Should I see a doctor for a fever in a newborn younger than three months?

Short answer

Why people ask this

Because infants younger than three months have immature immune systems, even a low-grade fever can signal a serious infection. Parents also hear that babies under 28 days with a fever should go to the ER and want to know if that applies to their situation. Caregivers wonder whether recent vaccines, warm clothing, or room temperature can explain the fever. They also want to know how to check temperature correctly (often rectally at this age) and when to call the pediatrician versus going to urgent care or the ER.

When it might be safe

• A rectal temperature below 100.4°F (38.0°C) with the baby feeding well, waking normally, and having normal diapers, while you continue close observation.
• A mild temperature elevation within 24–48 hours after routine vaccinations if the baby otherwise looks well—still call your pediatrician to confirm.
• Brief warmth from overdressing or a hot room that normalizes after adjusting clothing and rechecking a rectal temperature in 15–20 minutes.
• If your pediatrician already provided a specific fever plan for your newborn (e.g., late preterm follow-up) and the baby remains well-appearing.

When it is not safe

• Any rectal temperature of 100.4°F (38.0°C) or higher in an infant younger than 3 months—seek medical care the same day; under 28 days old usually means ER evaluation.
• Fever with poor feeding, lethargy, irritability, weak cry, or trouble waking the baby at any time in this age group.
• Fever with breathing problems, a rash that doesn’t blanch, bluish color, persistent vomiting, or signs of dehydration (fewer wet diapers, dry mouth).
• If the baby was born preterm, has a heart or lung condition, or is under 28 days old—even a single fever needs urgent assessment.
• If you cannot obtain a reliable rectal temperature or the baby simply “doesn’t look right.”

Possible risks

• Serious bacterial infections (e.g., urinary tract infection, bacteremia, meningitis) are more common in infants under 3 months.
• Rapid clinical changes can occur in newborns, making early evaluation important.
• Dehydration from poor feeding or vomiting can develop quickly in this age group.
• Misreading temperature with non-rectal devices may delay appropriate care.
• Giving fever-reducing medication without guidance can mask symptoms and complicate evaluation.

Safer alternatives

• Check a rectal temperature with a digital thermometer for accuracy; avoid ear/forehead devices for decisions at this age.
• Call your pediatrician or on-call nurse line for same-day guidance; describe age in days, rectal temperature, appearance, feeding, and diaper counts.
• Use telehealth if immediately available while preparing to go in if advised, especially for infants under 28 days.
• Ensure the baby isn’t overheated: remove extra layers, keep room 68–72°F (20–22°C), and recheck rectal temperature after 15–20 minutes.
• Track feeding frequency and wet/dirty diapers; bring this information to any visit.
• Avoid giving acetaminophen or ibuprofen unless your pediatrician specifically instructs you for this infant’s age and situation.

Bottom line

For infants younger than three months, a rectal temperature of 100.4°F (38.0°C) or higher generally needs prompt medical evaluation—immediately in babies under 28 days or if the infant looks unwell; when in doubt, call your pediatrician or go to the ER.

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