A baby’s normal body temperature is around 98.6°F (37°C), but healthy readings typically fall anywhere between 97°F and 100.3°F depending on where you measure and the time of day. A rectal temperature of 100.4°F (38°C) or higher is the standard threshold for fever in infants.
Normal Ranges by Measurement Site
The number on the thermometer depends on where you take the reading. A baby’s core body temperature (measured rectally) runs slightly higher than a reading from the armpit, which sits closer to the skin’s surface. Here’s how the fever cutoffs break down by location:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher indicates a fever
- Oral: 100°F (37.8°C) or higher indicates a fever
- Armpit: 99°F (37.2°C) or higher indicates a fever
Anything below those numbers falls within the normal range for that measurement site. So a rectal reading of 99.8°F, while it might feel warm to a worried parent, is perfectly normal. An armpit reading of 98.5°F is also normal. The key is knowing which threshold applies to the method you’re using.
Why Temperature Fluctuates Throughout the Day
Your baby’s temperature isn’t a fixed number. It shifts naturally over a 24-hour cycle, driven by the body’s internal clock. Research on children shows an average body temperature of 98.8°F, with a daily swing of nearly 2°F in either direction. Temperatures tend to be lowest in the early morning and peak in the late afternoon or evening.
This means a reading of 99.5°F after a nap in the late afternoon may be completely normal, while the same baby might register 97.5°F first thing in the morning. Physical activity, crying, and feeding can also temporarily bump the number up. If your baby has been swaddled heavily or dressed in warm layers, their temperature can rise as well. Signs of overheating include sweating, damp hair, flushed cheeks, heat rash, and rapid breathing. If you suspect overdressing pushed the reading up, remove a layer, wait 15 to 20 minutes, and take the temperature again.
Which Thermometer to Use at Each Age
Not every thermometer works well for every age. The American Academy of Pediatrics recommends specific tools depending on how old your baby is, because accuracy varies significantly.
For babies under 3 months, a digital rectal thermometer gives the most accurate reading. This matters because fever in very young infants is taken seriously, and you want a number your pediatrician can trust. Forehead (temporal artery) thermometers can be used as a first pass for babies 3 months and older, and Johns Hopkins Medicine notes they’re nearly as accurate as rectal readings with less discomfort. If your baby under 3 months shows signs of illness and you use a forehead thermometer first, your doctor will likely want a rectal reading to confirm.
Ear thermometers aren’t reliable for babies younger than 6 months because their ear canals are too narrow for an accurate reading. After 6 months, ear thermometers become a reasonable option. Armpit readings work at any age as a quick screening tool, but they’re the least accurate method. Fever-detecting forehead strips and color-changing pacifier thermometers are not reliable enough to trust.
Tips for a Rectal Reading
If you’ve never taken a rectal temperature, it’s simpler than it sounds. Apply a small amount of petroleum jelly to the tip of a digital thermometer, lay your baby on their back, and gently insert the tip about half an inch. Hold it in place until it beeps. The whole process takes under a minute, and most babies tolerate it well.
When a Fever Needs Immediate Attention
For any baby under 3 months old, a rectal temperature of 100.4°F or higher warrants a call to your pediatrician right away, even if the baby seems otherwise fine. Young infants don’t yet have mature immune systems, and fever at this age is evaluated more aggressively than in older children.
For babies older than 3 months, the temperature number matters less than how your baby is acting. A baby with a 101°F fever who is feeding well, making eye contact, and staying alert is in a very different situation than a baby with a 100.5°F fever who is limp and unresponsive. Symptoms that need prompt medical care at any age include:
- Difficulty breathing or unusually rapid breaths
- Skin or lips that appear blue, purple, or gray
- Unusual drowsiness, difficulty waking, or seeming less alert than normal
- Pain or fussiness that keeps getting worse or won’t let up
- A rash that appears alongside a fever
Fever paired with cold symptoms, coughing, or diarrhea is worth mentioning to your doctor sooner rather than later, especially if those symptoms are worsening. The severity of the accompanying symptoms, not just the number on the thermometer, guides how urgently your baby needs to be seen.
Getting an Accurate Reading
A few practical steps help ensure you’re getting a number you can trust. Wait at least 15 minutes after bathing, feeding, or heavy swaddling before taking a temperature, since all of these can skew the reading. If you get a number that seems surprisingly high, take it again. A single elevated reading after a crying spell or a warm car ride doesn’t necessarily mean fever.
When you call your pediatrician about a temperature, mention the exact number, where you measured it, and the time of day. A reading of 100.2°F taken rectally at 6 a.m. tells a different story than the same number taken under the arm at 5 p.m. That context helps your doctor decide next steps quickly.