A baby has a fever when their rectal temperature reaches 100.4°F (38°C) or higher. That’s the standard threshold used by pediatricians for infants of all ages. But you’ll often notice something is off before you ever reach for a thermometer: your baby feels warmer than usual against your skin, fusses more than normal, or seems sluggish and uninterested in feeding.
A thermometer gives you the definitive answer, and the type you use matters more than you might think, especially for very young babies.
Physical Signs That Suggest a Fever
Babies can’t tell you they feel sick, so you have to read their behavior. A feverish baby often feels noticeably warm to the touch, particularly on the forehead, back of the neck, or belly. But warmth alone isn’t reliable. Babies run hot after crying, being swaddled, or sitting in a warm room.
Look for a cluster of changes rather than any single sign:
- Fussiness that won’t quit. Crying and irritability that continue even after feeding, changing, and comforting efforts.
- Unusual sleepiness. Seeming weaker or more tired than normal, or being hard to wake up.
- Poor feeding. Refusing the breast or bottle, or eating noticeably less than usual.
- Skin changes. Appearing pale, flushed, or yellowish.
- Breathing changes. Breathing faster or harder than usual.
- Vomiting. Beyond the normal spit-up that happens after meals.
- New rash. Any skin rash that wasn’t there before.
Any of these signs, with or without confirmed fever, means it’s worth taking an actual temperature reading.
Which Thermometer to Use
Not all thermometers are equally accurate for babies. The best choice depends on your child’s age.
For babies under 3 months, a rectal (digital) thermometer is the gold standard. It gives the most accurate core body temperature reading, and accuracy is critical at this age because even a low fever in a newborn can signal a serious infection. A forehead (temporal artery) thermometer can be used on children of any age and is a reasonable screening tool, but if the reading looks elevated in a young infant, confirm it rectally. Ear thermometers are only reliable for babies 6 months and older because a younger baby’s ear canals are too small for an accurate reading. Oral thermometers aren’t appropriate until age 4.
Keep separate thermometers for rectal and oral use if your household needs both. Label them clearly so they never get mixed up.
How to Take a Rectal Temperature
It sounds intimidating, but rectal readings are quick and straightforward. Clean the thermometer tip before use, then apply a small amount of petroleum jelly or water-based lubricant to the end.
Lay your baby on their back and gently lift their thighs, or place them belly-down across your lap. Insert the lubricated tip about half an inch to one inch (1.3 to 2.5 centimeters) into the rectum. Never force it past any resistance. Hold the thermometer in place with one hand and keep your other hand on the baby so they stay still. Never walk away while the thermometer is inserted. Most digital thermometers beep within 10 to 20 seconds. Remove it, read the number, and clean the tip afterward.
If the reading is 100.4°F or above, your baby has a fever.
What the Temperature Means by Age
The urgency of a fever depends heavily on how old your baby is. The younger the infant, the more seriously a fever needs to be taken.
Under 3 months: Any fever at all, even 100.4°F, warrants an immediate call to your pediatrician. At this age, babies have immature immune systems and a fever can be the only sign of a bacterial infection that needs rapid treatment. Don’t wait to see if it goes away on its own.
3 to 6 months: Call your pediatrician if your baby has a temperature up to 100.4°F and seems unwell (poor feeding, excessive fussiness, lethargy). Also call for any temperature above 100.4°F, regardless of how your baby is acting.
6 to 24 months: A fever above 100.4°F that lasts more than one day needs a call to the doctor. If the fever is accompanied by other symptoms like a rash, persistent vomiting, or difficulty breathing, don’t wait the full day.
Signs That Need Immediate Attention
Some symptoms alongside a fever point to something more urgent than a typical virus. Watch for difficulty breathing, which can look like flared nostrils, visible rib outlines with each breath, or noticeably fast, labored breathing. A baby who is unusually hard to wake, goes limp, or has a weak cry needs prompt evaluation.
Dehydration is another concern during a fever. Signs include fewer wet diapers than usual, crying with fewer or no tears, a dry mouth, and a sunken soft spot on the top of the head. If you notice any of these, your baby needs fluids and medical attention quickly.
Managing a Fever at Home
For babies older than 8 weeks, infant acetaminophen (the active ingredient in Tylenol) can help bring a fever down and ease discomfort. The dose is based on your baby’s weight, not age, so check with your pediatrician or follow the weight-based chart on the packaging carefully. Ibuprofen (the active ingredient in Motrin or Advil) is not safe for babies under 6 months.
Beyond medication, keep your baby lightly dressed. Overdressing or heavy blankets can trap heat and push the temperature higher. Offer frequent feedings, whether breast milk or formula, to prevent dehydration. A lukewarm sponge bath can provide some comfort, but avoid cold water or alcohol rubs, which can cause shivering and actually raise the core temperature.
A fever itself isn’t dangerous in most cases. It’s the body’s way of fighting infection. Your goal isn’t necessarily to bring the number back to normal, but to keep your baby comfortable and hydrated while their immune system does its work. The number on the thermometer matters most for deciding whether to call the pediatrician, especially in those first few months of life when even a slight fever carries different weight than it does in an older child.