Guides · Health · 0–12 months · 3 min read

Fever: What to Do, When to Call

How to measure and manage a fever, and the clear red lines for calling the doctor — including under 3 months.

A fever is a temperature of 100.4°F (38°C) or higher, and it's usually a sign the immune system is doing its job — most childhood fevers are caused by everyday viruses. Your job is to keep baby comfortable and know the red lines.

The non-negotiable rule

**If your baby is under 3 months old and has a rectal temperature of 100.4°F (38°C) or higher, call your doctor or seek urgent care immediately** — even if they seem fine. Young infants can't fight infection well, and this always needs same-day assessment.

Taking a temperature

A rectal thermometer is the most accurate for babies under about 6 months; follow the device instructions and use a dab of petroleum jelly. Forehead (temporal) thermometers are a reasonable alternative. Skip ear thermometers before 6 months and don't rely on touch alone.

Caring for a feverish baby (3 months+)

• Treat the child, not the number — the goal is comfort, not a perfect reading.
• Offer plenty of breast milk or formula; hydration matters most.
• Dress them lightly; don't bundle or cold-sponge.
• Ask your pediatrician about infant acetaminophen dosing (from 3 months, weight-based). Ibuprofen is an option from 6 months. **Never give aspirin.**

Call the doctor if

• Fever in any baby under 3 months, as above.
• Fever above 104°F (40°C), or lasting more than 24 hours in a baby under 2, or persisting 2–3 days.
• Baby is unusually drowsy, floppy, inconsolable, refusing feeds, has few wet diapers, a rash that doesn't fade under pressure, trouble breathing, or a seizure — seek emergency care for these.

Trust your instincts: if they seem really unwell, call regardless of the thermometer.

*Always consult your pediatrician for advice specific to your baby.*