Fever in babies: when to worry, when to wait

A fever itself isn't dangerous — it's usually a sign the immune system is doing its job. What matters most is baby's age and how they're acting alongside the number on the thermometer.

What counts as a fever

Generally, a rectal temperature of 100.4°F (38°C) or higher is considered a fever in babies — rectal readings are the most accurate for infants, which is why pediatricians often ask for that specific number.

Age changes everything

• Under 3 months old: any fever of 100.4°F or higher is a same-day call to your pediatrician or urgent care, even if baby seems otherwise okay. A young infant's immune system can't yet reliably fight off infection the way an older baby's can, so providers take any fever seriously at this age.
• 3–6 months old: call your pediatrician for guidance, especially for fevers above 102°F (38.9°C) or a fever lasting more than a day.
• Over 6 months old: how baby is acting matters more than the exact number — a fever with a baby who's still drinking, alert, and consolable is generally lower urgency than a lower fever with a baby who's very listless or hard to console.

Signs that mean don't wait — seek care now, regardless of age

• Difficulty breathing, or breathing that looks labored.
• A rash that doesn't fade when pressed.
• Persistent vomiting, or signs of dehydration (very few wet diapers, no tears when crying, sunken soft spot).
• Extreme fussiness, or the opposite — unusually limp or hard to wake.
• A fever accompanied by a stiff neck, or baby seems to be in significant pain.

What generally helps at home for a milder fever in an older baby

• Fluids — keep offering breast milk, formula, or water depending on age.
• Light clothing, not bundled up.
• Fever-reducing medication (acetaminophen or, for babies over 6 months, ibuprofen) only at the dose and age your pediatrician has specifically approved — dosing by weight matters, and it's worth confirming rather than guessing.

The general rule

When in doubt, call. Pediatric offices field fever questions constantly, and a quick call is far better than guessing on something that's genuinely age-sensitive.