A healthy newborn’s temperature falls between 97.7°F and 99.5°F (36.5°C to 37.5°C) when measured rectally. The key number to remember: a rectal temperature of 100.4°F (38°C) or higher in a baby under 3 months old requires immediate medical attention. On the low end, anything below 97.7°F (36.5°C) suggests your baby may be getting too cold.
How to Take a Newborn’s Temperature
A rectal thermometer is the most accurate option for babies under 3 months old. Forehead thermometers can work as a first pass, but pediatricians often want a rectal reading to confirm if the number looks concerning. Ear thermometers aren’t reliable for newborns and are only recommended for babies older than 6 months.
Armpit (axillary) readings are convenient but can be misleading. While the average difference between an armpit reading and a rectal reading is only about 0.1°C, individual measurements can vary by as much as 1.0°C in either direction. That’s a big gap when you’re trying to determine whether your baby has crossed the 100.4°F fever threshold. If you get a concerning armpit reading, follow up with a rectal measurement.
Why Newborns Lose Heat So Quickly
Babies are born with a high surface-area-to-body-weight ratio, which means they lose heat faster than adults. They also can’t shiver to warm themselves up. Instead, newborns rely on a special type of body fat called brown fat, which acts like a built-in heater. Brown fat generates warmth by burning energy directly as heat rather than storing it. At full activation, it can produce up to 300 times more heat per unit of mass than any other tissue in the body, accounting for roughly 10% of a newborn’s daily heat production.
This system kicks in at birth, triggered by the sudden shift from the warm uterine environment to the cooler outside world. But it has limits. A baby who is underdressed, wet, or in a cold room can overwhelm this heating system and slip into hypothermia.
Hypothermia Warning Signs
Hypothermia in newborns is classified by severity. A temperature between 96.8°F and 97.7°F (36.0°C to 36.5°C) is considered potential (mild) hypothermia. Between 89.6°F and 96.8°F (32°C to 36°C) is moderate hypothermia. Anything below 89.6°F (32°C) is severe and life-threatening.
A cold baby may feel cool to the touch on the chest or back, not just on the hands and feet (which tend to run cool naturally). Lethargy, poor feeding, and weak crying are signs that a baby’s core temperature has dropped enough to affect normal function. Skin-to-skin contact and an extra layer of clothing can help a mildly cold baby warm up, but a temperature that stays below 97.7°F after warming efforts needs medical evaluation.
What Counts as a Fever
For any baby under 3 months old, a rectal temperature of 100.4°F (38°C) or higher is a fever that requires a call to your pediatrician, regardless of how well the baby seems. The American Academy of Pediatrics has specific clinical guidelines for evaluating fevers in infants broken into three age groups: 8 to 21 days, 22 to 28 days, and 29 to 60 days. Younger babies get a more aggressive workup because their immune systems are less developed and infections can progress quickly.
A fever in a newborn is treated differently than a fever in an older child or adult. In babies this young, even a “low-grade” fever of 100.4°F can signal a serious bacterial infection. Don’t wait to see if it resolves on its own, and don’t give fever-reducing medication before calling your baby’s doctor, as it can mask a temperature that the medical team needs to track.
Overheating Is Just as Risky
Overbundling is one of the most common causes of elevated temperature in newborns, and it can mimic a true fever. An overheated baby may sweat, breathe rapidly, and feel hot to the touch. As overheating worsens, babies can become weak, tired, pale, and may vomit. In extreme cases, when body temperature climbs above 104°F (40°C), seizures and loss of consciousness become risks.
The practical difference between overheating and an infection-related fever: if you remove a layer of clothing and bring your baby into a cooler room, an overheated baby’s temperature will drop within 15 to 20 minutes. A true fever won’t respond to environmental changes. If the temperature stays at or above 100.4°F after cooling measures, treat it as a fever.
Keeping the Room at the Right Temperature
The recommended nursery temperature is between 68°F and 78°F. A simple rule: if you’re comfortable in a t-shirt, the room is probably fine for your baby with one additional light layer. Keep air circulating with a fan on low, which also reduces the risk of SIDS. Avoid placing the crib near windows, heating vents, or direct sunlight, all of which can create temperature pockets that don’t match the rest of the room.
Dress your newborn in one more layer than you’d wear in the same environment. Skip hats indoors, as babies release a significant amount of excess heat through their heads, and covering it can contribute to overheating during sleep.
Signs That Need Immediate Attention
Beyond the numbers on the thermometer, certain behaviors signal that something is wrong. Call your baby’s doctor if your newborn refuses two or more feedings in a row, is sleeping far more than usual and is difficult to wake, seems floppy or unusually limp, or is crying in a way that’s markedly different from normal and can’t be soothed.
Get emergency care if your baby has trouble breathing, appears unresponsive or unusually withdrawn, or has skin or lips that look blue, purple, or gray. These symptoms can accompany both dangerously high and dangerously low temperatures, but they also indicate problems beyond temperature alone.