If your baby is under 3 months old and has a temperature of 100.4°F (38°C) or higher, call your pediatrician immediately — or go to the emergency room if you cannot reach them.
This is not a “wait and see” situation. In very young infants, a fever can be the only sign of a serious infection like meningitis, urinary tract infection, or bacteremia. Their immune systems are still developing and cannot fight infections the way an older child’s can.
Why Is Fever in Young Babies So Serious?
Babies under 3 months:
- Have immature immune systems that may not show other symptoms even during serious infections
- Cannot localize infections well — an infection can spread rapidly
- May develop serious bacterial infections including meningitis, UTI, or bloodstream infections
- Often show subtle signs that are easy to miss without medical training
What Temperature Counts as a Fever?
For babies under 3 months, any rectal temperature of 100.4°F (38°C) or higher is considered a fever.
Important notes on temperature:
- Rectal temperature is the most accurate for infants — armpit and forehead readings can be unreliable
- Do not add or subtract degrees — use the number you read
- A “low-grade” fever in an older child is still a medical urgency in a baby under 3 months
What to Do Right Now
- Take a rectal temperature to confirm the fever
- Call your pediatrician immediately — most have an after-hours line
- If you can’t reach your doctor, go to the ER — don’t wait until morning
- Do NOT give medication (Tylenol or Motrin) until instructed by a doctor
- Keep your baby comfortable — dress lightly, offer feeding if interested
What Happens at the Doctor/ER
Your baby will likely receive:
- A thorough physical examination
- Blood tests (CBC, blood culture)
- Urinalysis and urine culture
- Possibly a lumbar puncture (spinal tap) — this sounds scary but is a routine part of evaluating febrile neonates
- Possible admission for observation and IV antibiotics until cultures return (typically 48 hours)
When the Rules Change
Once your baby is 3 months or older, the approach shifts. A fever is still worth monitoring, but it no longer automatically requires ER evaluation. For babies 3-6 months:
- Fever under 102°F with normal behavior — call your pediatrician for guidance
- Fever over 102°F — contact your pediatrician promptly
- Signs of dehydration, lethargy, or difficulty breathing — seek immediate care
Not sure what to do? TriageNest’s symptom triage is designed for exactly this — it adjusts every threshold and recommendation based on your child’s exact age. A 2-month-old and a 6-month-old get completely different guidance. Try it free.
Common Questions
My baby feels warm but the thermometer says 99.5°F. Is that a fever?
No. For infants under 3 months, a fever is defined as a rectal temperature of 100.4°F (38°C) or higher. Below that threshold, monitor your baby but no urgent action is needed.
Can I give Tylenol to reduce the fever before going to the doctor?
Don’t give medication before consulting your doctor. The fever reading helps the medical team assess how serious the infection might be. Artificially lowering it before evaluation can mask important diagnostic information.
My baby had a vaccine today and now has a mild fever. Same rules?
Yes — even post-vaccine fevers in babies under 3 months should be reported to your pediatrician. While vaccine reactions are common in older babies, the under-3-month threshold applies regardless of suspected cause.
Frequently Asked Questions
What temperature is a fever in a baby under 3 months?
For babies under 3 months, any rectal temperature of 100.4°F (38°C) or higher is considered a fever and requires immediate medical attention. Rectal temperature is the most accurate method for infants — armpit and forehead readings can be unreliable.
Can I give Tylenol to a 2-month-old with fever?
Do not give acetaminophen (Tylenol) or any medication to a baby under 3 months with a fever without consulting your doctor first. The fever reading helps the medical team assess how serious the infection might be. Artificially lowering it before evaluation can mask important diagnostic information.
Should I go to the ER for a 3-month-old with fever?
If your baby is under 3 months old and has a rectal temperature of 100.4°F or higher, call your pediatrician immediately. If you cannot reach them, go to the ER — don’t wait until morning. This is standard medical guidance because young infants can have serious infections with fever as the only symptom.
What will happen at the ER for an infant with fever?
Your baby will likely receive a thorough physical exam, blood tests (CBC, blood culture), urinalysis, and possibly a lumbar puncture (spinal tap) to rule out meningitis. Many infants are admitted for 24–48 hours of observation and IV antibiotics until culture results return. This is routine protocol for febrile infants under 3 months.
My 2 month old has a fever of 100.4 — what should I do?
A fever of 100.4°F or higher in a 2-month-old requires immediate medical attention. Call your pediatrician right away, or go to the ER if you cannot reach them. Do not give medication before consulting a doctor. This threshold applies to all babies under 3 months — even a low-grade fever at this age needs evaluation.
What do I do if my 2 month old has a fever?
Take a rectal temperature to confirm. If it is 100.4°F (38°C) or higher, call your pediatrician immediately or go to the emergency room. Do not give Tylenol or any medication without a doctor’s instruction. Keep your baby lightly dressed and offer a feeding while you arrange to be seen.
Related Guides
- Newborn Low-Grade Fever: When to Worry — Even mild temperature elevations in newborns can be significant
- Is My Baby’s Fever Dangerous? — Understanding fever severity by age
- Baby Fever 101: When to Call the Doctor — Age-specific fever thresholds and what they mean
A fever in a baby under 3 months always warrants medical evaluation. This article is informational — please call your pediatrician or go to the ER if your infant has a fever. For age-specific pediatric guidance, use TriageNest.
