Red flags — this entire page is the red-flag block
- Any temperature of 38°C or more, measured in any way, in a baby under 3 months old, is on its own a reason for immediate emergency assessment.
- Do not wait for the baby to "seem sick" — fever in this age group is treated as an emergency even in a baby who is smiling, feeding, or appears alert between checks.
- Do not use paracetamol, or any other medication, as a way to manage the situation at home and delay care — the goal is diagnosis, not symptom control, until a clinician has assessed the baby.
Why age changes everything
Fever is the body's normal, useful response to infection: it is a sign that the immune system has detected something and is fighting it. In an older child or adult, a modest fever is often not concerning on its own. In a baby under three months old, the same finding is treated completely differently, because a very young infant's immune system has not yet built up the layered defences that older children and adults rely on, and because the same short list of serious bacterial infections — including infections of the blood, the coverings of the brain, or the urinary tract — can present with fever as one of the only early clues, sometimes with very few other obvious signs. A young baby can also look deceptively well in between fever spikes and still be seriously unwell underneath. Because it is very difficult, even for experienced clinicians, to tell a dangerous cause of fever apart from a mild one using only symptoms and a look at the baby, medical guidelines treat any fever in this age group as needing urgent, in-person assessment, every time, without exception.
What counts as a fever
A temperature of 38°C (100.4°F) or higher, however it is measured (rectal, underarm/axillary, or another validated method your clinician has advised), is treated as a fever in this age group. A baby does not need to seem distressed or unwell in other ways for this threshold to matter — the number itself, at this age, is the trigger for urgent care.
What to do
A fever of 38°C or more in a baby under 3 months is an emergency — do not give paracetamol and wait. Do not try to manage it at home, do not wait to see if it comes down on its own, and do not wait to see if other symptoms develop. Contact emergency services or go directly to an emergency department or the emergency line recommended by your clinic. Bring the baby as they are; there is no home step that substitutes for an in-person medical assessment at this age.
While arranging urgent care, you can undress the baby down to a light layer and offer normal feeds if the baby is willing, but this is comfort care only and does not replace the need for immediate assessment.
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Message us on WhatsAppClinical thresholds in this page were reviewed and signed off by Dr Didier Decamps on 17 August 2026. This is general information and does not replace an individual assessment by a qualified clinician.
Written with AI assistance. Reviewed, corrected and signed by Dr Didier Decamps, paediatrician.
Last reviewed: 17 August 2026