A temperature above 100.4°F (38.0°C) is the standard definition of a fever in children, according to the American Academy of Pediatrics. But the number that should concern you depends on your child’s age, and in some cases, temperatures slightly below that threshold still warrant attention in very young babies.
Fever Thresholds by Age
While 100.4°F is the widely accepted cutoff, younger children run slightly cooler than older kids, so a lower reading can still signal a fever. For babies 3 months and younger, a temperature above 99.4°F (37.4°C) is considered elevated. For children between 3 months and 3 years, fever starts above 99.6°F (37.6°C), and a “high fever” begins above 101.3°F (38.5°C). For kids older than 3, fever starts above 99.9°F (37.7°C), and high fever begins above 103.0°F (39.4°C).
These distinctions matter because a baby’s immune system is far less developed than an older child’s. A 2-month-old with a temperature of 100.4°F needs prompt medical evaluation, even if the baby seems fine. Hospitals routinely run blood work, urine tests, and sometimes a spinal tap on febrile infants under 8 weeks old because serious bacterial infections are harder to detect by appearance alone at that age.
When a Fever Becomes an Emergency
A temperature of 105°F (40.6°C) or higher is a medical emergency in any child and requires immediate attention. Below that level, the number on the thermometer matters less than how your child looks and acts. A child with 103°F who is drinking fluids and playing is generally less concerning than a child with 101°F who is limp and unresponsive.
Regardless of the temperature reading, certain warning signs call for urgent care:
- Extreme sleepiness or unresponsiveness: difficulty waking up or not reacting to your voice or touch
- Breathing trouble: fast, labored breaths, chest pulling inward, or blue-tinged lips
- Severe dehydration: dry mouth, no tears when crying, fewer than six wet diapers in 24 hours, or a sunken soft spot on a baby’s head
- Rash that doesn’t fade when pressed, or purple spots on the skin, which can indicate a serious bacterial infection like meningitis
- Stiff neck: your child resists bending their head forward or turning their neck
- Persistent, unusual crying: high-pitched and inconsolable, beyond normal fussiness
- Seizures: uncontrollable shaking or stiffening of the body (lay your child on their side and call for help)
For babies, a bulging soft spot on the head can signal increased pressure inside the skull, while a sunken soft spot points to dehydration. Both warrant a call to your pediatrician right away.
Febrile Seizures
Febrile seizures are one of the most frightening things a parent can witness, but they’re relatively common and usually harmless. Children between 6 months and 5 years are most susceptible, with the peak risk between ages 1 and 3. The typical febrile seizure lasts a few seconds to 15 minutes and happens only once within a 24-hour period.
Interestingly, the height of the fever doesn’t predict seizures as much as you might expect. Children who seize at a relatively low temperature are actually more likely to have another febrile seizure in the future. A family history of seizures and having the first seizure before 18 months also raise the odds of recurrence. If your child has a febrile seizure, lay them on their side, don’t put anything in their mouth, and contact your doctor once it passes.
Which Thermometer to Use
Rectal thermometers give the most accurate reading and are the recommended method for children under 3. Other methods can miss fevers entirely. Oral temperatures average about 1°F lower than rectal readings, but the gap can be as large as 2.9°F in either direction. Ear (tympanic) thermometers are even less reliable: only about half of children who register 100.4°F with an ear thermometer actually have a true rectal fever.
Forehead and underarm readings are convenient for a quick check but shouldn’t be relied on when accuracy matters. Underarm (axillary) readings are particularly unreliable and aren’t recommended for screening. If you get a borderline reading from any non-rectal method, and your child seems unwell, a rectal reading will give you the most trustworthy number.
Keeping Your Child Comfortable
Fever itself is a sign your child’s immune system is working, not something that needs to be eliminated at all costs. The goal of treatment is comfort, not hitting a specific number on the thermometer.
Dress your child in a single layer of lightweight clothing unless they’re shivering. Bundling up a feverish baby under 1 year old is particularly risky because infants overheat easily. Offer extra fluids frequently: water or cold drinks for older children, extra breast milk or formula for babies under 6 months. Small, frequent sips work better than trying to get a sick child to drink a full cup at once.
Fever-Reducing Medication
Acetaminophen (Tylenol) can be given every 4 to 6 hours, up to 5 doses in 24 hours. It’s safe for babies 3 months and older. Ibuprofen (Advil, Motrin) can be given every 6 to 8 hours, up to 4 doses per day, but only to children 6 months and older. Both medications are dosed by weight, not age, so check the packaging carefully or ask your pharmacist if you’re unsure.
As a rough guide for children’s liquid acetaminophen (160 mg per 5 mL): a child weighing 12 to 17 pounds gets 2.5 mL, 24 to 35 pounds gets 5 mL, and 48 to 59 pounds gets 10 mL. For children’s liquid ibuprofen (100 mg per 5 mL): a child weighing 24 to 35 pounds gets 5 mL, and 48 to 59 pounds gets 10 mL. Never give aspirin to children, as it’s linked to a rare but serious condition called Reye’s syndrome.