A normal baby temperature taken rectally ranges from 96.8°F (36°C) in the morning to 100.3°F (37.9°C) late in the day, with 98.6°F (37°C) as the average. That range is wider than many parents expect, and a reading on either end is perfectly healthy. Understanding where “normal” stops and a fever begins depends on your baby’s age and how you take the temperature.
The Normal Range by Measurement Method
Rectal temperature is the gold standard for babies, and it’s the method pediatricians rely on for accuracy. A normal rectal reading falls between 96.8°F and 100.3°F. That means a reading of 99.5°F after a warm bath or a fussy afternoon is not a fever.
Armpit (axillary) readings run about a degree lower than rectal temperatures, so a normal armpit reading generally falls in the 97.5°F to 99.3°F range. Armpit readings are the least accurate of the common methods, though. If your baby’s armpit temperature seems high or you’re unsure of the result, it’s worth confirming with a rectal reading.
Ear (tympanic) thermometers should not be used on babies younger than 6 months because the ear canal is too small for an accurate reading. Forehead (temporal artery) thermometers are a newer option, and recent research suggests they can provide accurate readings even in newborns. Digital pacifier thermometers and fever strips are not considered reliable enough to trust.
Why Your Baby’s Temperature Changes Throughout the Day
A baby’s body temperature naturally dips in the early morning and rises in the late afternoon and evening. This is the same circadian pattern adults have, just more noticeable in infants. Seeing a reading of 97°F at 6 a.m. and 100°F at 5 p.m. on the same healthy baby is completely normal.
Several things can temporarily push a reading higher or lower without any illness involved. A baby who’s been swaddled in a warm room, crying hard, or just finished feeding may register slightly warmer. On the flip side, a baby fresh out of a bath can lose 2 to 3 degrees through wet skin evaporation alone, which can make a reading look artificially low. Even a bare head matters: babies lose a large amount of heat through their scalp, which is why hospitals put hats on newborns right away.
Which Thermometer to Use at Each Age
For babies from birth to 3 months, a standard digital thermometer used rectally is the recommended choice. This age group is the one where accurate readings matter most, and rectal measurement is the most reliable way to get one. A temporal artery thermometer across the forehead is also considered acceptable for newborns.
From 3 months to 4 years, you have more options. A rectal reading is still the most accurate, but armpit and forehead readings become reasonable alternatives for routine checks. Ear thermometers enter the picture at 6 months, once the ear canal is large enough for a proper reading.
When a Temperature Becomes a Fever
A fever in infants is defined as a rectal temperature of 100.4°F (38°C) or higher. This threshold is consistent across major children’s hospitals and pediatric guidelines. It applies regardless of time of day or how warm the room is.
The urgency of a fever depends heavily on age. For any baby under 2 months old, a rectal temperature of 100.4°F or higher is treated as a potential emergency. At this age, a baby’s immune system is still immature, and a fever can signal a serious infection that isn’t obvious from outward symptoms alone. Babies under 2 months with a fever this high should be evaluated in an emergency department.
For babies between 3 and 6 months, a fever still warrants a call to your pediatrician, but it’s less likely to require emergency care. By 6 months, a baby’s immune system is more developed, and fevers are more commonly tied to routine illnesses like colds or teething-related inflammation. The number on the thermometer matters, but so does how your baby is acting: feeding normally, making eye contact, and staying alert are all reassuring signs even when a mild fever is present.
When a Temperature Is Too Low
While most parents worry about fevers, a temperature that’s too low can also be a concern, especially in newborns. The World Health Organization defines neonatal hypothermia as a temperature below 97.7°F (36.5°C). Even full-term, healthy newborns can struggle to maintain their body temperature if the environment is too cold.
When a baby’s skin temperature drops just one degree below that 97.7°F threshold, their oxygen consumption can increase by 10%. The baby’s body burns through energy and oxygen trying to generate warmth, which puts extra strain on a system that doesn’t have large reserves to draw from. Keeping the room comfortably warm, dressing your baby in layers with a hat, and drying them quickly after baths all help prevent unnecessary cold stress.
Getting an Accurate Reading
For a rectal reading, apply a small amount of petroleum jelly to the tip of a digital thermometer and insert it about half an inch. Hold it in place until the thermometer beeps. Most digital thermometers take 10 to 30 seconds. Label any thermometer used rectally so it doesn’t get mixed up with one used orally later on.
For an armpit reading, place the thermometer tip in the center of your baby’s armpit with their arm held snugly against their body. Keep in mind that if the arm shifts or the thermometer isn’t firmly in contact with skin, the reading will skew low. If an armpit reading comes back borderline or higher than expected, switch to a rectal check before making any decisions based on the number.
Timing matters too. Taking a temperature right after a feeding, a crying spell, or a warm bath can give you a misleadingly high number. Waiting 15 to 20 minutes after any of these activities gives you a more representative baseline. If you’re tracking a pattern, try to take readings at the same time each day so the natural daily fluctuation doesn’t confuse the picture.