A rectal temperature of 100.4°F (38°C) or higher is considered a fever in an infant. This threshold applies regardless of the baby’s age, but what you should do about it depends heavily on how old your baby is. For infants under 2 months old, a temperature at or above 100.4°F requires immediate medical evaluation, even if your baby looks perfectly fine.
The 100.4°F Threshold
The American Academy of Pediatrics and the Mayo Clinic both define an infant fever as a rectal, ear, or temporal artery temperature of 100.4°F (38°C) or higher. This is the number pediatricians use to make clinical decisions, and it’s the number you should have in mind when checking your baby’s temperature at home.
Normal body temperature in infants fluctuates throughout the day, typically sitting somewhere between 97°F and 100.3°F. A reading of 99°F or 100°F can feel alarming, but it falls within the normal range. The 100.4°F cutoff is where the medical definition of fever begins.
Why Your Baby’s Age Matters
A fever of 100.4°F in a 4-month-old and the same fever in a 2-week-old are treated very differently. The AAP breaks febrile infant evaluation into three age groups: 8 to 21 days, 22 to 28 days, and 29 to 60 days. Younger babies have immature immune systems, which means a fever can be the only visible sign of a serious infection.
For babies under 8 weeks (56 days), a recorded temperature of 100.4°F or higher within the past 24 hours warrants emergency evaluation, even if the baby appears well. At this age, doctors will typically run blood work and urine tests to rule out bacterial infections. This isn’t optional or overly cautious. It’s standard care because young infants can deteriorate quickly without clear warning signs.
Once your baby is older than 2 to 3 months, a fever becomes less automatically urgent. At that age, how your baby is acting matters more than the number on the thermometer. A 4-month-old with a 101°F fever who is feeding well, making eye contact, and producing wet diapers is in a very different situation than one who is limp and refusing to eat.
How to Take an Accurate Temperature
For babies under 3 years old, a rectal thermometer gives the most accurate reading. The AAP advises against using ear (tympanic) thermometers for infants younger than 3 months because the ear canal is too small to get a reliable measurement. Armpit readings are convenient but less precise, and they tend to read lower than the actual core body temperature.
To take a rectal temperature, apply a small amount of petroleum jelly to the tip of a digital thermometer, lay your baby face-down across your lap or on a flat surface, and gently insert the tip about half an inch. Hold it in place until the thermometer beeps. If you get a reading of 100.4°F or higher and your baby is under 2 months old, that number alone is enough to call your pediatrician or head to the emergency room.
Common Causes of Infant Fever
Most fevers in infants are caused by viral infections: colds, respiratory viruses, and stomach bugs. These fevers tend to resolve on their own within a few days as the immune system fights off the virus. Bacterial infections are less common but more concerning, which is why doctors in the youngest age groups run tests to distinguish between the two.
Teething is often blamed for fevers, but the evidence is thin. One study in the Archives of Disease in Childhood found that some infants did develop temperatures above 99.5°F around the day a tooth erupted, but the elevations were mild and short-lived. Teething does not cause high fevers. If your baby has a temperature of 100.4°F or above, it’s worth looking for another cause rather than chalking it up to a new tooth.
Vaccinations can also trigger a low-grade fever within 12 to 24 hours of the shot. This is a normal immune response and usually resolves within a day or two.
Signs a Fever Needs Urgent Attention
Beyond the temperature number itself, certain behaviors signal that something more serious is going on. Watch for these in a febrile infant of any age:
- Extreme drowsiness or difficulty waking. A sick baby will sleep more than usual, but you should still be able to rouse them.
- Fewer wet diapers than normal. Less than half the usual number of wet diapers suggests dehydration.
- A bulging soft spot (fontanelle). The soft spot on top of the head should be flat or slightly sunken. If it’s visibly bulging, that needs immediate evaluation.
- Inconsolable crying. Fussiness is expected, but crying that won’t stop no matter what you try is a red flag.
- Skin color changes. Pale, mottled, or bluish skin warrants emergency care.
Febrile Seizures
About 3 to 4 out of every 100 children will experience a febrile seizure, a convulsion triggered by a rapid rise in body temperature. These are most common with fevers of 102°F (38.9°C) or higher, but they can happen with milder fevers too. Febrile seizures typically occur in children between 6 months and 5 years old.
Watching your baby have a seizure is terrifying, but most febrile seizures last less than a few minutes and do not cause lasting harm. If it happens, lay your child on their side on a safe surface, don’t put anything in their mouth, and time the episode. A seizure lasting longer than 5 minutes needs emergency care. Even for a short seizure, it’s worth calling your pediatrician afterward, especially if it’s the first one.
Keeping Your Baby Comfortable at Home
For babies older than 2 to 3 months with a fever that doesn’t require emergency evaluation, comfort care at home focuses on hydration, appropriate clothing, and keeping the environment cool.
Hydration is the priority. Breastfed babies should nurse more frequently. Formula-fed babies should continue their normal feeding schedule. If your baby is old enough for supplemental fluids, a pediatric electrolyte solution can help prevent dehydration. The goal is to keep wet diapers coming at a normal pace.
Dress your baby in lightweight, loose-fitting clothes. It’s tempting to bundle up a baby who has chills, but piling on blankets traps heat and can push the temperature higher. One light layer and a single blanket is enough. Turning the thermostat down a degree or two at home also helps.
Lukewarm baths can bring temporary relief, but avoid cold water or alcohol rubs, both of which can cause shivering and actually raise core temperature. Let the water be comfortably warm, not cool enough to make your baby uncomfortable.