A healthy newborn’s body temperature sits around 97.7°F to 99°F (36.5°C to 37.2°C), depending on where and how you measure it. Underarm readings for infants average about 98.2°F (36.8°C). Because newborns can’t regulate their own body heat well, keeping them in the right temperature zone, both on their body and in their environment, matters more in the first few months than at any other stage of life.
Normal Body Temperature for a Newborn
Newborns run slightly warmer than older children and adults. An underarm (axillary) reading of around 98.2°F (36.8°C) is typical. Rectal readings tend to be about half a degree to a full degree higher than underarm readings, so a rectal temperature near 99°F is perfectly normal.
The World Health Organization defines hypothermia in newborns as a core temperature below 97.7°F (36.5°C). On the other end, an underarm reading of 99°F (37.2°C) or higher is considered a fever. For rectal readings, the fever threshold is 100.4°F (38°C). That 100.4°F number is especially important for babies under three months old: a rectal temperature at or above it warrants an immediate call to your pediatrician, because young infants have immature immune systems and even a modest fever can signal a serious infection.
How to Take a Newborn’s Temperature
Rectal thermometers give the most accurate core body temperature reading in infants, which is why pediatricians rely on them for babies under three months. Digital thermometers work quickly, usually in under 30 seconds. Underarm readings are less invasive and fine for a quick check, but they read lower than core temperature. If you get an underarm reading that seems high, follow up with a rectal reading before deciding whether to call your doctor.
Forehead (temporal artery) thermometers are convenient but less reliable in newborns. Ear thermometers aren’t recommended for infants because their ear canals are too small for an accurate reading.
Best Room Temperature for Sleep
The ideal room temperature for a baby’s sleep environment is 68°F to 72°F (20°C to 22°C), regardless of season. This range helps prevent both overheating and cold stress. Overheating during sleep is a known risk factor for sudden infant death syndrome (SIDS), so erring on the cooler side of that range is generally safer than pushing above it.
You don’t need a nursery thermometer to get this right, though inexpensive room thermometers can help. A good test: if the room feels comfortable for you in a single layer of clothing, it’s likely fine for your baby with one additional layer.
How to Dress a Newborn for the Temperature
The NHS recommends giving your baby one extra layer of clothing compared to what you’re comfortably wearing. If you’re in a T-shirt and sweater, your baby would be comfortable in a bodysuit, sleepsuit, and a cardigan or light sweater. If you feel chilly, your baby probably does too.
For sleep, a wearable blanket or sleep sack replaces loose blankets, which aren’t safe in a crib. Choose the tog rating (thickness) based on your room temperature: a lighter sack for rooms at 72°F, a warmer one for rooms closer to 68°F. Skip hats indoors. Babies lose excess heat through their heads, and covering the head during sleep can cause overheating.
Signs Your Baby Is Too Hot
Newborns don’t always show obvious distress signals when they’re overheating. They may simply look unwell or act fussier than usual. More specific signs include skin that feels very warm to the touch (especially on the chest or back of the neck), irritability, and fatigue. A body temperature above 100°F (38°C) from environmental heat, rather than illness, is a sign of mild to moderate heat-related illness.
Severe overheating looks more alarming: hot, flushed skin or very pale skin, extreme sleepiness that’s hard to rouse, and a body temperature above 102°F (39°C). Dehydration can accompany overheating, and in infants the signs include no tears when crying, a dry mouth, a sunken soft spot on top of the head, faster heartbeat, and shallow breathing. If you notice any of these, move your baby to a cooler environment, remove a layer of clothing, and contact your pediatrician right away.
Signs Your Baby Is Too Cold
Cold stress can begin even before a baby’s temperature drops below the hypothermia threshold of 97.7°F. When a newborn gets cold, their body burns extra calories to generate heat through a process that uses a special type of fat (brown fat) that adults don’t have much of. This can double or triple a baby’s metabolic rate and oxygen demand, which is a significant strain on a tiny body.
In practical terms, a cold baby may feel cool to the touch on the chest, belly, or back of the neck. Hands and feet alone aren’t reliable indicators because they’re often cooler than the rest of the body in healthy newborns. A cold-stressed baby may be fussy, feed poorly, or seem unusually still. Prolonged cold stress that goes unrecognized can divert calories away from growth and lead to low blood sugar. For premature babies or those with breathing difficulties, cold stress is especially dangerous because it increases oxygen needs that their lungs may not be able to meet.
If your baby feels cold, add a layer, hold them skin-to-skin against your chest, or move them to a warmer room. Recheck their temperature after 15 to 20 minutes to make sure it’s climbing back into the normal range.
Quick Reference by Temperature Type
- Normal underarm temperature: around 97.5°F to 98.6°F (36.4°C to 37°C)
- Underarm fever threshold: 99°F (37.2°C) or higher
- Rectal fever threshold (under 3 months): 100.4°F (38°C) or higher, requires immediate medical attention
- Hypothermia: below 97.7°F (36.5°C)
- Room temperature for sleep: 68°F to 72°F (20°C to 22°C)