A baby with a fever is one of the most anxiety-provoking situations for new parents. The worry is understandable — fever in a young infant can occasionally signal something serious. But the vast majority of infant fevers are caused by common viral illnesses, resolve on their own, and require only comfort measures. Knowing what's actually concerning versus what's normal is what makes the difference between a calm, informed response and a panicked one. Here's the complete guide.
What Is a Fever in a Baby?
Fever is defined as a rectal temperature of 38°C (100.4°F) or higher. This is the clinical threshold — not 37.5°C, not "feeling warm." The rectal temperature is the gold standard for infants because it's the most accurate method, particularly in the first 3 months when accuracy is critical.
Temperature varies naturally throughout the day — it's typically lowest in the early morning and highest in the late afternoon. A baby who feels warm to touch but has a rectal temperature below 38°C does not have a clinical fever.
How to Take a Baby's Temperature Accurately
By Age and Method
| Age | Recommended Method | Fever Threshold |
|---|---|---|
| 0–3 months | Rectal (most accurate) | ≥38.0°C / 100.4°F |
| 3–12 months | Rectal or temporal artery | ≥38.0°C / 100.4°F |
| 12+ months | Rectal, temporal, or tympanic (ear) | ≥38.0°C / 100.4°F |
Avoid: forehead touch, axillary (armpit) in young infants, oral in children under 4 years. Temporal artery (forehead strip) thermometers are acceptable from 3 months and convenient for ongoing monitoring. Ear thermometers require correct technique and are less reliable under 3 months.
When to Call a Doctor Immediately
Fever management in infants is age-dependent, and the thresholds matter significantly:
Under 3 Months: Call Immediately
A rectal temperature of 38°C or higher in a baby under 3 months is a medical emergency that requires same-day evaluation — call your pediatrician immediately or go to an emergency department. Do not wait to see if it comes down on its own, do not give fever medication and reassess. Go now.
Why: The immune system in young infants is not yet able to contain bacterial infections effectively, and bacterial infections (meningitis, sepsis, urinary tract infection) can progress rapidly. The same fever that's benign in a 6-month-old can be life-threatening in a 6-week-old.
3–6 Months
Fever ≥38°C warrants a call to your pediatrician the same day. Most fevers in this age group are viral, but evaluation is appropriate.
Over 6 Months
Fever alone is less immediately alarming, but call your pediatrician if:
- Fever is above 39°C (102.2°F)
- Fever has lasted more than 2–3 days
- Baby appears very unwell, lethargic, or inconsolable
- Baby has a febrile seizure (call 999/911 immediately)
- Rash appears alongside the fever
- Baby is not producing wet diapers (dehydration)
Red Flag Symptoms Requiring Emergency Care at Any Age
- Difficulty breathing, fast or labored breathing
- Purple or red spots that don't fade when pressed (possible meningococcal rash — emergency)
- Bulging fontanelle (the soft spot on top of the head)
- Stiff neck
- Extreme lethargy — difficult to rouse, unresponsive
- Seizure
- Blue lips or fingertips
What Causes Fever in Babies
The most common cause by far is viral infection — the same cold and flu viruses that adults get, adapted to an infant's immune system. Common causes include:
- Upper respiratory infections: Rhinovirus (common cold), RSV, influenza
- Roseola: Very common in babies 6–12 months. High fever for 3–5 days, then a distinctive pink rash as fever resolves. Benign.
- Ear infections: Common from 6 months; often follows a cold
- Urinary tract infections: More common in girls and uncircumcised boys; fever may be the only symptom in infants
- Post-vaccination: A low-grade fever in the 12–24 hours following vaccination is common and expected. Not a cause for concern unless it exceeds 39°C or is accompanied by other symptoms.
Teething does not cause fever. Studies consistently show that the maximum temperature elevation from tooth eruption is less than 38°C — not a clinical fever. A baby who is teething and has a genuine fever has an infection that coincides with teething, not fever from teething. See our teething guide for more on this distinction.
Managing Fever at Home
Medication
- Paracetamol (acetaminophen / Tylenol): Safe from birth (with pediatric guidance under 3 months). Dose by weight, not age. Follow the packaging dosing chart precisely.
- Ibuprofen: Safe from 6 months. Slightly longer-acting than paracetamol. Do not give to babies under 6 months.
- Alternating: Some pediatricians recommend alternating paracetamol and ibuprofen for sustained high fever. Always follow your pediatrician's specific guidance on this.
- Never give aspirin: Associated with Reye's syndrome in children. Never appropriate.
Comfort Measures
- Keep baby lightly dressed: Overdressing traps heat and worsens fever. A single cotton layer is appropriate — not bundled in blankets.
- Maintain hydration: Offer breast or formula feeds more frequently. For older babies, water sips in addition.
- Lukewarm sponge bath: Can provide temporary comfort. Never cold water or alcohol rubs — both can cause dangerous temperature fluctuations in infants.
- Keep the room cool but not cold: 18–20°C is appropriate.
The Goal of Fever Treatment
Fever itself is not the enemy — it's a sign that the immune system is responding. The goal of treatment is not to normalize the temperature but to keep baby comfortable and prevent dehydration. A fever that comes down with paracetamol but returns when it wears off is normal. This does not mean the fever is "stronger" or that something is wrong with the medication.
For broader newborn health context, see our new parent guide and our guide on baby skincare for managing other common infant skin conditions.