A normal newborn temperature is around 97.5°F to 99.3°F (36.4°C to 37.4°C) when taken rectally. A rectal reading of 100.4°F (38°C) or higher is considered a fever, and in newborns under three months old, any fever at that threshold needs prompt medical attention because young babies can’t fight infections the way older children can.
Normal Range by Measurement Method
Where you take the temperature changes what numbers you should expect. A rectal reading is the gold standard for newborns and runs slightly higher than other methods. An armpit (axillary) reading typically runs about a degree lower. Here’s what counts as a fever depending on the method:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Armpit: 99°F (37.2°C) or higher
These thresholds apply to all children, but they carry extra urgency in the first three months of life. A temperature that might warrant a wait-and-see approach in a toddler requires a call to your pediatrician when the baby is a newborn.
Which Thermometer to Use
For babies three months and younger, a standard digital thermometer used rectally gives the most accurate reading. Forehead (temporal) thermometers are convenient but less reliable in this age group and are only recommended for babies three months and older. Digital ear thermometers are best suited for babies six months to one year old, since a newborn’s ear canal is too small for a consistent reading.
To take a rectal temperature, apply a small amount of petroleum jelly to the tip and insert it about half an inch. Hold it in place until the thermometer beeps. Most digital thermometers give a reading in under 30 seconds. If you’re using the armpit method as a quick check, tuck the thermometer snugly against bare skin and hold your baby’s arm gently against their body until it beeps. Just keep in mind that an armpit reading is a rough estimate, so if it seems elevated, follow up with a rectal reading to confirm.
Why Fever in a Newborn Is Serious
Babies under three months have immature immune systems, which means even a mild infection can escalate quickly. A rectal temperature of 100.4°F or above in this age group is treated as a potential emergency. Your pediatrician will likely want to see the baby right away or direct you to an emergency department, even if your baby otherwise seems fine.
Beyond fever, signs that something is wrong include trouble breathing, skin or lips that look blue, purple, or gray, unusual fussiness or lethargy, or a baby who seems less alert or responsive than usual. A newborn who won’t wake up, refuses to feed, or seems limp needs immediate care regardless of what the thermometer says.
Too Cold Is Also a Concern
While most parents worry about fever, a temperature below 97.5°F (36.4°C) can also signal a problem. Newborns lose heat quickly because they have a high surface-area-to-body-weight ratio and very little insulating fat. A persistently low temperature can be a sign of infection in newborns, sometimes even before a fever develops. If your baby feels cool to the touch on the chest or belly (not just the hands and feet, which are normally cooler), take a rectal reading and contact your pediatrician if it falls below 97.5°F.
Keeping the Room at the Right Temperature
The recommended room temperature for a baby’s sleep environment is 68°F to 72°F (20°C to 22°C). The NHS suggests a slightly wider range of 61°F to 68°F (16°C to 20°C) as comfortable and safe for sleeping babies. Either way, the goal is to keep the room cool enough that your baby doesn’t overheat, since overheating is a known risk factor for sudden infant death syndrome (SIDS).
A good way to check is to feel your baby’s chest or the back of their neck. If the skin there feels hot or sweaty, remove a layer of clothing or bedding. Cold hands and feet on their own are normal and don’t necessarily mean your baby is too cold.
How to Dress a Newborn for Sleep
The general rule is to dress your baby in one more layer than what you’re comfortable wearing in the same room. In a room that’s around 68°F to 72°F, a sleepsuit with a lightweight sleep sack is typically enough. If the room is warmer, a onesie alone with a light blanket or thin sleep sack may be plenty. In cooler rooms, choose a sleep sack with a higher warmth rating (manufacturers label these by “tog,” which is a measure of thermal resistance).
Avoid hats indoors. Babies regulate a significant portion of their body heat through their heads, and covering it during sleep can cause overheating. Loose blankets, heavy quilts, and stuffed animals also raise both overheating and suffocation risks and should be kept out of the crib.