets, which can actually trap heat and push the temperature higher.
Lukewarm baths, once a common recommendation, turn out to offer very little benefit. Research shows they lower temperature by only about 0.3°C on average compared to fever-reducing medicine alone, and they cause significant discomfort. The temperature difference between the water and the baby’s overheated body triggers shivering and crying. Skipping the bath and relying on medication and fluids is a more effective, more comfortable approach.
Remember that fever itself is not the enemy. It’s the body’s way of fighting infection. The goal of treatment is to keep your baby comfortable, not to force the temperature back to exactly 98.6°F. If your baby is sleeping peacefully with a low fever, there’s no need to wake them for medicine.