A fever in an infant is a rectal temperature of 100.4°F (38°C) or higher. That single number is the standard threshold used by pediatricians regardless of your baby’s age, and it applies whether the temperature is taken rectally, in the ear, or with a temporal artery thermometer. Armpit readings run lower, so a fever by that method is 99°F (37.2°C) or higher.
Why the Thermometer Method Matters
For babies under 3 months old, a rectal thermometer gives the most reliable reading and is the method pediatricians recommend. Forehead (temporal artery) and ear thermometers are convenient but can be less precise in very young infants. Armpit readings are the least accurate, though they work as a quick screening. If an armpit reading looks elevated, follow up with a rectal reading to confirm.
Different methods produce slightly different numbers because they measure temperature at different points on the body. That’s why the fever threshold shifts depending on the method: 100.4°F rectally, 100°F orally, and 99°F under the arm. When in doubt, rectal is the gold standard for infants.
Age Changes Everything
The same 100.4°F reading carries very different weight depending on how old your baby is. In the first 12 weeks of life, any fever can signal a serious infection and needs immediate medical attention. For babies under 3 months, call your pediatrician right away, even if your baby looks fine otherwise. Young infants have immature immune systems, and infections can escalate quickly without obvious warning signs.
The youngest newborns face the highest risk. Babies under 3 weeks old with a fever of 100.4°F or higher are evaluated urgently because their ability to fight bacterial infections is still extremely limited. Premature babies and those with complex medical histories also carry additional risk factors that make any fever more concerning.
For older infants (3 months and up), fever is still worth monitoring, but it becomes less automatically alarming. At that age, the fever itself isn’t the danger. It’s the body’s natural response to infection. What matters more is how your baby is acting and whether other worrying symptoms are present.
Symptoms That Need Immediate Attention
Beyond the fever number itself, certain signs mean your baby needs care right away:
- Trouble breathing or rapid, labored breaths
- Skin or lips that look blue, purple, or gray
- Unresponsiveness or unusual drowsiness, acting strangely or seeming less alert than normal
- Repeated vomiting
- Pain or fussiness that keeps getting worse or won’t let up
A sluggish, floppy baby with a fever is more concerning than a fussy one who is still making eye contact and feeding. Trust your instincts. If something feels off about the way your baby looks or behaves, that’s reason enough to call.
Teething Does Not Cause Fevers
This is one of the most common misconceptions. Teething can nudge your baby’s temperature slightly above its normal baseline, but it will not push it to 100.4°F or higher. If your baby’s temperature hits that threshold, something else is going on. A true fever is a sign of infection, not teething. Dismissing a fever as “just teething” can delay care for an illness that actually needs treatment.
Fever After Vaccines
A low-grade fever after vaccinations is common and generally harmless. Most post-vaccine fevers show up within a day or two and resolve within one to two days. The timing varies by vaccine. DTaP shots, for example, can trigger a mild fever within the first few days, while the MMR vaccine can cause one up to two weeks later.
That said, a fever in a baby under 3 months old still warrants a call to your pediatrician, even if it started right after a vaccine visit. For older infants, a brief post-vaccine fever that resolves on its own is typically nothing to worry about.
Keeping Your Baby Comfortable at Home
For infants older than 3 months with a mild fever who are otherwise acting normally, home care focuses on comfort and hydration. Dress your baby in lightweight, breathable clothing. If they have chills, a light blanket is fine. Offer frequent, small feedings, whether breast milk or formula, to keep fluids up. A lukewarm bath can help your baby feel better, but skip cold water or ice baths. Cold water triggers shivering, which actually generates more body heat and works against you.
Watch for signs of dehydration, which can develop when a fever causes fluid loss. In infants, dehydration shows up as a sunken soft spot on the top of the head, fewer tears when crying, noticeably less activity, or increased irritability. If you notice any of these, contact your pediatrician.
Fever-Reducing Medication for Infants
Acetaminophen (Tylenol) can be given to infants, but for babies under 2 years old, you should get dosing guidance from your pediatrician first. The dose is based on your baby’s weight, not age, and getting it right matters. Liquid infant acetaminophen comes in a concentration of 160 mg per 5 mL, and you can give a dose every 4 hours as needed, with a maximum of 5 doses in 24 hours.
Ibuprofen (Advil, Motrin) is not safe for babies under 6 months old. After 6 months, it can be given every 6 to 8 hours, again dosed by weight. Never give aspirin to an infant or child. And avoid combination medicines (products that treat multiple symptoms at once) for children under 6.
One important note: for babies under 12 weeks old, do not give fever-reducing medication and assume the problem is handled. A fever at that age needs medical evaluation regardless of whether the temperature comes down with medicine.