A normal temperature for a baby is around 97.7°F to 99.5°F (36.5°C to 37.5°C) when taken rectally. The classic “98.6°F” is a useful benchmark, but healthy babies fluctuate above and below that number throughout the day. A reading of 100.4°F (38°C) or higher, taken rectally, is considered a fever.
Normal Range by Measurement Method
The number you see on the thermometer depends on where you take the reading. A rectal temperature reads higher than an armpit temperature because it measures your baby’s core body heat directly, while the armpit only captures surface warmth. Here’s what counts as normal and what counts as a fever for each method:
- Rectal: Normal is roughly 97.7°F to 99.5°F (36.5°C to 37.5°C). A fever starts at 100.4°F (38°C).
- Armpit (axillary): A fever starts at 99°F (37.2°C), but this method is the least accurate because it’s not measuring internal body temperature. Adding half a degree to a full degree Fahrenheit gives you a closer estimate of the true core temperature.
- Ear or temporal artery (forehead): A fever starts at 100.4°F (38°C), the same threshold as rectal.
For babies 3 years old and younger, a rectal reading is the most accurate option with a standard digital thermometer. Ear and forehead thermometers are faster and easier, but rectal is the gold standard pediatricians rely on, especially for very young infants where precision matters most.
Why Baby Temperatures Fluctuate
Your baby’s temperature isn’t a fixed number. It shifts throughout the day, running slightly lower in the morning and slightly higher in the late afternoon or evening. Physical activity like crying, feeding, or being bundled in heavy clothing can also push the reading up temporarily. A warm bath, a hot room, or overdressing can all raise your baby’s temperature without any illness being present.
This is why a single reading slightly above or below 98.6°F isn’t automatically a concern. What matters is whether the number crosses that 100.4°F threshold rectally, and whether your baby is showing other signs of being unwell.
How to Take a Rectal Temperature
Use a standard digital thermometer labeled for rectal use. Apply a small amount of petroleum jelly to the tip, lay your baby face-down on a firm surface or across your lap, and gently insert the tip about half an inch to one inch into the rectum. Hold it in place until the thermometer beeps. Keep a hand on your baby’s lower back so they don’t squirm or roll.
Clean the thermometer with rubbing alcohol or soap and water afterward, and label it so it doesn’t get mixed up with one used orally. The whole process takes under a minute with a digital thermometer.
Fever Thresholds by Age
The fever number itself, 100.4°F, stays the same across infancy. But what that fever means changes dramatically depending on your baby’s age. The American Academy of Pediatrics breaks this down into specific windows: 8 to 21 days old, 22 to 28 days old, and 29 to 60 days old. The younger the baby, the more seriously a fever is taken, because very young infants have immature immune systems and fewer obvious signs when something is wrong.
A fever of 100.4°F or higher in a baby under 2 months old needs immediate medical evaluation. This isn’t a “wait and see” situation. In older infants, around 3 to 6 months, a fever still warrants a call to the pediatrician, but there’s more room to observe how your baby is acting overall. By the time a child is over 6 months, a low fever in an otherwise happy, eating, drinking baby is less urgent, though it still deserves monitoring.
When a Baby Is Too Cold
Most parents worry about fever, but a temperature that’s too low also matters. The World Health Organization defines hypothermia in newborns as a core temperature below 97.7°F (36.5°C). Babies, especially premature or low-birth-weight newborns, lose heat quickly because they have a large surface area relative to their body weight and limited fat for insulation.
Mild cold stress can go unnoticed but still has consequences. When a baby’s body diverts energy to producing heat, those calories aren’t going toward growth. Prolonged hypothermia in newborns can lead to drops in blood sugar and, in severe cases, increase the risk of serious infection. Cool hands and feet alone aren’t reliable signs, since babies commonly have cool extremities even when their core temperature is fine. A rectal reading is the best way to check.
Signs That Matter More Than the Number
A thermometer gives you one piece of information. How your baby looks and acts gives you the rest. A baby with a temperature of 100.5°F who is feeding well, making eye contact, and has normal wet diapers is in a very different situation than a baby with the same reading who is limp, refusing to eat, or difficult to wake.
Watch for these red flags regardless of what the thermometer says:
- Feeding changes: Missing two or more feedings in a row, or eating very poorly.
- Unusual sleepiness: Sleeping much more than normal, being hard to wake, or seeming floppy.
- Dehydration signs: Fewer wet diapers, crying without tears, dry mouth, or a sunken soft spot on the head.
- Breathing trouble: Fast or labored breathing, flaring nostrils, or skin pulling in around the ribs with each breath.
- Skin color changes: Skin or lips that look blue, purple, or gray.
- Rash: Especially one that appears quickly, blisters, or looks infected.
- Persistent vomiting or diarrhea: More than three very watery stools, or vomiting after feedings repeatedly.
A baby who shows any of these symptoms needs prompt medical attention, whether or not a fever is present. Conversely, a mild fever in an otherwise content baby who is eating, sleeping, and producing wet diapers normally is often the body doing exactly what it’s supposed to do: fighting off a minor infection.