A healthy newborn’s temperature should fall between 97.7°F and 99.5°F (36.5°C to 37.5°C) when taken rectally. This is the standard range used by pediatricians worldwide. Anything below 97.7°F is considered hypothermia, and a rectal reading of 100.4°F (38°C) or higher is classified as a fever in babies under three months old.
Why Rectal Readings Are the Standard
For newborns, a rectal thermometer gives the most accurate picture of core body temperature. Armpit (axillary) readings tend to run about 0.1°C lower on average, but the real problem is inconsistency. In a study of preterm infants published in Frontiers in Pediatrics, armpit readings could swing as much as 1.3°F above or below the rectal temperature for the same baby. For a healthy baby with a normal temperature, this gap is small enough to not matter much. But when a baby is running cold, the difference widens, which is exactly when accuracy matters most.
Forehead and ear thermometers are convenient for older children, but they aren’t reliable enough for newborns. If you’re checking your baby’s temperature because something seems off, use a rectal thermometer.
What a Fever Means in the First Three Months
A rectal temperature of 100.4°F (38°C) or higher in a baby under three months old is a medical emergency, full stop. This threshold comes from both the American Academy of Pediatrics and the Mayo Clinic. Unlike in older children or adults, a fever in a newborn can signal a serious bacterial infection, and young babies don’t always show obvious signs of illness beyond the temperature itself.
Don’t wait to see if the fever comes down on its own. Don’t give fever-reducing medication first. A baby this young with a temperature at or above 100.4°F needs to be evaluated right away.
When a Baby Is Too Cold
The World Health Organization defines neonatal hypothermia as a core temperature below 97.7°F (36.5°C). Newborns lose heat fast because they have a high surface area relative to their body weight and very little insulating fat. Unlike adults, they can’t shiver to warm up. Instead, their bodies burn a special type of fat to generate heat, a process that doubles or triples their oxygen consumption and energy use. This is a real strain, especially for premature or small babies.
Signs that a newborn is getting too cold include skin that feels cool to the touch (especially the hands, feet, and belly), sluggish or weak feeding, less movement than usual, and a weak cry. Persistent low body temperature can cause blood sugar to drop and increases the risk of infection. If your baby’s temperature reads below 97.7°F, warm them with skin-to-skin contact and an extra layer, then recheck in 15 to 20 minutes. If the temperature doesn’t come up, seek medical attention.
Signs of Overheating
Overheating is harder to catch than you might expect, because newborns often don’t show the early warning signs that older children do. They may simply look unwell or seem fussier than normal. As overheating worsens, you might notice hot, flushed skin, a faster heartbeat, rapid shallow breathing, or heavy sweating followed by no sweating at all (which is a more dangerous sign). Heat rash, small red bumps typically on the neck, chest, or diaper area, is another clue that your baby is too warm.
Overheating during sleep is a known risk factor for SIDS (sudden infant death syndrome). The AAP recommends dressing your baby in no more than one extra layer compared to what you’d wear comfortably in the same room. Sleep sacks are a good option. Avoid blankets, hats indoors, and heavy swaddles in warm environments, and never cover your baby’s head during sleep.
Room Temperature for Sleep
You’ll find recommendations of 68°F to 72°F (20°C to 22°C) repeated widely online, but the AAP’s actual 2022 safe sleep guidelines don’t specify a number. Their review of the research found that while overheating clearly raises SIDS risk, the studies defined “overheating” in different ways, making a single target temperature hard to pin down. The practical guidance they do give is straightforward: dress your baby for the room you’re in, and use your own comfort as a benchmark. If you’d be comfortable in a T-shirt, your baby likely needs that plus one light layer.
A good habit is to feel the back of your baby’s neck or their chest. These areas give a better sense of core warmth than hands and feet, which tend to run cool in newborns even when their body temperature is perfectly normal.
How to Take Your Newborn’s Temperature
Use a digital rectal thermometer with a flexible tip. Apply a small amount of petroleum jelly to the end, lay your baby face-down across your lap or on a flat surface, and insert the tip about half an inch (no more). Hold it in place until it beeps, usually 10 to 20 seconds. Clean the thermometer with rubbing alcohol or soap and water afterward, and label it so it doesn’t get mixed up with oral thermometers in the house.
If your baby is calm and has been resting (not just finished a crying spell or a warm bath), you’ll get the most accurate reading. Crying and bundling can temporarily push a baby’s temperature slightly higher, so if a reading seems borderline, give it a few minutes and try again.