A normal body temperature for a newborn is around 97.9°F to 99.5°F (36.6°C to 37.5°C) when taken rectally. A rectal reading of 100.4°F (38°C) or higher is considered a fever. Because newborns can’t regulate their own body heat well, keeping them in the right temperature range involves more than just watching the thermometer. Room temperature, clothing, bath water, and how you take a reading all matter.
Normal Body Temperature Range
For newborns, a rectal temperature between about 97.9°F and 100.3°F is considered normal. Armpit (axillary) readings run lower, so a fever threshold by that method is 99°F (37.2°C) rather than 100.4°F. The numbers shift slightly depending on time of day, how recently your baby ate, and how bundled up they are, but anything consistently outside that range is worth paying attention to.
For babies under 3 months old, any fever at all warrants a call to your pediatrician. At this age, even a low-grade fever can signal a serious infection because a newborn’s immune system is still immature. Don’t wait to see if it resolves on its own.
Why Newborns Lose Heat So Quickly
Inside the womb, temperature stays constant. At birth, a baby enters an environment that’s roughly 18°F (10°C) cooler. That sudden drop is a real challenge. Newborns have a large skin surface area relative to their body weight, which means they radiate heat faster than adults do. Smaller babies lose heat even more quickly because their surface-to-volume ratio is higher.
Newborns do have a built-in heating system: a special type of fat called brown fat, concentrated between the shoulder blades and around the kidneys, that burns calories to generate warmth. But this supply is limited, especially in premature babies who didn’t have enough time in the womb to build up those fat stores. In the first 36 hours of life, newborns are particularly vulnerable to dropping too cold because their energy reserves can deplete quickly. Over the coming weeks, as your baby gains weight and their body mass increases relative to their skin surface, holding onto heat becomes easier.
How to Take an Accurate Temperature
Rectal thermometers are the gold standard for newborns. A study comparing rectal and armpit readings in infants found that rectal temperatures ran an average of 0.7°C (about 1.3°F) higher than armpit readings, and the gap between the two methods varied by as much as 3°C in some cases. That kind of inconsistency means an armpit reading could miss a real fever or falsely suggest one.
To take a rectal reading, apply a small amount of petroleum jelly to the tip of a digital thermometer and insert it about half an inch. Hold it in place until it beeps. If you’re using an armpit reading as a quick check, keep in mind it’s a rough estimate. Ear (tympanic) thermometers share the same fever threshold as rectal, 100.4°F, but they’re less reliable in very young infants because the ear canal is so small.
Best Room Temperature for Sleep
The ideal room temperature for a sleeping newborn is 68°F to 72°F (20°C to 22°C). This range keeps your baby comfortable without increasing the risk of overheating, which is a known risk factor for sudden infant death syndrome (SIDS). A simple room thermometer near the crib helps you stay in the right zone, especially overnight when temperatures can drop.
What your baby wears to bed matters just as much as the room temperature. Sleep sacks are rated by a TOG number, which measures thermal resistance. The higher the TOG, the warmer the sack. Here’s a practical guide:
- 75°F+ (24°C+): Use a 0.2 to 0.5 TOG sack with just a diaper or short-sleeve bodysuit underneath.
- 72°F to 75°F (22°C to 24°C): A 0.5 to 1.0 TOG sack with a short-sleeve bodysuit.
- 68°F to 72°F (20°C to 22°C): A 1.0 TOG sack with a long-sleeve bodysuit or lightweight pajamas.
- 64°F to 68°F (18°C to 20°C): A 2.0 to 2.5 TOG sack with a long-sleeve bodysuit and warmer pajamas.
- Below 64°F (below 18°C): A 2.5 to 3.5 TOG sack with a warm base layer plus pajamas. Consider warming the room.
A good rule of thumb: dress your baby in one more layer than you’d be comfortable wearing in that room.
Signs Your Baby Is Too Hot or Too Cold
Newborns can’t tell you they’re uncomfortable, so you need to check. Place your hand on your baby’s chest or the back of their neck. These areas give you a better read on core temperature than hands or feet, which are often cool even when a baby is perfectly warm.
Signs of overheating include skin that feels hot or damp on the chest, flushed or red skin, rapid breathing, irritability, and sweating (especially on the head). In more serious cases, your baby may seem unusually sleepy, difficult to wake, or limp. A body temperature above 100°F (38°C) alongside these symptoms suggests heat-related illness rather than infection, particularly in a warm environment.
Signs your baby is too cold include cool skin on the chest or belly (not just the hands), unusually quiet or still behavior, and in more serious cases, lethargy or weak feeding. Mild coolness in the hands and feet alone is normal and doesn’t necessarily mean your baby needs another layer.
Safe Bath Water Temperature
Aim for bath water around 100°F (38°C), which feels warm but not hot when you test it with the inside of your wrist or elbow. A baby’s skin is thinner and more sensitive than an adult’s, so water that feels comfortably warm to your hand can actually be too hot for them.
To prevent accidental scalding, set your home water heater to below 120°F (49°C). A bath thermometer takes the guesswork out of it, but the wrist test works in a pinch. Keep baths short for newborns, since they lose body heat quickly in water, and have a warm towel ready before you lift them out.