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What counts as a fever in a baby under 3 months?

In a baby under 3 months, a temperature of 100.4 °F (38 °C) or higher is a fever, and the AAP says to call your pediatrician right away. The NHS uses the same threshold; this page quotes the AAP sentence, which is the one we have checked word for word.

Call your pediatrician right away (UK: your GP or NHS 111) if your baby:

  • Under 3 months (12 weeks), 100.4 °F (38.0 °C) or higher
Fever threshold under 3 months
AgeTemperatureActionSource
Under 3 months (12 weeks)100.4 °F (38.0 °C) or higherCall your pediatrician right awayAAP

What the sources say

Is younger than 3 months (12 weeks) and has a temperature of 100.4°F (38.0°C) or higher

AAP / HealthyChildren.orgFever: When to Call the PediatricianPage date: December 2, 2022Checked on October 23, 2026

At 3 AM

Take the temperature before you call if it takes under a minute: a digital thermometer, the reading as shown, and how you took it. Say the number and the method, and do not round it.

Under 3 months the threshold is low on purpose, and the action is the same at 3 AM as at 3 PM: call. Most pediatric practices have an after-hours line, and the number is usually on the voicemail. In the UK, the NHS urgent page puts this under 999 or A&E.

If your baby was born early, say so, and give both the age since birth and the due date. Let the clinician decide which line applies.

Have these ready: the baby's age in weeks, the temperature and how it was taken, when the fever started, what you have given and when, how the baby is feeding, wet diapers today, and whether they are waking and responding as usual.

Write each reading in the log with its time as soon as you have it. You will be asked for the first reading and the latest one, and the gap between them.

Where the sources differ

The AAP says call the pediatrician right away. The app's reviewed table records that the NHS fever page uses the same 38 °C threshold and that the NHS urgent page lists this fever under 999 or A&E; we quote only the AAP sentence here, because it is the one we have checked word for word. Where the sources differ, the stronger action wins.

You know your baby best. If you think something is seriously wrong, get help now.

This page is general information, not medical advice. If you're worried, call your pediatrician.

Baby Tracker for Dads keeps this threshold and the rest of the list on its free When to call a doctor page, next to your baby's log.

Written by The Baby Tracker for Dads teamChecked on October 23, 2026Medical review: pending. Every number on this page is quoted from the AAP, CDC or NHS; the links are above.