A rectal temperature of 100.4°F (38°C) or higher is considered a fever in a baby. This is the standard threshold used by pediatricians, and rectal readings are the most accurate way to check a baby’s temperature, especially in the first year of life. How urgently you need to respond depends on your baby’s age and how they’re acting.
Why Rectal Temperature Is the Standard for Babies
A rectal thermometer measures your baby’s core body temperature more reliably than any other method. Armpit readings, by comparison, miss about 27% of actual fevers in infants. In one study of hospitalized babies, 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 individual cases. That kind of inconsistency makes armpit readings unreliable when accuracy matters most.
Forehead and ear thermometers are convenient for older children, but for babies under 3 months, a rectal reading is what your pediatrician will ask about. It’s the number that drives clinical decisions.
Normal Rectal Temperature in Babies
A healthy baby’s rectal temperature typically falls between 97.9°F and 100.4°F (36.6°C to 38°C). That’s a wider range than most parents expect. Body temperature naturally fluctuates throughout the day, running lower in the morning and slightly higher in the late afternoon. A reading of 99.5°F after a warm bath or a bout of crying doesn’t necessarily mean your baby is sick. The key number is 100.4°F: anything at or above that threshold counts as a true fever.
Age Makes a Big Difference
The younger the baby, the more seriously a fever needs to be taken. Here’s how the response changes by age:
- Under 8 days old: Newborns in their first week are at higher risk for serious infections, including those acquired during birth. Any fever in this age range warrants immediate medical evaluation.
- 8 days to 2 months old: A rectal temperature of 100.4°F or higher in this age group is treated as a potential emergency, even if the baby looks fine. Infants this young can have serious bacterial infections without showing obvious signs of illness beyond the fever itself.
- 3 to 6 months old: Call your pediatrician if the temperature reaches 100.4°F, particularly if your baby seems unusually fussy, lethargic, or is feeding poorly.
- 6 to 24 months old: A fever above 100.4°F that lasts more than one day warrants a call. For any baby in this range, a fever lasting more than three days should be evaluated regardless of how they’re acting.
The reason for the heightened concern in very young infants is straightforward: their immune systems are immature, and infections can escalate quickly. Older babies with fevers are more likely fighting a routine virus, and their behavior, whether they’re still drinking, making eye contact, and wetting diapers, tells you a lot about how serious the situation is.
How to Take a Rectal Temperature
You’ll need a digital thermometer labeled for rectal use. Apply a small amount of petroleum jelly to the tip for comfort. There are two safe positions to choose from: lay your baby tummy-down across your lap with your palm resting on their lower back, or place them face-up and gently bend their legs toward their chest, resting your free hand against the back of their thighs. Either position keeps the baby stable and gives you a clear line of sight.
Insert the thermometer tip about half an inch to one inch into the rectum. Don’t force it. Hold it in place until the thermometer beeps, which typically takes 10 to 20 seconds with a modern digital model. Keep a gentle but firm hold on both the baby and the thermometer the entire time, since babies tend to squirm.
A few practical tips: take the reading when your baby is relatively calm rather than right after crying or a feeding, since both can temporarily raise body temperature. Clean the thermometer with rubbing alcohol or soap and water afterward, and label it so it’s never accidentally used orally.
What the Number Means in Context
Fever itself isn’t a disease. It’s the immune system’s response to an infection, and in most cases it means your baby’s body is doing exactly what it should. The height of the fever doesn’t always correspond to how sick a baby is. A baby with a temperature of 101°F who is feeding well and making normal eye contact is often in better shape than a baby at 100.5°F who is limp and unresponsive.
That said, the combination of a high fever and behavioral changes is what to watch for. Signs that something more serious may be going on include refusing to eat, fewer wet diapers than usual, a weak or high-pitched cry, difficulty waking up, or a rash that appears alongside the fever. In babies under 3 months, the fever alone is reason enough to call, regardless of how the baby seems.