Baby Health · Illness
Baby Fever — When to Worry
Fever (≥38°C / 100.4°F) in babies. Under 3 months = always emergency. Older babies by symptoms not just temperature. Safe paracetamol / ibuprofen doses. Meningitis red flags. NICE NG143.
Last reviewed 2 June 2026
Baby fever decision tool
When to worry about your baby's temperature
🚨 Red flags — emergency now if ANY present
⚠️ Amber flags — same-day GP / NHS 111
What temperature counts as a fever?
- ≥ 38 °C (100.4 °F) = fever, regardless of age.
- Under 3 months — any fever is a same-day emergency.
- 3–6 months — fever ≥ 39 °C OR amber features = same-day GP.
- Over 6 months — comfort-treat; act on the child not the number.
Thermometer accuracy: NICE recommends electronic axillary (under arm) for under-4-weeks and infrared tympanic (ear) or axillary thereafter. AVOID forehead strips (inaccurate). Rectal is most accurate but rarely needed at home.
Treating fever — the modern approach
- Treat comfort, not numbers. NICE explicitly: don't routinely use antipyretics with the sole aim of reducing temperature. Use them if the child is distressed.
- Paracetamol 15 mg/kg every 4–6 hours (max 4 doses / 24 hours). From birth (under specialist guidance) or routinely from 2 months + > 4 kg.
- Ibuprofen 10 mg/kg every 6–8 hours (max 3 doses / 24 hours). ONLY for babies ≥ 3 months AND > 5 kg. Avoid in dehydration, chickenpox, kidney issues.
- Don't alternate routinely. NICE and AAP both advise choosing one. Alternating doesn't reduce fever more and increases dosing errors.
- NEVER aspirin under 16 with fever. Risk of Reye's syndrome.
- Avoid tepid sponging. Unhelpful and can cause shivering which raises temperature.
- Light clothing. Don't bundle up. Cool room, normal layers.
- Plenty of fluids. Breast / formula / water on demand.
- Watch the child, not the thermometer. A child who is alert, taking fluids, and engaging with you at 39 °C is doing better than one who is listless at 38 °C.
Common fever questions
- "How high is too high?" The NUMBER matters less than the CHILD. A well-looking 18-month-old at 40 °C with no other features is less worrying than a quiet, withdrawn 4-month-old at 38.2 °C. Always assess the child, not just the thermometer.
- "Will the fever damage my child's brain?" No. Fevers up to 42 °C from infection do not cause brain damage. The fever is the body's defence, not the problem. Brain damage from heat requires temperatures > 42 °C from external heat sources (heatstroke), not infection.
- "What about febrile convulsions?" Happen in ~3% of children, peak age 6 months to 5 years. Most are brief (< 5 minutes), self-resolve, and don't predict epilepsy. NICE: if convulsion lasts > 5 min, call 999. After a febrile convulsion, child should be seen by GP. Paracetamol does NOT prevent febrile convulsions in subsequent illnesses (proven in trials).
- "How do I take a baby's temperature?" Under 4 weeks: electronic axillary (under arm). Over 4 weeks: tympanic (ear) or axillary. NICE advises AGAINST forehead strip thermometers (inaccurate). Add 0.5 °C if axillary if comparing to oral / rectal. Don't use mercury (banned).
- "My baby feels hot but the thermometer reads normal." Sweating, overheating from blankets, recent feed, hot car, or post-cry can all cause feeling-hot without true fever. Strip a layer, wait 10 minutes, recheck. Trust the thermometer.
- "Should I undress my baby to cool them down?" Light clothing, yes. Stripping completely can cause shivering, which raises temperature. One layer + nappy in a cool room is the goal.
- "Fever with rash — meningitis?" Glass test: press a clear glass against the rash. If the rash does NOT fade under pressure (non-blanching), call 999 — this is a meningococcal sign. Most rashes with fever are viral and blanching — but the glass test takes 5 seconds and you should do it.
- "How long should I wait before calling?" ANY fever in a baby under 3 months — same day, not next day. 3-6 months + ≥ 39 °C — same day. Over 6 months + well-looking, low-grade fever — wait 24-48 hours and call if not improving or amber features develop.
- "Teething fever?" Teething can cause mild warmth (up to 38 °C) but does NOT cause fever ≥ 38 °C. Calling something a 'teething fever' has been linked to delays in spotting real infection. If ≥ 38 °C, assume infection.
- "Post-vaccination fever?" Common in the 24-48 hours after vaccinations. Paracetamol if uncomfortable; usually settles. After MenB vaccine specifically, NICE recommends prophylactic paracetamol (the only vaccine where this is advised) because high fever is very common. Discuss with practice nurse.
- "Roseola — when does the rash appear?" Classic pattern: high fever for 3-5 days in a 6-24 month old, then fever BREAKS and a pink rash appears. The rash means recovery, not a new worry. Confirmed with timing — fever during rash phase suggests different cause.
- "My child has fevers every few weeks." Common between 6 months and 3 years (mostly nursery / daycare viruses, ~8-12 viral illnesses per year is normal). Frequent fevers WITH growth concerns, frequent bacterial infections, unusual infections, or family history of immunodeficiency = GP review.
- "COVID / RSV / flu — same approach?" Yes. Antipyretics for comfort, fluids, monitor for breathing changes (RSV especially — see paediatric services if any respiratory distress). Antiviral treatment for specific situations only.
Educational tool only — not medical advice. Babies under 3 months with any fever ≥ 38 °C need same-day emergency assessment. Always trust your gut — if you're worried, get help.
What does this mean?
The single most important fever rule is the under-3-months rule: any temperature ≥ 38 °C (100.4 °F) in a baby less than 3 months old is a SAME-DAY emergency, regardless of how well the baby looks. Their immune systems aren’t mature enough to handle bacterial infection without help, and many young infants with sepsis or meningitis present with fever as the only feature. AAP 2021 + NICE NG143 are explicit. Don’t treat with paracetamol first; don’t wait overnight to see how it settles. For older babies and children, the modern approach — treat comfort, not numbers — comes from the 2011 AAP and NICE NG143 reframing of antipyretics. The fever itself is the body’s immune defence; it does NOT damage the brain (true even at 42 °C from infection). The goal of paracetamol or ibuprofen is to help your child feel less awful, not to chase a number. A well-looking 2-year-old at 40 °C is less worrying than a quiet, withdrawn 4-month-old at 38.2 °C. Watch the child, not the thermometer: alertness, colour, breathing, responsiveness, hydration (wet nappies, tears, moist mouth). The NICE traffic-light system gives a structured way to know when to call. Red (emergency now): non-blanching rash, mottled or blue, stiff neck, bulging soft spot, grunting or severely laboured breathing, no response to social cues, unrousable, or under 3 months with any fever. Amber (same-day GP): less active but rousable, taking less than half normal feeds, dry mouth or fewer wet nappies, fever ≥ 5 days, joint swelling, not weight-bearing. Green (comfort care): normal colour, responding normally, smiling or content, normal cry, hydrated. Some practical myths worth dropping: (1) Don’t alternate paracetamol and ibuprofen routinely — pick one; alternating doesn’t reduce fever more and increases dosing errors. (2) Forehead strip thermometers are unreliable — use electronic axillary or tympanic. (3) Tepid sponging is unhelpful — causes shivering which raises temperature. (4) Paracetamol does NOT prevent febrile convulsions in subsequent illnesses (proven in RCTs). (5) Teething does NOT cause fever ≥ 38 °C — assume infection. (6) NEVER aspirin in under-16s with fever — Reye’s syndrome risk. The glass test for non-blanching rash (press a glass against the rash; if it doesn’t fade under pressure = call 999) takes 5 seconds and could save a life.
What counts as a fever?
≥38°C (100.4°F) universal cut-off. Below isn’t fever — just raised temperature.
When to worry by age
- Under 3 months: ANY fever ≥38°C = same-day A&E / emergency.
- 3-6 months: ≥39°C or amber feature = same-day GP.
- Over 6 months: assess by symptoms + how baby looks.
Watch the child, not the thermometer.
Why under-3-months fever is serious
Immature immune system. Up to 10% of well-looking febrile babies under 3 months have serious bacterial infection (UTI, bacteraemia, meningitis). AAP 2021: any ≥38°C = sepsis workup.
How to take temperature
- Under 4 wk: axillary or rectal (by HCP).
- 4 wk-5 yr: axillary OR ear (>3 months).
- Avoid forehead strips (less accurate).
- Don’t put thermometer in mouth in young child.
- Normal: ~36.5-37.5°C.
Safe doses (per dose)
- Paracetamol from 2 months: 15 mg/kg every 4-6h; max 60 mg/kg/24h.
- Ibuprofen from 3 months + ≥5 kg: 5-10 mg/kg every 6-8h; max 30 mg/kg/24h.
- Can alternate; don’t use simultaneously routinely.
- NEVER aspirin (Reye’s syndrome).
Call 999 if
- Non-blanching rash (glass test).
- Difficult to wake / very drowsy.
- Unresponsive / floppy.
- Very fast / laboured breathing.
- Blue lips / tongue.
- Stiff neck.
- Bulging fontanelle.
- Fits >5 min.
- High-pitched cry.
- Baby <3 mo looking unwell.
Febrile seizures
~3% of children 6 months-6 years. Brief (<5 min), generalised, usually harmless. Lie baby on side; protect from injury; time; don’t put anything in mouth; stay calm. 999 if first / >5 min / not coming round.
Meningitis signs
- Non-blanching rash (glass test).
- Stiff neck.
- Bulging fontanelle (under 1).
- Fits.
- High-pitched cry.
- Very lethargic / hard to wake.
- Severe headache.
- Vomiting.
- Cold hands + feet with hot core.
Rash NOT always present. 999 if suspected.
Vaccine fever — normal
Mild fever 24-48h after some vaccines. MenB causes more — NICE recommends prophylactic paracetamol at vaccination + 4-8h later.
Different scenarios
Scenario 1: 6-wk baby, temp 38.2, otherwise OK
Under 3 months — A&E. Sepsis workup standard.
Scenario 2: 14-mo, temp 39.5, runny nose, feeding well
Likely viral. Paracetamol if distressed. Watch. Hydrate.
Scenario 3: 4-mo, temp 39.5 + non-blanching rash
999. Meningococcal sepsis until proven otherwise.
Scenario 4: 2-yo febrile seizure 3 min, recovering
Usually benign. GP / 111 to check cause. Antipyretic + reassurance.
Scenario 5: 10-mo, post-MenB vaccine, fever 38.5
Expected. Prophylactic paracetamol per NICE. Comfort.
Care guidance — fever
- Watch baby, not just thermometer.
- Under 3 mo + fever = emergency.
- Light clothing, cool room, fluids.
- Paracetamol / ibuprofen for comfort, not chasing temperature.
- NO tepid sponging (NICE).
- Glass test for rash.
- Meningitis can have NO rash early.
Sources
- NICE NG143. Fever in under 5s.
- AAP. Clinical Practice Guideline: febrile infants 8-60 days old (2021).
- NHS. Fever in children.
- Meningitis Now UK. meningitisnow.org.
Frequently asked questions
What counts as a fever in a baby?
≥38°C (100.4°F) universal cut-off (NHS, AAP, CDC). Below this isn't fever — just raised temperature. UNDER 3 MONTHS: even 38°C is a same-day emergency. OLDER babies + children: number matters less than how the child LOOKS — feeding, drinking, lethargic, responsive.
When should I worry about baby's fever?
ALWAYS: any baby UNDER 3 MONTHS with ≥38°C — same-day A&E / emergency assessment. 3-6 MONTHS with ≥39°C or amber feature — same-day GP. OVER 6 MONTHS with red-flag signs (non-blanching rash, mottled, stiff neck, bulging fontanelle, grunting / laboured breathing, no response to social cues) — emergency. WATCH the child, not the thermometer.
Why is fever in a newborn so serious?
Under 3 months have immature immune systems + can't reliably contain bacterial infections. Up to 10% of well-looking febrile babies under 3 months have a serious bacterial infection (UTI, bacteraemia, meningitis). AAP 2021 CPG: any temperature ≥38°C in this age group warrants same-day medical review + usually a SEPSIS workup (bloods, urine, sometimes lumbar puncture).
How do I take baby's temperature?
(1) Under 4 weeks: electronic axillary (under arm) or rectal (if HCP); (2) 4 weeks-5 years: axillary OR ear (tympanic, >3 months); (3) AVOID forehead strips (less accurate); (4) DON'T put thermometer in mouth in young child. NORMAL: ~36.5-37.5°C. HOLD baby still; reading takes 30-60 sec. RECHECK if uncertain. NHS NICE NG143.
Paracetamol or ibuprofen — which?
BOTH safe + effective. PARACETAMOL: from 2 months for fever; 15 mg/kg every 4-6 hours; max 60 mg/kg/24h. IBUPROFEN: from 3 months AND ≥5 kg; 5-10 mg/kg every 6-8 hours; max 30 mg/kg/24h; AVOID if dehydrated, kidney issues, asthma sensitive. CAN ALTERNATE both. NICE NG143: don't use them simultaneously routinely (alternate if needed). NEVER aspirin (Reye's syndrome). FEVER itself doesn't usually need treatment unless distress.
Do I need to treat the fever?
FEVER IS PROTECTIVE — boosts immune response. AIM: comfort, not normal temperature. If baby DISTRESSED (irritable, not feeding, sleep disrupted): give paracetamol. If COMFORTABLE: monitor, hydrate, light clothing, observe. Don't 'chase' a temperature down to normal. SPONGING / cold baths NOT recommended (NICE) — can cause shivering + worse.
What about febrile seizures?
AFFECT ~3% of children aged 6 months-6 years. USUALLY brief (<5 min), all-over (generalised) convulsion during fever. SCARY but USUALLY HARMLESS — no brain damage; no link to epilepsy in most. DURING SEIZURE: protect from injury; LIE on side; TIME it; don't put anything in mouth; stay calm. CALL 999 if: first seizure; lasts >5 min; not coming round; difficulty breathing. ASSESS underlying cause.
What's the underlying cause of fever?
MOSTLY VIRAL infections (cold, flu, RSV, hand-foot-mouth, roseola, COVID). BACTERIAL: tonsillitis, ear infection, UTI, chest infection, meningitis, sepsis. POST-VACCINATION fever common 24-48 hours after some vaccines. RARELY: Kawasaki disease, autoimmune, malignancy. WORKUP based on signs + age.
When to call NHS 111 / out-of-hours?
(1) Under 3 months with any fever; (2) Persistent fever >5 days; (3) Fever + no clear cause + worsening; (4) High fever + unusual symptoms; (5) Worried but not 999-emergency; (6) Need advice on home care. 111 ROUTES: phone, online, app. Triage to GP / A&E / home care. AVAILABLE 24/7. AVOIDS unnecessary trips to A&E.
What signs need 999?
(1) NON-BLANCHING rash (doesn't fade under pressed glass — purpura test); (2) Difficult to wake / very drowsy; (3) Unresponsive / floppy; (4) BREATHING very fast or laboured; (5) Blue lips / tongue / face; (6) NECK STIFFNESS; (7) BULGING fontanelle; (8) Fits lasting >5 min; (9) HIGH-PITCHED, abnormal cry; (10) BABY <3 months looking unwell; (11) Severe headache (older child).
Could it be meningitis?
Signs: NON-BLANCHING rash (glass test); STIFF NECK; BULGING FONTANELLE (under 1); fits; high-pitched cry; very lethargic / hard to wake; severe headache; vomiting; cold hands + feet with hot core; fast breathing. RASH NOT ALWAYS PRESENT — can develop late. 999 if suspected. NHS pamphlet 'glass test' for non-blanching rash. UK has free meningococcal vaccines (MenB, MenACWY).
Should I undress baby with fever?
LIGHT CLOTHING (one layer + cotton). DON'T heavily wrap; DON'T strip naked + cold (shivering). KEEP ROOM cool (18-20°C). FLUIDS — offer milk / water often. PARACETAMOL if uncomfortable. NICE recommends gentle approach — not tepid sponging or cold baths.
Vaccine fever — what's normal?
Mild fever (<39°C) in 24-48 hours after vaccines common + expected. Means immune system responding. SPECIFICALLY: MenB causes more fever — NICE recommends PROPHYLACTIC paracetamol at vaccination + 4-8 hours later. PERSISTENT fever >48 hours or HIGH FEVER (>39°C) or with other symptoms: assess for other cause. Vaccine fever doesn't mean baby's unwell from vaccine — just immune response.
How does this relate to other calculators on BumpBites?
Companion: /calculators/baby-cough; /calculators/pediatric-dose; /calculators/croup-westley; /calculators/vaccine-scheduler; /calculators/vaccine-reaction; /calculators/baby-colic; /calculators/hand-foot-mouth; /calculators/muac-cds.