A normal temperature for a baby is between 97.7°F and 99.5°F (36.5°C to 37.5°C) when measured rectally. The exact number you see on the thermometer depends on how you take it, what time of day it is, and how active your baby has been. A reading of 100.4°F (38°C) or higher taken rectally is considered a fever.
Normal Ranges by Measurement Method
Not all thermometers give the same number, so the fever threshold shifts depending on where you take the reading. A rectal thermometer gives the most accurate core body temperature in infants. Here’s how the numbers break down:
- Rectal, ear, or forehead: Normal is below 100.4°F (38°C). Anything at or above that is a fever.
- Oral: Normal is below 100°F (37.8°C).
- Armpit (axillary): Normal is below 99°F (37.2°C).
Armpit readings tend to run lower than the actual core temperature, so they’re useful as a quick screen but not the most reliable. Forehead (temporal artery) thermometers are convenient, but studies show they miss about 28% of fevers in children compared to more invasive methods. For babies under 3 months, a rectal reading is the standard your pediatrician will want.
Why Your Baby’s Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It follows a natural 24-hour cycle with roughly a 1°F swing from the lowest point to the highest. Your baby’s temperature dips to its lowest during sleep at night and peaks in the late afternoon or early evening. This is driven by the body’s internal clock and normal metabolic activity.
Physical activity, crying, a warm bath, bundling up, or being in a hot room can all push the reading a bit higher without meaning anything is wrong. If you get a borderline reading, wait 15 to 20 minutes, make sure your baby isn’t overdressed, and check again.
When a Temperature Is Too Low
Most parents focus on fevers, but a temperature that’s too low also matters. The World Health Organization defines infant hypothermia as a core temperature below 97.7°F (36.5°C). Newborns are particularly vulnerable because they lose heat quickly and can’t shiver to warm themselves. Instead, they burn a special type of fat tissue to generate heat, which dramatically increases their oxygen demand and energy use.
Persistent low temperature in a newborn can lead to low blood sugar, stress on the organs, and a higher risk of infection. If your baby feels cold to the touch on the chest or back of the neck (not just the hands and feet, which are normally cooler), add a layer of clothing or a lightweight blanket and recheck.
Keeping Your Baby at a Comfortable Temperature
The recommended room temperature for a sleeping baby is 61°F to 68°F (16°C to 20°C), which feels cooler than most adults expect. At that range, a lightweight sleep sack or one layer of light bedding is enough. You can check whether your baby is comfortable by feeling their chest or the back of their neck. If the skin feels hot or sweaty, remove a layer.
A few practical tips that make a difference: don’t put a hat on your baby indoors, since babies release excess heat through their heads. Keep the crib away from radiators, heaters, and direct sunlight. On hot nights, open a window if it’s safe and use lighter clothing. It’s rarely necessary to leave the heating on overnight, but if you do, keep it at or below 68°F (20°C).
Fever Guidelines by Age
The same fever means very different things depending on how old your baby is. Age is the single biggest factor in how urgently a fever needs attention.
For babies under 3 months, any fever of 100.4°F (38°C) or higher needs a call to your pediatrician right away, even if your baby seems fine otherwise. Young infants have immature immune systems, and a fever can be the only sign of a serious infection that needs prompt evaluation.
For babies 3 to 6 months old, call if the temperature reaches 100.4°F or if your baby has a lower temperature but seems unusually sleepy, irritable, or unwell.
For babies 6 to 24 months old, a fever above 100.4°F that lasts longer than one day warrants a call. At this age, a brief fever with an otherwise happy, feeding baby is often just the immune system doing its job. But if the fever stretches past three days at any age, check in with your pediatrician regardless of other symptoms.
How to Get an Accurate Reading
For babies under 3 months, a digital rectal thermometer is the gold standard. Apply a small amount of petroleum jelly to the tip, insert it about half an inch, and hold it in place until it beeps. It takes under a minute and gives the most reliable number.
Once your baby is around 6 months old, a digital ear thermometer becomes a reasonable option, though results can vary if the probe isn’t seated properly in the ear canal. Forehead thermometers work well for a quick, no-fuss check, but if the reading looks borderline or high, confirm it with a rectal measurement before making decisions.
Whichever method you use, avoid checking right after a bath, a feeding, or heavy bundling. Wait at least 15 minutes so the reading reflects your baby’s actual core temperature rather than the environment.