Fever in a Baby: What to Do, Step by Step

For an infant, one number decides everything. The American Academy of Pediatrics says to call the pediatrician immediately for a baby younger than 3 months (12 weeks) with a temperature of 100.4 °F (38.0 °C) or higher — regardless of how well the baby looks, how brief the fever is, or whether anything else is wrong. At that age, fever is evaluated, not observed.

Step 1 — Take the temperature the right way

Method changes the number, and for a newborn the number changes the plan. The AAP’s guidance by age:

Method	AAP guidance
Rectal	”Rectal temperatures are the most accurate” — recommended under 3 months, and generally under 3 years
Temporal artery (forehead)	Can be used at all ages; second most accurate
Tympanic (ear)	Age 6 months and older; ear canals are too narrow before that
Oral	Not recommended until age 4 or older
Axillary (armpit)	The least accurate; use for quick screening at any age, not for a decision

That last row is the one that trips people up. An underarm reading of 99.8 °F in a six-week-old is not reassurance — it is a screening result that needs confirming rectally before you decide anything.

Step 2 — Apply the age rule

• Younger than 3 months, 100.4 °F (38.0 °C) or higher: call now. Do not wait for morning, do not medicate first.
• 3 to 12 months: MedlinePlus sets a contact threshold at 102.2 °F (39 °C) or higher, plus any fever lasting more than 24 to 48 hours in a child 2 years or younger.
• Any age, repeatedly above 104 °F (40 °C): call, per the AAP.

The AAP’s 2021 clinical practice guideline for well-appearing febrile infants covers ages 8 to 60 days with a fever of 38.0 °C or higher, and splits its recommendations across 8–21, 22–28, and 29–60 days. Infants younger than 8 days sit outside the guideline entirely — they are handled more aggressively still. The takeaway for parents is simply that pediatrics treats each of those windows as a separate problem, which is why “my baby has a fever” is always followed by “how old?”

Step 3 — Watch the baby, not the thermometer

Beyond the age thresholds, appearance carries the weight. The AAP says call if the baby looks very ill, is unusually drowsy, or is very fussy, and lists a stiff neck, severe headache, throat pain, ear pain, breathing problems, an unexplained rash, and repeated vomiting or diarrhea among the warning signs — along with heat exposure, an immune system condition, a heart problem, or a history of seizures.

Practical things to watch at home:

• Feeding. Is the baby taking the breast or bottle roughly as usual?
• Wet diapers. Fewer than usual is the first sign of dehydration.
• Alertness. Hard to wake, or not settling between peaks, both count.
• Breathing. Fast, labored, or with visible effort.
• Color. Blue lips, tongue, or nails is a 911 call, per MedlinePlus.

Step 4 — Comfort measures that are still recommended

1. Fluids frequently — breast milk or formula, offered more often rather than in bigger volumes.
2. A single layer of clothing. The AAP says do not overdress; if the baby is shivering or has chills, add a blanket.
3. Fever reducers only for discomfort, at a dose set by weight, and after you have called if the baby is under 3 months. MedlinePlus keeps ibuprofen away from infants under 6 months, and aspirin away from children entirely.
4. Skip the cooling rituals. The AAP is direct: alcohol baths, ice packs, and sponging “are no longer recommended. These can actually have adverse effects for your child.”

Go to the emergency room for

• Any fever in a baby younger than 3 months, if you cannot reach the pediatrician promptly.
• A seizure.
• Trouble breathing, or blue lips, tongue, or nails.
• Unresponsiveness, or a baby you cannot wake properly.
• Inconsolable crying, or a cry that sounds nothing like the usual one.
• A rash that does not fade when you press on it.
• Refusal to feed with fewer wet diapers.

For the rules once your child is past infancy, see high fever in kids: when to worry. For what causes fever in the first place, see fever causes, and for converting between scales, fever: Fahrenheit to Celsius.

This content is informational and does not replace medical evaluation, diagnosis, or treatment. Seek care immediately for any fever in a newborn.