A normal newborn temperature taken rectally falls between 97.7°F and 100.4°F (36.5°C to 38°C). Anything below 97.7°F is considered hypothermia by the World Health Organization, and a rectal reading at or above 100.4°F meets the American Academy of Pediatrics’ definition of fever in young infants. That 2.7-degree window is narrower than most parents expect, which is why knowing how to measure accurately matters as much as knowing the number itself.
Why Newborns Lose Heat So Quickly
Inside the womb, your baby’s temperature was regulated entirely by your body. At birth, they’re suddenly exposed to air that’s 20 or more degrees cooler than what they’re used to. Newborns have a large skin surface relative to their body weight, thin skin, and very little insulating fat, so they lose heat fast through evaporation, radiation, and direct contact with cool surfaces.
To compensate, newborns rely on a special tissue called brown fat, concentrated around the shoulders, neck, and kidneys. Unlike regular body fat, brown fat acts like a built-in heater. When activated by cold exposure, it can generate up to 300 times more heat per unit of mass than any other tissue in the body. This process kicks in immediately after birth and burns through stored energy rapidly. It’s effective but costly: a baby fighting to stay warm diverts calories away from growth and, in premature or sick infants, can deplete blood sugar and oxygen reserves quickly.
How to Measure a Newborn’s Temperature
For babies under 3 months, a rectal reading with a standard digital thermometer is the most accurate method. Johns Hopkins Medicine and the Mayo Clinic both recommend it as the go-to for this age group. To take one, apply a small amount of petroleum jelly to the tip, gently insert it about half an inch into the rectum, and hold it in place until it beeps.
Forehead (temporal artery) thermometers are a reasonable secondary option. Recent evidence suggests they can provide accurate readings in newborns, though many pediatricians will still want a rectal confirmation if the baby seems unwell. Ear thermometers are not recommended for newborns or young infants because a baby’s ear canal is too small and curved for a reliable reading. They’re generally considered appropriate only after 6 months. Pacifier thermometers and fever strips are not reliable enough to use at any age.
Armpit Readings Run Lower
An armpit (axillary) temperature is the least accurate method, but it’s also the least invasive, so many parents try it first. In babies with a normal temperature, armpit readings average about 0.1°C (roughly 0.2°F) lower than rectal readings. That gap can widen to about 0.3°C in babies who are already running cold. The bigger issue is inconsistency: the difference between the two methods can swing by more than a full degree Fahrenheit in either direction. If an armpit reading concerns you at all, follow up with a rectal check.
What Counts as a Fever
In a baby under 3 months old, a rectal temperature of 100.4°F (38°C) or higher is a fever. This threshold is treated seriously at this age because young infants have immature immune systems and fewer visible signs of serious infection. A fever in this age group always warrants a call to your pediatrician, even if the baby seems fine otherwise. The concern isn’t the temperature itself so much as what might be causing it.
For babies 3 to 6 months old, the same 100.4°F number applies, but the urgency depends more on how the baby is acting. A slightly elevated temperature in a baby who is feeding well and alert is managed differently than the same reading in a baby who is lethargic or refusing to eat.
When a Temperature Is Too Low
A core temperature below 97.7°F (36.5°C) qualifies as hypothermia in a newborn. Cold stress can actually begin before the thermometer drops that far, as the baby’s body ramps up heat production to compensate for heat loss. You might notice cool hands and feet, pale or mottled skin, or unusual fussiness.
Mild hypothermia is common right after birth and usually resolves with skin-to-skin contact and warm blankets. Prolonged hypothermia is more dangerous. A baby stuck in cold stress burns through glucose and glycogen stores, which can lead to low blood sugar, poor weight gain, and in severe cases, increased risk of infection. Premature babies are especially vulnerable because they have less brown fat and thinner skin.
Keeping Your Baby’s Temperature Stable
The recommended room temperature for a sleeping baby is 61°F to 68°F (16°C to 20°C). That range may feel cool to you, but paired with appropriate clothing and a lightweight sleep sack, it helps prevent overheating, which is a risk factor for sudden infant death syndrome (SIDS). Avoid heavy blankets, hats worn indoors, and multiple layers that could push your baby’s temperature above the normal range.
A good rule of thumb: dress your baby in one more layer than you’d wear comfortably in the same room. Feel the back of their neck or their chest to gauge warmth. Hands and feet often feel cooler than the rest of the body in newborns, so they’re not the best indicator. If the skin on their chest or back feels hot or sweaty, remove a layer. If it feels cool or clammy, add one.
Signs That Need Immediate Attention
Any fever in a baby under 3 months old deserves a phone call to your pediatrician, day or night. Beyond temperature, the following signs in a newborn call for emergency care regardless of what the thermometer says:
- Breathing changes: labored breathing, flaring nostrils, or pauses longer than a few seconds
- Color changes: skin or lips that appear blue, purple, or gray
- Altered consciousness: extreme sleepiness, difficulty waking, or seeming “out of it”
- Escalating distress: pain or fussiness that keeps getting worse and won’t settle
A single normal temperature reading doesn’t rule out illness, and a slightly elevated reading doesn’t always mean something is wrong. What matters most is the pattern over time and how your baby looks and acts between readings. If something feels off, trust that instinct and check the temperature rectally to get the most reliable number you can.