Baby fever: when to worry and when to wait

Fever is defined as a temperature of 38°C (100.4°F) or above.¹ In older children, fever is rarely dangerous in itself — it is the body mounting an immune response to infection. In young babies, the same temperature carries significantly higher risk, because their immune system is not yet able to reliably contain infection. The age of your baby is the most important factor in deciding what to do.

What fever actually is

Temperature rises when the immune system releases chemicals called pyrogens in response to infection or inflammation. Fever is a defence mechanism, not a disease. The number matters most for young babies; after 6 months, how your baby is acting — responsiveness, eye contact, feeding, consolability — often matters as much as the temperature reading.

Age-based thresholds

Why this threshold is so strict: Young infants cannot localise infection well. A fever that would indicate a mild viral illness in an older child can, in a 4-week-old, indicate serious bacterial infection — meningitis, sepsis, or a urinary tract infection — which can deteriorate very rapidly. The AAP and NHS are in full agreement: any fever under 3 months warrants immediate same-day medical assessment at minimum, with 111 or A&E if there is any doubt about the setting.¹²

3–6 months:

• Temperature 38.0–39.0°C with no other concerning signs: call your GP for a same-day appointment
• Temperature above 39.0°C: seek urgent care — call 111 or go to A&E
• Any fever with warning signs listed below: don't wait for a GP slot — call 111¹

Over 6 months:

• Fever alone in a baby who is alert, feeding, and consolable: monitor at home; call GP if it lasts beyond 5 days or your baby is getting worse rather than better
• Fever alongside any warning sign below: act the same day¹

The NHS and AAP both note that in this older age group, accompanying behaviour and symptoms matter as much as temperature. A baby who is clearly unwell with a temperature of 38.5°C is more concerning than a briefly feverish baby who is playing and feeding normally.

How to take a temperature accurately

Axillary (armpit): Most practical for home use. Hold the digital thermometer firmly in the armpit for the full recommended time. This method is reliable and appropriate for all ages.

Tympanic (ear): Reliable from 3 months; not recommended under 3 months as the ear canal is too small. Ensure you are placing the probe correctly — following the device's instructions matters for accuracy.

Rectal: The most clinically accurate method, rarely needed at home. If a healthcare professional requests a rectal reading, they will guide you.

Avoid: Forehead strips and non-contact infrared thermometers are not reliable enough for clinical decision-making. If you are deciding whether to call 111, use a digital axillary or tympanic thermometer.

What helps

Paracetamol (acetaminophen): Appropriate from 2 months (over 4 kg, not premature). Reduces temperature and relieves discomfort. The dose is weight-based — always follow the package instructions exactly, or use the NHS dose calculator. Do not estimate or use adult packaging as a reference.¹

Ibuprofen: Appropriate from 3 months (over 5 kg). Also weight-based dosing from the package or NHS guidance. Can be alternated with paracetamol if one alone is not controlling your baby's distress — check with your GP or pharmacist before doing this. Do not give ibuprofen to a baby with chickenpox, or to a baby showing signs of dehydration.¹

Never give aspirin to a child under 16. Aspirin in children is associated with Reye's syndrome — a rare but serious condition affecting the brain and liver.¹

Keep feeding. Fever increases fluid requirements. Breastfed babies should be offered feeds more frequently. Older babies on formula or solids should be offered additional water.

Room and clothing: Keep the room comfortably cool (around 18°C). Dress your baby in light clothing and avoid heavy blankets. Tepid sponging is not recommended — it can trigger shivering, which raises core temperature rather than reducing it.

When fever signals something serious

The temperature reading is only part of the picture. The following signs alongside fever require emergency action — call 999 or go straight to A&E:

Call 111 or your GP same-day if fever is accompanied by:

• Persistent vomiting
• Signs of dehydration: no wet nappies for 8+ hours, sunken fontanelle, very dry mouth
• Your baby is less than 3 months old — any fever, any time¹²

The teething myth

Teething does not cause a temperature of 38°C or above.³ A very small rise — occasionally reported in the 24 hours around tooth eruption — does not reach the threshold of true fever. If your baby has a fever during teething, the teething is coincidental. Assess and respond to the fever on its own terms, using the thresholds above.

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Also in this cluster: When to call the doctor · Newborn jaundice

Sources

1. NHS. "High temperature (fever) in children." NHS, 2024. https://www.nhs.uk/conditions/high-temperature/
2. American Academy of Pediatrics. "Fever and Your Baby." HealthyChildren.org. https://www.healthychildren.org/English/health-issues/conditions/fever/Pages/Fever-and-Your-Baby.aspx
3. NHS. "Teething." NHS, 2023. https://www.nhs.uk/conditions/teething/