A fever in a baby is a rectal temperature of 100.4°F (38°C) or higher. That single number applies whether your baby is two weeks old or ten months old, but what you should do about it changes significantly depending on your baby’s age. For newborns under 3 months, even a low fever can signal something serious. For older babies, fever is usually the immune system doing its job against a common infection.
The Temperature That Counts as a Fever
The threshold is 100.4°F (38°C) measured rectally. Anything below that, even 100.2°F, is not technically a fever. This matters because parents sometimes worry about temperatures in the 99 to 100°F range, which fall within the normal fluctuation for babies throughout the day.
Different thermometer methods give slightly different readings. A rectal thermometer is the most accurate for babies and the one pediatricians rely on. Ear and forehead (temporal artery) thermometers also register a fever at 100.4°F, but an armpit reading of 99°F (37.2°C) or higher can indicate fever, and an oral reading of 100°F (37.8°C) qualifies. For any baby under 3 months, a rectal reading is the only one precise enough to guide decisions.
Why Age Changes Everything
A fever in a 2-week-old is treated very differently from a fever in an 8-month-old. That’s because newborns have immature immune systems, and a bacterial infection in the first weeks of life can escalate fast with few visible warning signs.
Here’s how the age brackets break down:
- Under 28 days: Any rectal temperature of 100.4°F or higher typically triggers a full evaluation, which can include blood work, urine tests, and a spinal fluid sample. This sounds alarming, but it’s standard because serious infections at this age don’t always produce obvious symptoms.
- 29 days to 3 months: A fever still warrants prompt medical attention. Doctors will examine your baby, run a urine test, and decide on further steps based on how your baby looks and acts.
- 3 to 6 months: A temperature up to 101°F (38.3°C) may not require an urgent visit if your baby is acting normally, eating well, and making eye contact. If your baby seems unusually irritable, sleepy, or uncomfortable, or the temperature goes above 101°F, it’s time to call.
- 6 to 24 months: A fever above 101°F that lasts more than a day without other symptoms, or any fever paired with concerning behavior, warrants a call to your pediatrician.
What Causes Fevers in Babies
Viral infections are by far the most common trigger. Colds, respiratory viruses, and stomach bugs all produce fevers that typically resolve within a few days. Bacterial infections like urinary tract infections or ear infections are less common but may need treatment. In babies under 3 months with no obvious source of infection, doctors specifically look for urinary tract infections because they can be present without symptoms like pain or foul-smelling urine.
Vaccinations are another frequent cause. Fever is one of the most common side effects of routine childhood shots. The timing varies by vaccine. DTaP vaccines tend to cause fevers within the first few days. The measles, mumps, and rubella vaccine can trigger a fever up to two weeks later, and the chickenpox vaccine can cause one as late as two to four weeks after the shot. Most vaccine fevers are mild, last about a day, and resolve on their own.
Teething Does Not Cause True Fever
This is one of the most persistent myths in parenting. Teething can raise a baby’s temperature slightly, but it stays below 100.4°F. If your baby has a temperature at or above that threshold, something else is going on. The timing is confusing because babies start teething around 4 to 6 months, which is also when the passive immunity they got during pregnancy starts to fade, making them more susceptible to infections. The teething and the illness are coincidental, not connected.
Signs That Need Immediate Attention
Beyond the temperature number itself, certain physical signs suggest a more serious infection in a baby of any age. These include being difficult to wake or unusually drowsy, a rash of small purple or red dots that don’t fade when you press on them, breathing that seems labored or faster than normal, and skin that looks pale or mottled. If you press gently on your baby’s skin and the color takes more than three seconds to return, that’s another warning sign.
A baby who is feeding poorly, producing fewer wet diapers than usual, or who just looks “off” to you in a way that’s hard to articulate is worth a call to your doctor. Parents’ instinct that something is wrong turns out to be a genuinely useful clinical signal.
Bringing a Fever Down at Home
For babies older than 3 months with low-grade fevers who are otherwise acting like themselves, you don’t always need to treat the fever. Fever itself is not dangerous in most cases. It’s a sign the immune system is responding to an invader. The goal of treatment is comfort, not hitting a specific number on the thermometer.
Acetaminophen (Tylenol) can be given to babies 8 weeks and older. Ibuprofen (Advil, Motrin) is not recommended until 6 months of age. Both are dosed by weight, not age, so check the packaging or ask your pediatrician for the right amount. Never give aspirin to babies or children.
A lukewarm sponge bath can also help. Use water between 90°F and 95°F (32 to 35°C) and sponge for 20 to 30 minutes. Stop immediately if your baby starts shivering, because shivering actually raises body temperature. Never use cold water, ice, or rubbing alcohol, all of which cool the body too fast and can be dangerous. If your baby doesn’t tolerate the sponge bath or doesn’t seem more comfortable afterward, skip it next time.
Keep your baby hydrated with breast milk, formula, or an electrolyte solution if your doctor recommends one. Dress your baby in light clothing and keep the room at a comfortable temperature. Bundling a feverish baby in blankets can trap heat and push the temperature higher.
How to Take a Rectal Temperature
Use a digital thermometer labeled for rectal use. Apply a small amount of petroleum jelly to the tip. Lay your baby face-down on your lap or face-up on a firm surface, and gently insert the tip about half an inch into the rectum. Hold it in place until it beeps, which usually takes about 10 to 20 seconds. Clean the thermometer with rubbing alcohol or soap and water afterward, and label it so it’s never accidentally used orally.
If you get a reading of 100.4°F or higher in a baby under 3 months, don’t wait to see if it comes down. Contact your pediatrician or go to the emergency room right away. For older babies, use the age-based guidelines above to decide your next step, and trust your read on how your baby is acting overall.