A fever in a baby is a rectal temperature of 100.4°F (38°C) or higher. That single number is the standard threshold used by pediatricians, and rectal measurement is the most reliable way to check a baby’s temperature. While a fever itself is a sign that your baby’s immune system is fighting an infection, the age of your baby and the symptoms accompanying the fever matter far more than how high the number climbs.
Temperature Thresholds by Measurement Site
Not every thermometer reads the same, and the fever cutoff shifts slightly depending on where you take the temperature. Your baby has a fever if you see:
- Rectal, ear, or forehead (temporal artery): 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit (axillary): 99°F (37.2°C) or higher
For babies under 3 months, rectal readings are the gold standard. Forehead and armpit thermometers are easier to use, but they’re less accurate in the range that matters most. A study comparing methods found that temporal artery (forehead) thermometers read about 0.2°C lower than rectal on average, and that gap widened to more than 0.5°C in children who actually had fevers. Armpit thermometers performed even worse, catching only about 12% of fevers that a rectal thermometer would detect. Forehead thermometers caught about 62%.
If your baby is under 3 months old and feels warm, use a rectal thermometer to get a number you can trust. For older babies, a forehead or ear thermometer is reasonable for everyday monitoring, but if the reading is borderline, confirm it rectally.
Why Age Changes Everything
A fever that’s routine in a one-year-old can be a medical emergency in a newborn. Any fever of 100.4°F or higher in a baby under 3 months old is considered serious and needs immediate medical evaluation, regardless of how well the baby appears to be acting. Young infants have immature immune systems, and infections like group B strep or bacterial meningitis can progress quickly with few obvious warning signs in the early weeks of life.
The American Academy of Pediatrics has specific clinical guidelines for evaluating febrile infants from 8 to 60 days old precisely because this age group is so vulnerable. Babies in their first week of life carry even higher risk due to the possibility of infections acquired during birth. If your newborn has a fever, don’t wait to see if it resolves on its own.
For babies older than 3 months, a fever is common and usually caused by a viral illness. At this age, how your baby is acting becomes a better guide than the number on the thermometer. A baby with a 102°F fever who is still nursing, making eye contact, and responding normally is generally in a safer position than a baby with a 100.5°F fever who is limp and unresponsive.
Signs That Need Immediate Attention
Beyond the under-3-months rule, certain symptoms alongside a fever signal something more serious at any age:
- Extreme sleepiness or unresponsiveness: difficulty waking your baby, or no reaction to your voice or touch
- Breathing problems: fast, labored, or shallow breaths, chest pulling inward with each breath, or blue-tinged lips
- A rash that doesn’t fade when pressed, or purple spots on the skin, which could indicate a bacterial infection like meningitis
- A bulging soft spot on the top of the head, which may signal increased pressure inside the skull
- A stiff neck or resistance to bending the head forward
- Persistent, high-pitched crying that you can’t soothe
- A fever lasting more than five days, even a low one
If your baby has a seizure, stops breathing, or becomes unresponsive, call 911.
Keeping Your Baby Hydrated
Fever increases fluid loss, so dehydration is the most common complication to watch for. If you’re breastfeeding, nurse more frequently than usual. If your baby is on formula, continue making it at full strength unless you’re told otherwise by a pediatrician. Do not give plain water, juice, or sports drinks to young infants.
For babies with vomiting or diarrhea on top of the fever, a pediatric oral rehydration solution may be recommended, but check with your baby’s doctor before using one. These are formulated specifically for infants and replace electrolytes in proportions that water or juice cannot.
Watch for these signs of dehydration:
- Fewer wet diapers than usual (fewer than six in 24 hours is a warning sign)
- No tears when crying
- A sunken soft spot on top of the head
- Sunken eyes
- Unusual drowsiness or irritability
Fever-Reducing Medication by Age
Acetaminophen (Tylenol) is the go-to option for babies, but it should not be given to infants under 8 weeks old. Babies that young with a fever of 100.4°F or higher need to be seen by a doctor, not treated at home. For babies older than 8 weeks, dose acetaminophen based on your baby’s weight, not age. The weight-based dosing on the package or provided by your pediatrician is the safest guide.
Ibuprofen (Motrin, Advil) is not recommended for babies under 6 months old. Once your baby passes that threshold, ibuprofen becomes a useful second option, also dosed by weight.
A few practical notes: fever-reducing medication treats discomfort, not the underlying illness. If your baby is sleeping comfortably and eating normally with a mild fever, you don’t necessarily need to bring the temperature down. The goal is to help your baby feel better, not to chase a perfect number on the thermometer. Light clothing and a comfortable room temperature also help. Skip alcohol baths and ice baths, which can cause shivering and actually raise core temperature.
How to Take a Rectal Temperature
Lay your baby face-down across your lap or face-up on a firm surface. Apply a small amount of petroleum jelly to the tip of a digital rectal thermometer. Gently insert the tip about half an inch to one inch into the rectum. Hold the thermometer in place until it beeps. The entire process takes under a minute, and while babies often fuss, it’s safe and gives you the most accurate reading available.
Label this thermometer and keep it separate from any others in your household. Clean it with rubbing alcohol or soap and water after each use.