GUIDE
A rectal temperature of 100.4°F (38°C) or higher is a fever at any age.
That is the universal threshold. But what you do about it depends on your baby's age, which thermometer you used, and how your baby is acting. This guide explains the numbers, the measurement differences, and why context matters as much as the reading.
The number you need to know is 100.4°F, which is 38°C. A rectal temperature at or above this threshold is a fever in a baby of any age. This is not a rough guideline or a suggestion — it is the clinical definition used by the American Academy of Pediatrics, emergency departments, and pediatricians everywhere.
Why 100.4°F specifically? Because decades of research have shown that this is the point where a temperature elevation is reliably caused by an infection or inflammatory process, rather than normal daily variation. Below this number — say, 99°F or even 100°F — you could be seeing the effect of a warm room, a heavy blanket, vigorous crying, or just the natural late-afternoon temperature peak that all humans experience. Above 100.4°F, something is going on biologically.
But here is the critical nuance that trips up many parents: 100.4°F means very different things depending on your baby's age. In a newborn under 28 days, it means emergency room, tonight, no exceptions. In a six-month-old who is playful and drinking well, it means monitor at home and call the pediatrician if it climbs. The number is the starting point. Your baby's age and behavior tell you what to do about it.
| Method | Normal Range | Fever Threshold | Notes |
|---|---|---|---|
| Rectal | 97.9°F - 100.3°F (36.6°C - 37.9°C) | 100.4°F (38°C) | Gold standard for babies. Most accurate. Recommended for all babies under 3 months. |
| Oral | 97.6°F - 99.6°F (36.4°C - 37.6°C) | 100.0°F (37.8°C) | Not appropriate for babies or young toddlers. Requires keeping mouth closed for several minutes. |
| Axillary (armpit) | 96.6°F - 98.0°F (35.9°C - 36.7°C) | 99.0°F (37.2°C) | Least accurate method. Reads 1-2°F lower than rectal. Useful for screening, not diagnosis. |
| Temporal artery (forehead) | 97.2°F - 100.1°F (36.2°C - 37.8°C) | 100.4°F (38°C) | Good accuracy in babies over 3 months. Quick and non-invasive. Sweat on the forehead can affect readings. |
| Tympanic (ear) | 97.5°F - 100.0°F (36.4°C - 37.8°C) | 100.4°F (38°C) | Not reliable in babies under 6 months due to small ear canals. Earwax and positioning affect accuracy. |
If you have ever taken your baby's temperature with a forehead thermometer, gotten 99.8°F, then taken it rectally and gotten 100.6°F, you are not doing anything wrong. Different methods measure at different body sites, and each site runs at a slightly different temperature.
Rectal temperature is the closest approximation of core body temperature — the temperature inside your baby's body where the organs are. It is the most accurate and the reference standard against which all other methods are compared.
Temporal artery (forehead) thermometers measure the blood flowing through the temporal artery just beneath the skin. They are fast and non-invasive, which is why parents love them. They are reasonably accurate for babies over 3 months, but they can be affected by sweat, ambient temperature, and technique. If your baby has a sweaty forehead, the reading may be falsely low.
Axillary (armpit) temperatures are consistently lower than rectal — typically by 1 to 2 degrees Fahrenheit, though the exact difference varies. This is because the armpit is an external measurement point and is affected by ambient temperature and how tightly the arm is held against the body. Armpit readings are useful as a quick screen ("does this baby feel warm to me?"), but they are not reliable enough to make clinical decisions, especially in young infants.
Tympanic (ear) thermometers measure infrared heat radiating from the eardrum. They can be accurate in older children and adults, but they are not recommended for babies under 6 months. An infant's ear canal is small and curved, making it difficult to aim the sensor correctly. Earwax can also interfere with readings.
The takeaway: for babies under 3 months, use rectal. For babies over 3 months, rectal or temporal artery are both acceptable. For a deeper look at technique, see our step-by-step guide on how to take a baby's temperature.
Most of us grew up hearing that normal body temperature is 98.6°F (37°C). That number comes from a German study published in 1868 — and while it was a reasonable average at the time, modern research has refined the picture considerably.
Normal body temperature is not a single number. It is a range, and it fluctuates throughout the day based on your baby's circadian rhythm. In healthy babies, rectal temperature typically ranges from about 97.9°F in the early morning to about 100.3°F in the late afternoon. This means a reading of 100.0°F at 5 PM can be completely normal, while the same reading at 5 AM would be worth noting.
Other factors that can affect your baby's temperature without indicating illness include:
None of these represent a true fever. If you get a borderline reading (99.5-100.3°F), rule out these causes first. Undress your baby to a single layer, wait 15-20 minutes in a comfortable room, and retake the temperature.
| Age | Fever (Rectal) | Recommended Response | Key Point |
|---|---|---|---|
| 0-28 days (newborn) | 100.4°F (38°C) | Go to the ER immediately | Any fever in this age group is a medical emergency. Do not give fever reducers. Do not wait. |
| 1-3 months | 100.4°F (38°C) | Call pediatrician immediately | Doctor will likely want to evaluate the baby in person. ER visit is common at this age for any fever. |
| 3-6 months | 100.4°F (38°C), call at 102°F | Monitor at 100.4°F; call pediatrician at 102°F | Behavior becomes an important factor. Watch for feeding, wet diapers, and alertness. |
| 6-24 months | 100.4°F (38°C), call at 102°F+ | Monitor and manage comfort; call if fever lasts 24+ hours or behavior concerns | Watch the baby, not the thermometer. Behavior and hydration matter more than the number. |
| 24+ months | 100.4°F (38°C), call at 104°F+ | Home management with fever reducers; call if lasting 3+ days or high fever | Most fevers at this age are viral and self-limiting. Focus on hydration and rest. |
Log each reading in tinylog with the exact temperature, method (rectal, forehead, armpit), and timestamp. Over the course of an illness, this creates a fever curve that tells you and your pediatrician whether things are improving, holding steady, or getting worse.
Fever myths are remarkably persistent, passed from parent to parent with the confidence of established fact. Let us clear up the most common ones.
| Common Myth | The Reality |
|---|---|
| You should add a degree to an armpit temperature | Do not add or subtract from any reading. Report the number as-is along with the method used. The offset between methods varies by individual, so arbitrary adjustments can be misleading. |
| Normal body temperature is always 98.6°F | 98.6°F is an average, not a constant. Normal temperature ranges from about 97°F to 100.3°F (rectal) and fluctuates throughout the day — lowest in early morning, highest in late afternoon. |
| A higher fever means a more serious illness | Not necessarily, especially in babies over 3 months. Some minor viral infections produce high fevers (103-104°F), while serious bacterial infections can sometimes present with lower fevers. Behavior is a better indicator of severity. |
| Teething causes high fevers | Research shows teething may cause a very slight temperature elevation (up to about 100.3°F) but not a true fever above 100.4°F. If your teething baby has a fever, another cause should be investigated. |
| You need to bring the fever down to 98.6°F | Fever is the immune system working. The goal of fever reducers is to improve comfort, not to normalize the temperature. Reducing a 103°F fever to 101°F, where your baby feels better and takes fluids, is a successful outcome. |
| A fever will cause brain damage | Fevers from infection do not cause brain damage. The body has built-in mechanisms to prevent fever from climbing to dangerous levels. Only extreme hyperthermia (above 107°F), usually caused by external factors like heatstroke, can cause neurological injury. |
These signs suggest your baby's immune system is doing its job. Continue monitoring, manage comfort, and maintain hydration.
If any of these apply, do not wait. For newborns under 28 days, any fever is an emergency — see our guide on fever in newborns under 3 months for detailed information.
It is Saturday afternoon. Your four-month-old feels warm, so you grab the forehead thermometer: 100.1°F. Concerning? Maybe. But you know that forehead thermometers can be affected by skin temperature, and your baby just woke from a nap on your chest. You wait 15 minutes, undress her to a onesie, and try again: 100.3°F. Still borderline.
Because she is four months old — past the critical newborn window but still young — you decide to confirm with a rectal temperature. The rectal reading: 100.8°F. That is a fever. Not a high one, but a real one.
You check the age chart: at 3-6 months, the threshold for calling your pediatrician is 102°F. Your baby is at 100.8°F. She is alert, took a bottle 30 minutes ago, and had a wet diaper an hour ago. You note the time and temperature in your log, keep her in light clothing, and recheck in two hours. By evening, the rectal temperature is 101.4°F. She is fussier but still eating. You give a dose of acetaminophen, log it, and plan to recheck before bed.
If the temperature climbs to 102°F or above, you will call the after-hours line. If she stops eating, stops making wet diapers, or becomes difficult to console, you will call sooner. But right now, you have a plan, you have data, and you know exactly what would change your next step.
For a full age-by-age action framework, see our baby fever chart by age.
When you call your pediatrician, saying '100.4 rectal' tells them something very different from '100.4 armpit.' The method determines how to interpret the reading. Write down both the temperature and how you took it every time. This avoids confusion and helps your doctor give accurate guidance.
We understand that taking a rectal temperature is not anyone's favorite activity. But for babies under 3 months, it is the only method accurate enough to make clinical decisions. A forehead thermometer that reads 99.8°F could actually be 100.4°F or higher rectally — and that difference matters enormously in a young infant. Learn the rectal method before you need it.
A temperature of 100.0-100.3°F (rectal) is not technically a fever, but it is not nothing. It can mean your baby is fighting off something, is overdressed, or just had a vigorous cry. If it persists, recheck in 30 minutes with your baby in light clothing. If it climbs to 100.4°F or above, you have a fever and should respond based on your baby's age.
Body temperature naturally dips in the early morning hours (around 4-6 AM) and peaks in the late afternoon and evening (around 4-6 PM). This means a baby fighting an infection will often have their highest fever readings in the evening — which is why parents so often discover fevers at bedtime. A reading of 99.5°F at 7 AM and 101°F at 5 PM can represent the same underlying illness.
Take a few temperature readings when your baby is well and log them in tinylog. Knowing that your baby's normal afternoon rectal temperature is 99.4°F means you will not panic when you see that number during cold season — and you will recognize a real fever faster.
This guide is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. If your baby has a fever and you are unsure how to respond, contact your pediatrician. For any fever in a baby under 3 months old, seek immediate medical attention. Call 911 if your baby is having difficulty breathing, is unresponsive, or is having a seizure.
