A fever in an infant is a rectal temperature of 100.4°F (38°C) or higher. That number is the same regardless of your baby’s age, but what you should do about it changes dramatically depending on how old your baby is. For infants under 2 months, a temperature at or above 100.4°F is a medical emergency that warrants an immediate trip to the emergency department.
Why Age Matters More Than the Number
The 100.4°F threshold applies across the board, but the urgency behind that number shifts as your baby grows. Newborns have immature immune systems, which means even a low-grade fever can signal a serious bacterial infection. The American Academy of Pediatrics breaks down its clinical guidelines into narrow age windows for this reason: 8 to 21 days, 22 to 28 days, and 29 to 60 days, each with its own evaluation steps.
Once your baby is past 3 months, the risk profile changes. Guidelines from the UK’s National Institute for Health and Care Excellence place babies under 3 months with any fever at or above 100.4°F in a high-risk category for serious illness. Babies between 3 and 6 months move into an intermediate-risk category, and only at higher temperatures of 102.2°F (39°C) or above. After 6 months, a fever is still worth monitoring, but healthy babies in this age range are better equipped to fight off common infections on their own.
How to Take Your Baby’s Temperature
Rectal thermometers are the gold standard for babies under 3 months. A forehead (temporal artery) thermometer is nearly as accurate and much easier to use, making it a reasonable option for older infants. Ear thermometers work quickly but aren’t recommended for babies younger than 6 months because the small ear canal makes readings unreliable. Earwax can also throw off results.
Skip fever strips that stick to the forehead and pacifier thermometers with color-changing dots. These aren’t reliable enough to trust when accuracy matters most. If you’re using a forehead thermometer outdoors, keep in mind that sunlight and cold air can skew the reading. For the most dependable number, especially in a very young baby, a rectal reading taken indoors is your best bet.
Signs That Need Immediate Attention
A fever on its own is your baby’s immune system doing its job. What matters more than the exact number is how your baby looks and acts. There are specific warning signs that call for urgent care:
- Unusual sleepiness. If your baby is sleeping far more than normal, is difficult to wake, or seems floppy and limp when you pick them up.
- Dehydration. No wet diapers for three hours or more is a red flag. A sunken soft spot on the top of the head is another sign of fluid loss.
- Persistent crying. Crying that’s more intense than usual and very difficult to calm, especially combined with a fever.
- Breathing trouble. Fast or labored breathing, or any sign your baby is working hard to get air.
- Rash. A rash that appears suddenly, blisters, or looks infected alongside a fever.
- Seizure. Any shaking, stiffening, or loss of consciousness during a fever needs emergency care.
- Frequent watery stools. Three or more very loose, watery bowel movements can accelerate dehydration in a small baby.
Febrile Seizures
Febrile seizures are one of the most frightening things a parent can witness, but they’re almost always harmless. They occur in children between 6 months and 5 years old, with the highest risk between 12 and 18 months. Most last only a few seconds to 15 minutes and don’t cause any lasting damage or indicate an underlying neurological problem.
These seizures often happen within the first 24 hours of a fever and can actually be the first sign that your child is sick. You may not even know your baby has a fever until the seizure occurs. If it happens, place your child on a safe surface, turn them on their side, and don’t put anything in their mouth. Time the seizure if you can. Any seizure that lasts longer than five minutes or is a first-time event warrants emergency care.
Post-Vaccination Fevers
A mild fever after routine immunizations is common and not a cause for alarm. These fevers are self-limiting, meaning they resolve on their own without complications. They typically start about 9 to 15 hours after the shot and last an average of 2.5 to 17 hours depending on the vaccine. Nearly all post-vaccination fevers clear within 48 hours.
The temperature tends to be modest. Pneumococcal vaccines produce an average peak of about 101.3°F, while hepatitis A vaccines can push a bit higher, averaging around 102.2°F at their peak. These numbers are averages, so your baby’s response may be slightly higher or lower. A post-vaccination fever that climbs above 102.2°F, lasts more than 48 hours, or is accompanied by any of the warning signs listed above is worth a call to your pediatrician. The critical exception still applies: if your baby is under 2 months old, any rectal temperature of 100.4°F or higher requires emergency evaluation, even if it follows a vaccination.
Keeping Your Baby Comfortable
Fever itself isn’t the enemy. It’s a sign that the immune system is actively fighting an infection. Your main job during a fever is to keep your baby hydrated and comfortable. Offer breast milk or formula more frequently than usual, since babies lose extra fluid through sweat and faster breathing. Track wet diapers closely. Fewer than the usual number, or none for three hours, means your baby needs more fluids and possibly medical attention.
Dress your baby in lightweight clothing and keep the room at a comfortable temperature. Bundling a feverish baby in blankets can trap heat and push the temperature higher. A lukewarm bath can help bring mild relief, but avoid cold water, which can cause shivering and actually raise the body’s core temperature. If your baby is older than 3 months and clearly uncomfortable, infant-appropriate fever reducers can help, but dosing is based on weight, not age, so confirm the right amount with your pediatrician if you’re unsure.