A fever in a baby is a rectal temperature of 100.4°F (38°C) or higher. That single number applies regardless of your baby’s age, but what you should do about it changes dramatically depending on whether your child is a few weeks old or closer to a year. Understanding those age-based differences is the most important thing a parent can take away from this topic.
The 100.4°F Threshold
Any temperature below 100.4°F is not technically a fever. Babies’ body temperatures fluctuate throughout the day, and a reading of 99°F or even 100°F can be completely normal, especially after a warm bath, a crying spell, or time spent bundled up. The 100.4°F cutoff is the standard used by pediatricians to distinguish a true fever from normal variation.
This matters because parents sometimes panic over readings just above “normal” that don’t actually qualify as a fever. It also matters in the other direction: a temperature of 100.5°F in a two-week-old is a genuine medical situation, even though the number itself doesn’t sound alarming.
How to Get an Accurate Reading
For babies from birth to 3 months, a rectal temperature with a standard digital thermometer is the most reliable method. It’s the measurement pediatricians will ask about if you call, and other methods aren’t accurate enough at this age to guide decisions. Forehead (temporal artery) thermometers are showing promise for newborns in recent studies, but rectal remains the standard.
Digital ear thermometers shouldn’t be used until your baby is at least 6 months old. The ear canal is simply too small before that point to get a consistent reading. After 6 months, ear and forehead thermometers are reasonable options for a quick check, though rectal still gives the most precise number when precision matters.
Why Age Changes Everything
A fever in a newborn is treated far more seriously than the same temperature in a 9-month-old. The reason is straightforward: very young babies have immature immune systems, and serious infections like urinary tract infections or bacterial bloodstream infections can develop quickly without obvious symptoms. Before about 8 weeks of age, it’s genuinely difficult, even for experienced doctors, to tell whether a baby is mildly ill or dangerously ill just by looking at them. Young infants haven’t yet developed the social responses (like a social smile) that help clinicians assess how sick a child really is.
Here’s the age-based breakdown for when to call your pediatrician:
- Under 3 months: Call for any fever of 100.4°F or higher, regardless of how your baby is acting. Don’t give fever-reducing medication and wait to see what happens. This age group needs prompt evaluation.
- 3 to 6 months: Call if your baby has a temperature up to 100.4°F and seems unwell, or if the temperature is higher than 100.4°F.
- 6 to 24 months: Call if a fever above 100.4°F lasts more than one day.
- Any age: Call if a fever persists beyond three days.
For babies under 2 months who are brought in with a fever, hospitals typically run blood tests, urine tests, and sometimes a spinal fluid analysis. This sounds aggressive, but it’s standard practice because bacterial infections at this age can progress rapidly and aren’t always obvious from a physical exam alone.
Red Flags Beyond the Number
Temperature is just one piece of information. Certain symptoms alongside a fever signal that something more serious may be going on:
- Unusual sleepiness: Sleeping far more than normal, being difficult to wake, or seeming floppy or limp.
- Dehydration: Fewer wet diapers than usual (babies over a week old should have at least six to eight per day), a dry mouth, fewer tears when crying, or a sunken soft spot on the head.
- Breathing changes: Fast or labored breathing, or a persistent cough with difficulty breathing.
- Color changes: Skin or lips that look blue, purple, or gray.
- Inconsolable crying: Fussiness or pain that keeps getting worse and won’t respond to comfort.
What About Teething?
Teething is one of the most common reasons parents assume their baby has a fever, but research consistently shows that teething does not cause true fevers. Studies have found that babies may have a slight uptick in body temperature when a tooth is coming in, but the increase stays below the 100.4°F threshold. If your baby is teething and has a rectal temperature of 100.4°F or higher, something else is going on, likely a coincidental viral infection.
Comfort Measures at Home
For babies older than 3 months with a low-grade fever who are otherwise acting normally (eating, making eye contact, producing wet diapers), you can often manage things at home while monitoring. Dress your baby in lightweight, breathable clothing. If they have chills, a light blanket is fine. Offer frequent small feedings to keep them hydrated, whether breast milk, formula, or water for older babies. Let them rest as much as they want to.
A lukewarm bath can help your baby feel more comfortable, but avoid cold water or ice baths. These cause shivering, which actually raises the body’s core temperature and works against you.
Fever-Reducing Medication by Age
Acetaminophen (Tylenol) should not be given to babies younger than 3 months without a doctor’s specific instruction. After 3 months, it can be used, and dosing is based on your baby’s weight rather than age. For a baby weighing 12 to 17 pounds, the typical dose is 2.5 mL of infant liquid suspension.
Ibuprofen (Advil, Motrin) is off-limits until 6 months of age. After that, it becomes another option, again dosed by weight. Never give aspirin to a baby or child of any age.
One important point: the goal of medication isn’t necessarily to bring the temperature back to normal. Fever itself is part of the immune response and isn’t inherently dangerous in most cases. The purpose of medication is to help your baby feel comfortable enough to sleep, eat, and stay hydrated. If your baby has a fever of 101°F but is nursing well, playing, and making wet diapers, medication may not even be necessary.
Febrile Seizures
Some babies and toddlers experience seizures triggered by fever, called febrile seizures. These are frightening to witness but are usually not harmful. The most common type, a simple febrile seizure, lasts from a few seconds up to 15 minutes and happens only once within a 24-hour period. They’re caused by the rapid rise in temperature rather than the peak temperature itself, which is why they sometimes occur before a parent even realizes their child has a fever. If your baby has a seizure, lay them on their side on a safe surface and note how long it lasts. A first febrile seizure always warrants a call to your pediatrician or a trip to the emergency room for evaluation.