A temperature of 100.4°F (38°C) or higher is considered a fever in children. But the number that’s “too high” depends heavily on your child’s age. For babies under 3 months, 100.4°F already warrants urgent medical attention. For older children, the true emergency threshold is 105°F (40.6°C), according to the American Academy of Pediatrics.
Fever Thresholds by Age
A child’s age changes how seriously you need to take any given temperature. Here’s how it breaks down:
- Newborns under 3 months: A rectal temperature of 100.4°F (38°C) or higher is considered high-risk for serious illness. Babies this young have immature immune systems, and fever can be the only sign of a dangerous infection. Infants under 21 days old with a confirmed fever typically undergo blood, urine, and spinal fluid testing and are hospitalized for 24 to 48 hours while doctors rule out bacterial infections.
- Babies 3 to 6 months: A fever of 102.2°F (39°C) or higher places them in an intermediate-risk category. A temperature between 100.4°F and 102.2°F still deserves a call to your pediatrician, but the urgency is lower than with a newborn.
- Children over 6 months: Fever above 100.4°F is still defined as a fever, but most fevers in this age group are caused by common viral infections and resolve on their own. The number to watch for is 105°F (40.6°C), which is a medical emergency requiring immediate care.
A temperature reading at home counts. If your thermometer registers 100.4°F or above, whether taken rectally or under the arm, it should be treated as a real fever. Underarm readings actually average about half a degree lower than rectal readings, so a high underarm number is particularly meaningful.
How to Get an Accurate Reading
Rectal thermometers give the most accurate reading of core body temperature, and they’re the recommended method for any child under 3 months. Ear (tympanic) thermometers are safer and easier to use, but they read slightly lower, about 0.1°C (roughly 0.2°F) less than rectal on average. That small gap matters when you’re right at the 100.4°F cutoff for a young baby.
For children over 6 months, ear or forehead thermometers are reasonable options. Oral thermometers work well once a child is old enough to hold one under their tongue without biting it, usually around age 4 or 5.
Warning Signs That Matter More Than the Number
Temperature alone doesn’t tell the whole story. A child with 103°F who is drinking fluids and playing between naps is generally in better shape than a child with 101°F who is limp, unresponsive, or refusing to drink. Pay attention to how your child looks and acts, not just the number on the thermometer.
Certain symptoms alongside a fever signal a true emergency regardless of temperature:
- Trouble breathing: Fast breathing, grunting, bluish lips, or skin pulling in between the ribs with each breath.
- Stiff neck: Your child can’t touch their chin to their chest.
- Purple or blood-red spots on the skin: Especially spots that don’t fade when you press on them. Combined with fever, this can indicate a serious bloodstream infection.
- Bulging soft spot: In babies, a tense or bulging fontanelle suggests pressure on the brain.
- Severe dehydration: No tears when crying, dry mouth and tongue, or a sunken soft spot in infants.
- Extreme lethargy: Your child is difficult to wake, won’t make eye contact, or seems confused.
Febrile Seizures
Febrile seizures are one of parents’ biggest fears, and they affect 2% to 5% of children in the U.S. and Europe. They occur most often between 6 months and 5 years of age, peaking around 12 to 18 months. Despite what many people assume, there’s no specific temperature that triggers a seizure. Every child has a different threshold, and seizures can happen with fevers as low as 100.4°F. They often occur as a temperature is rising rapidly, before a parent even realizes the child has a fever.
Simple febrile seizures involve whole-body shaking and last less than 15 minutes, followed by brief drowsiness. They look terrifying but don’t cause brain damage or epilepsy. Complex febrile seizures last longer than 15 minutes, affect only one side of the body, or happen more than once in 24 hours. These need prompt medical evaluation. You cannot prevent febrile seizures by giving fever-reducing medication early, since the seizure often occurs before the fever is detected.
Managing Fever at Home
Fever itself is not dangerous in most cases. It’s the body’s way of fighting infection, and bringing the number down doesn’t cure anything. The goal of treating a fever is comfort: helping your child feel well enough to rest, eat, and drink.
Acetaminophen (Tylenol) can be given to babies 3 months and older. Ibuprofen (Advil, Motrin) is safe for babies 6 months and older. Both are dosed by weight, not age, so check the packaging carefully or ask your pharmacist. Do not give either medication to a baby younger than 3 months without a doctor’s guidance, because fever at that age needs evaluation first, not treatment at home.
Keeping your child hydrated matters more than lowering the fever. A good rule of thumb for a dehydrated child is roughly 1½ to 2½ ounces of fluid per pound of body weight over 24 hours, using an oral rehydration solution. For mild cases, watch for wet diapers. Fewer wet diapers than usual, a dry mouth, or no tears when crying are early signs of dehydration. Offer small, frequent sips rather than large amounts at once.
Lukewarm baths can help a child feel more comfortable, but avoid cold water or rubbing alcohol, both of which can cause shivering and actually raise core temperature. Dress your child in light clothing and keep the room at a comfortable temperature.
When to Seek Immediate Care
The simplest framework: any fever in a baby under 3 months needs same-day medical evaluation, even if the baby looks fine. For children 3 months and older, a temperature of 105°F or above is an emergency. Between those extremes, the decision depends on symptoms. A fever lasting more than 3 days, a child who won’t drink fluids, or any of the red-flag symptoms listed above all warrant a call to your pediatrician or a trip to urgent care.