A normal temperature for an infant is around 97.5°F to 99.5°F (36.4°C to 37.5°C) when taken rectally, though readings vary depending on where you measure. A rectal temperature of 100.4°F (38°C) or higher is the standard threshold for fever in infants. Anything below 97.7°F (36.5°C) is considered low enough to qualify as hypothermia.
Normal Range by Measurement Method
Where you place the thermometer changes what “normal” looks like. Rectal readings run highest because they measure core body temperature most directly. Armpit readings run lower because the skin surface doesn’t fully reflect what’s happening inside. Here’s how the fever thresholds break down:
- Rectal, ear, or forehead: Fever starts at 100.4°F (38°C)
- Oral: Fever starts at 100°F (37.8°C)
- Armpit: Fever starts at 99°F (37.2°C)
For babies under 3 months, a rectal thermometer is the most reliable option. Armpit readings can be a quick screening tool, but if the number looks elevated, follow up with a rectal reading to confirm. Ear thermometers aren’t ideal for very young infants because their ear canals are too small for an accurate measurement.
Why Your Baby’s Temperature Fluctuates
A baby’s temperature isn’t a fixed number. It shifts throughout the day in a predictable pattern. Studies measuring children’s temperatures around the clock have found a daily swing of nearly 2°F. Temperatures tend to hit their lowest point in the early morning, around 5 to 6 a.m., and peak in the late afternoon between 4 and 6 p.m. So a reading of 99.3°F at 5 p.m. might be perfectly normal, while the same number at 6 a.m. could signal something worth watching.
Beyond time of day, a baby’s temperature can rise after feeding, crying, being bundled in heavy clothing, or spending time in a warm room. If you get a reading that seems high, wait 15 to 20 minutes with a layer of clothing removed and check again before assuming it’s a fever.
How Babies Control Body Heat
Infants regulate their temperature differently than adults. Adults shiver to generate heat when they’re cold. Newborns can’t shiver effectively, so they rely on a special type of tissue called brown fat, which sits between the shoulder blades and around the kidneys. Brown fat acts like a built-in heater. When activated, it can produce up to 300 times more heat per unit of mass than any other tissue in the body, accounting for roughly 10% of a newborn’s total daily heat production.
This system kicks in at birth, when a baby moves from the warm uterus into the cooler outside air. It works well under normal conditions, but it has limits. The process burns through calories and oxygen quickly, doubling or tripling a newborn’s metabolic rate. That’s one reason newborns are more vulnerable to both overheating and cold stress than older children. Their thermostat works, but it’s running at full capacity much of the time, leaving less margin for error.
When a Low Temperature Is a Problem
Most parents worry about fevers, but a temperature that’s too low also deserves attention. The World Health Organization defines infant hypothermia as a core temperature below 97.7°F (36.5°C). Even temperatures slightly below that threshold can cause cold stress, where the baby’s body diverts calories away from growth toward generating heat.
Prolonged hypothermia in newborns can lead to low blood sugar and increases the risk of serious infection. Premature and low-birth-weight babies are especially vulnerable because they have less brown fat and thinner skin. If your baby feels cool to the touch, has mottled or bluish skin, or seems unusually sluggish, take a temperature. A consistently low reading, particularly below 97°F, needs medical attention.
Keeping the nursery at a comfortable room temperature and dressing your baby in one more layer than you’d wear yourself are simple ways to prevent cold stress. Avoid overdoing it in the other direction, though. Overheating during sleep is a risk factor for SIDS.
Fever Thresholds by Age
The same fever doesn’t carry the same weight at every age. A baby’s immune system is still developing in the first few months, so infections can escalate quickly. The age-based guidelines reflect that reality:
- Under 3 months: Any rectal temperature of 100.4°F (38°C) or higher warrants a call to your pediatrician, even if your baby seems fine otherwise. At this age, a fever can be the only visible sign of a serious bacterial infection.
- 3 to 6 months: A temperature up to 100.4°F paired with unusual fussiness, poor feeding, or lethargy is worth a call. Any temperature above 100.4°F at this age also warrants contact with your doctor.
- 6 to 24 months: A fever above 100.4°F that lasts more than 24 hours should be evaluated, especially if it comes with other symptoms like rash, vomiting, or refusal to drink.
- Any age: A fever lasting more than three days needs medical attention regardless of how mild it seems.
The number on the thermometer matters, but so does how your baby is acting. A baby with a 100.5°F fever who is feeding well and alert is in a very different situation than one with the same temperature who is limp and unresponsive. Both the reading and the behavior tell the story.
Tips for Getting an Accurate Reading
Timing and technique make a real difference. Take your baby’s temperature when they’ve been calm for at least 15 minutes, not right after a feeding, bath, or crying spell. For a rectal reading, lubricate the tip of a digital thermometer, insert it about half an inch, and hold it in place until it beeps. For an armpit reading, tuck the thermometer snugly into the bare armpit and hold your baby’s arm against their side.
Keep a dedicated thermometer for rectal use and label it so it doesn’t get mixed up with one used orally. Digital thermometers give results in about 10 to 20 seconds, which makes the process easier with a squirming baby. Mercury thermometers are no longer recommended because of the breakage risk.
If you’re comparing readings over time, try to use the same method and the same location each time. Switching between armpit and rectal measurements makes it hard to spot a true trend, since the two methods can differ by a full degree or more.