Health & safety

Fever and taking a temperature

For a baby 3 months old or younger, a rectal temperature of 100.4°F (38°C) or higher needs immediate medical attention—even if baby seems well. Call the pediatrician now and arrange urgent in-person evaluation; if you cannot reach them promptly, go to an emergency department.

Full guide

The dated summary changes with the card you choose. The complete reference below stays available for context and safety.

What to check right now

Warm skin, flushed cheeks, fussiness, sleepiness, or feeding changes may make you suspect illness, but touch cannot confirm a fever. Normal temperature varies with age, activity, and time of day. Use a digital thermometer and write down the exact number, time, and where it was taken; do not rely on a forehead strip.

A rectal reading is the most reliable for an infant, especially under 3 months. Underarm readings are the least accurate, and ear thermometers are not reliable before 6 months because a young baby’s ear canal is too narrow. The American Academy of Pediatrics (AAP) explains the differences in its guide to taking a child’s temperature.

What changes next

Before the 3-month mark: the fever threshold stays the same. One rectal reading of 100.4°F (38°C) or higher is enough to act; do not wait for a second reading or for other symptoms to appear. The AAP’s fever guidance for babies says to call immediately.
At 3 months: keep treating 100.4°F (38°C) or higher as an immediate call while you confirm the next plan. The AAP fever article says “3 months of age or younger,” while its thermometer guide says “2 months old or younger”; ask the pediatrician which age cutoff and measuring method to use going forward.
From about 6 months: an ear thermometer becomes an option when positioned correctly. A digital rectal or forehead thermometer can still be used.
From about age 4 years: most children can cooperate with an oral temperature. Until then, choose a method recommended for the child’s age.

Reassuring signs

Look at the whole pattern, not only one number:

The measured temperature is below the fever threshold.
Baby wakes or can be gently roused for feeds and has some alert periods.
Feeding effort and wet diapers remain close to baby’s usual pattern.
Breathing looks comfortable, and lips and tongue keep their usual color.

These signs are encouraging, but a normal temperature does not rule out illness in a newborn. Call promptly if a young baby looks or acts distinctly different, feeds poorly, is unusually sleepy or fussy, or is getting worse.

When to call or get help

Call the pediatrician

You want to confirm which thermometer and after-hours plan the practice recommends before the first illness.
Two thermometers or measuring sites keep giving different readings, without any prompt or emergency sign below.
An older baby has a fever and you are unsure which age-specific advice applies.

Get prompt medical advice

Call now, even without a measured fever, if baby:

looks very ill, is unusually drowsy or fussy, or is getting worse;
feeds poorly, has significantly fewer wet diapers, or repeatedly vomits or has diarrhea;
has breathing difficulty or a new unexplained rash; or
has an immune-system or heart condition that changes the care plan.

Get urgent help now

For a baby 3 months old or younger with a temperature of 100.4°F (38°C) or higher: call the pediatrician immediately and arrange urgent in-person evaluation. The AAP’s current guide to choosing a care setting says to go to an emergency department; go if the pediatrician cannot direct you promptly. Do not wait to see whether the temperature falls.

Call 911 or your local emergency number if baby is unresponsive or cannot be woken, has a seizure, is struggling to breathe, or has blue or gray lips, tongue, or face. Do not wait for a callback when baby appears to have a life-threatening emergency.

Usually not a worry

Not exactly 98.6°F: normal temperature varies during the day and from one child to another. Watch the trend and the baby, not a single “perfect” number.
Warm skin with a reading below 100.4°F: touch is an imprecise screen. Recheck with a reliable method if baby still seems warm or unwell, and call if other concerning signs appear.
Slightly different readings by different methods: the sites are not equally accurate. Report the exact number and method rather than adding or subtracting degrees.
Teething later in infancy: it can cause a small rise within the normal range, but the AAP says it does not cause a true fever. Do not explain away 100.4°F (38°C) or higher as teething.

Useful today

Keep a working digital thermometer, petroleum jelly, and the instructions together; label the thermometer “rectal.”
Save the pediatrician’s daytime and after-hours numbers and know which emergency department the care team recommends.
If you call, share baby’s age, exact temperature, measuring method and time, feeding and wet diapers, other symptoms, and any medicine already given.
Get medical guidance before giving a young baby fever medicine. The AAP says acetaminophen should not be given under age 2 without a doctor’s guidance because fever in the first 12 weeks needs immediate evaluation; it also says not to give ibuprofen under 6 months unless a doctor directs it. See the AAP’s acetaminophen guidance and ibuprofen guidance.