If a baby under 3 months has a temperature of 38 ℃, go straight to the hospital and don't watch at home

High value
Evidence A
Buys life
For: Child under 3, Pregnant or trying
Costs a little
Takes minutes
No willpower

In plain words

If a child under 3 months has a temperature of 38 ℃, go straight to the hospital. Don't watch at home, and don't give fever medicine first and wait and see. The American Academy of Pediatrics has set out a specific management plan for feverish babies who look reasonably well, so "looks alert enough" is no reason to wait at home. The UK guideline also classes a child under 3 months with a temperature of 38 ℃ as at high risk of serious illness.

Cost: A few hundred yuan for registration and tests.
What you get: The American Academy of Pediatrics has a dedicated guideline for full-term infants 8 to 60 days old who look reasonably well. The guideline defines fever as "temperature ≥38.0 ℃." It divides them into three bands, 8 to 21 days, 22 to 28 days and 29 to 60 days, with one management path for each. Infants under 8 days old are not within the scope of this guideline. The UK NICE fever guideline covers children under 5. It states that children under 3 months with a temperature of 38 ℃ or above are in the "high-risk group for serious illness."
Notes: At this age, a child with a serious bacterial infection may show only a fever. So looking "alert enough" is no reason to watch at home. And don't give fever medicine on your own first and then wait and see.
Evidence grade: A : Specific numbers from meta-analyses, large cohorts or randomized trials; for law and policy, the statute or official text.
Sources: Pantell RH, Roberts KB, Adams WG, et al. (2021). Evaluation and Management of Well-Appearing Febrile Infants 8 to 60 Days Old. Pediatrics, 148(2), e2021052228. https://doi.org/10.1542/peds.2021-052228
NICE (2019, updated 2021). Fever in under 5s: assessment and initial management (NG143), 1.2.12. https://www.nice.org.uk/guidance/ng143/chapter/Recommendations

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Translated from the Chinese original. Original book, section 20, tip 8. This section is about health and medicines. It is general information, not a substitute for a doctor’s diagnosis or prescription. Content may be out of date and rules differ by place. When you actually deal with an agency, a doctor or a court, follow what local officials, doctors and lawyers tell you. Disclaimer