A healthy baby’s normal body temperature is around 98.6°F (37.0°C), but anything between 97.7°F and 99.5°F (36.5°C to 37.5°C) taken rectally falls within the normal range. That range shifts depending on the time of day, how your baby is dressed, and where on the body you take the reading.
Normal Temperature Range by Measurement Site
Not every thermometer gives the same number, and that’s expected. A rectal reading of 36.5 to 37.5°C (97.7 to 99.5°F) is considered normal for both full-term and premature infants. An armpit reading runs about half a degree to a full degree Fahrenheit lower than a rectal one, so you’ll need to mentally add 0.5 to 1°F to get a closer picture of your baby’s core temperature. Ear and forehead thermometers fall somewhere in between but aren’t reliable enough in very young babies to be the go-to method.
Which Thermometer to Use at Each Age
For babies under 3 months, a rectal thermometer is the most accurate option and the one most pediatricians will want you to use. A forehead (temporal artery) thermometer can serve as a quick first check at any age, but if it flags a possible fever in a baby under 3 months, a rectal reading should confirm it.
Armpit thermometers are fine as a screening tool from 3 months onward, though they’re the least reliable of all methods. Ear thermometers become accurate after 6 months. Oral thermometers shouldn’t be used until a child is at least 4 years old.
Why Your Baby’s Temperature Changes Throughout the Day
A baby’s body temperature isn’t a fixed number. It naturally rises and falls over the course of 24 hours. In children, body temperature can swing by nearly 2°F during a single day, with the lowest readings in the early morning and the highest in the late afternoon. So a reading of 99°F at 4 p.m. might be perfectly normal, while the same number at 6 a.m. could be more meaningful.
Activity, crying, a recent feeding, and warm clothing can all push a reading slightly higher. If you’ve just unwrapped your baby from a car seat blanket or taken off a snowsuit, wait 15 to 20 minutes before checking.
What Counts as a Fever
The threshold depends on where you measure. A rectal temperature of 100.4°F (38°C) or higher is widely considered a fever in infants. An armpit reading of 99°F (37.2°C) or higher meets the fever cutoff. These thresholds apply to babies and children up to age 5.
Fever itself isn’t a disease. It’s your baby’s immune system responding to an infection or other trigger. The American Academy of Pediatrics emphasizes that the goal of treating a fever should be improving your child’s comfort, not forcing the number back to 98.6°F. There’s no evidence that bringing a fever down reduces the severity of an illness or prevents complications like febrile seizures. What matters more than the number on the thermometer is how your baby looks and acts: whether they’re feeding, making eye contact, and staying reasonably alert between naps.
Most pediatricians consider treatment reasonable once a temperature passes 101°F (38.3°C) and the child seems uncomfortable. If your baby is sleeping with a fever, 80% of pediatricians say there’s no reason to wake them just to give a fever reducer.
When a Fever Needs Immediate Attention
Age is the single biggest factor in how seriously to take a fever. The younger the baby, the narrower the margin.
- Under 3 months: Any fever at all, even a low one, warrants a call to your baby’s provider. At this age, fever can be the only visible sign of a serious infection. A temperature that drops below 97.7°F (36.5°C) rectally is also a concern and should prompt the same call.
- 3 months and older: Contact a provider if the temperature climbs above 104°F (40°C), or if a fever doesn’t respond to a fever-reducing medication.
- Any age: A fever lasting more than five days needs further investigation regardless of how high it is.
When a Baby’s Temperature Is Too Low
The World Health Organization defines hypothermia in newborns as a core temperature below 97.7°F (36.5°C). Even temperatures slightly above that cutoff can cause “cold stress,” where the baby’s body burns extra calories just to stay warm. Over time, unrecognized cold stress can interfere with growth and, in premature or medically fragile babies, lead to drops in blood sugar or an increased risk of infection.
The signs aren’t always obvious. A baby who feels cool to the touch on the chest or belly, seems unusually sluggish, or feeds poorly may be getting too cold. Hands and feet alone aren’t reliable indicators since they’re normally cooler than the rest of the body.
Room Temperature and Overheating
The environment your baby sleeps in directly affects their body temperature. The recommended room temperature for infant sleep is 61 to 68°F (16 to 20°C). Keeping the room within this range helps lower the risk of sudden infant death syndrome (SIDS), because overheating is an independent risk factor.
To check whether your baby is too warm, feel their chest or the back of their neck. If the skin feels hot or sweaty, remove a layer of clothing or bedding. A few practical guidelines help prevent overheating:
- Skip the hat indoors. Babies release a significant amount of heat through their heads, so a hat worn inside can trap that heat.
- Avoid duvets and quilts. Lightweight blankets or a sleep sack are safer options.
- Keep the crib away from radiators, heaters, and direct sunlight.
- On hot nights, use lighter bedding, open a door or window if safe, or run a fan pointed away from the baby.
You rarely need to keep the heating on all night. If the room is cooler than the recommended range, adding one extra layer of clothing is usually enough.