PediaPulse

Fever in Newborns and Babies Under Two Months

Authored by Ebenezer Adebiyi, MD, MPH, FAAP | Last medically reviewed: June 2026

A rectal temperature of 100.4°F (38.0°C) or higher in a baby under two months old is always treated as urgent. At this age, the immune system is still immature, and fever may be the only sign of a serious infection. Prompt evaluation is recommended — even if the baby appears well.
A newborn baby being checked with a thermometer by a parent
Illustrative image.

Why is fever in babies under two months treated differently?

  • Babies under two months have immature immune systems — they cannot fight infections the way older children can
  • Fever may be the only visible sign of a serious bacterial infection, including bloodstream infections (bacteremia — bacteria in the blood) or meningitis (infection of the fluid around the brain and spine)
  • Well-appearing newborns with fever can still have serious infections — physical appearance alone is not enough to rule out danger at this age
  • Approximately 8 to 13 percent of febrile young babies have a bacterial infection — and about 4 percent of those under 28 days have an invasive bacterial infection such as bacteremia or meningitis
  • The younger the baby, the higher the concern — babies under 21 days are generally considered at the highest risk
  • Prompt evaluation is standard practice regardless of how the baby looks or behaves
A reference table showing that the pediatric definition of fever is a rectal temperature of 100.4 degrees Fahrenheit or 38 degrees Celsius or higher across all age groups per AAP and ACEP guidelines while noting that clinical significance and urgency of evaluation vary by age.Fever — universal pediatric definition (AAP / ACEP)Rectal temperature ≥ 100.4°F (38.0°C) — this threshold applies to all age groupsAge groupFever thresholdBest methodClinical urgencyUnder 2 monthsNewborn≥ 100.4°F (38.0°C)Rectal thermometermost accurateHighest urgencyCall promptly2–6 monthsYoung infant≥ 100.4°F (38.0°C)Rectal or temporalartery thermometerHigh urgencyBehavior key6–24 monthsOlder infant≥ 100.4°F (38.0°C)Rectal temporalor axillaryWatch alertnessand feeding2–5 yearsToddler≥ 100.4°F (38.0°C)Temporal earor oralDuration andbehavior keyThe number is only one piece of informationBehavior appearance and feeding matter as much as the temperature readingAlways tell your pediatrician the exact reading the method used and the time taken

What should parents watch for in a baby under two months with fever?

  • Any rectal temperature of 100.4°F (38.0°C) or higher — even a single reading — warrants immediate medical attention
  • Poor feeding, weak cry, unusual sleepiness, or difficulty waking are concerning signs alongside fever
  • A baby who feels limp, looks pale or mottled (blotchy skin with patches of different color), or has cool hands and feet needs urgent evaluation
  • Faster breathing, grunting, or chest pulling in alongside fever adds to the concern
  • Fewer wet diapers, no tears when crying, or a sunken soft spot on the top of the head may suggest dehydration
  • A bulging soft spot (fontanelle — the soft area on top of a newborn's head) alongside fever is a warning sign
  • Parents should not wait to see if the fever goes away on its own — prompt evaluation is recommended at this age
A four-step visual guide showing parents how to safely take a rectal temperature in a baby including correct positioning lubricating the tip correct insertion depth and how to read and record the result.Step 1Step 2Step 3Step 4Position babyLubricate tipInsert gentlyRead and recordLay baby on backHold both anklesLift legs toward chestHave thermometerwithin easy reachApply a small amountof petroleum jellyto the tip onlyDo not use salivaor waterhalf inch onlyInsert no more thanhalf an inchHold still and waitfor the beepNever force it100.4 F38.0 CNote the readingWrite down the timeNote the method usedShare all three withyour pediatricianAlways share with your pediatricianThe exact reading — which method was used — and the time it was takenRectal is the most accurate method for babies under 3 years per AAP guidelines

How do pediatricians generally evaluate a baby under two months with fever?

  • Evaluation typically includes a thorough physical examination assessing the baby's overall appearance, activity, and color
  • Blood tests — including a complete blood count and inflammatory markers such as C-reactive protein and procalcitonin — are commonly used to help assess infection risk
  • A urine sample is generally collected to check for urinary tract infection, which is the most common serious bacterial infection in this age group
  • In the youngest babies — especially those under 21 to 28 days — a lumbar puncture (spinal tap — a test of the fluid around the brain and spine) may be performed to check for meningitis
  • Blood and urine cultures are typically sent to identify specific bacteria, with results generally available within 24 to 36 hours
  • Hospital admission for observation and precautionary treatment is common while awaiting culture results
  • The goal is to identify or rule out serious bacterial infection as quickly as possible

See fever context for babies under 2 months

How old is your child?

Select an age group to see age-appropriate context for what you are observing.

Under 2 months: Babies younger than 2 months may show fever through subtle changes in alertness, feeding, or skin color rather than obvious fussiness. A temperature of 100.4°F (38.0°C) or higher in this age group is typically evaluated promptly by a healthcare professional, even when a baby otherwise appears well.

Observation Summary

Age group selected: Under 2 months.

Why Pediatricians Ask About This

Age changes how pediatricians interpret fever and which questions they prioritize. In infants younger than 2 months, a temperature of 100.4°F (38.0°C) or higher is typically evaluated promptly by a healthcare professional, even when a baby otherwise appears well. Recording age ensures home observations are reviewed with the right clinical context.

Other Things Parents Often Notice

  • Date of birth or corrected age if your child was born early
  • Whether this is your child’s first fever or a recurring pattern
  • Recent vaccinations or new medications started
  • Whether your child has a condition your pediatrician already follows

Share this observation with your child's pediatrician.

Back to Fever in Babies and ToddlersView visual guide →

Related Learning

High Temperature ReadingWhat counts as a fever and what causes it in babies and young childrenFever in Babies 3 to 6 MonthsAge-specific fever context for babies 3 to 6 months oldVery Sleepy — Hard to WakeWhat concerning drowsiness alongside fever looks like

Check Your Understanding

Tap the answer that best fits each scenario.

A 3-week-old has a rectal temperature of 100.6°F. The baby is sleeping and appears comfortable. There are no other visible symptoms.

How would you describe this situation?

A 4-week-old has a rectal temperature of 101°F. The baby feels slightly limp when picked up, is feeding less than usual, and has had fewer wet diapers in the last six hours.

What does this combination of signs describe?

A parent of a 6-week-old calls the doctor about a rectal temperature of 100.8°F. The doctor's office asks the parent to describe what they are observing.

Which observations are most useful to describe?

PediaPulse is an independent, visual educational initiative founded by Ebenezer Adebiyi, MD, MPH, FAAP. It is not affiliated with, endorsed by, or representative of the views or clinical practices of any hospital network or medical institution. Dr. Adebiyi's work on PediaPulse is strictly educational, does not constitute the establishment of a doctor-patient relationship, and does not provide medical advice or diagnostic triage. Always consult your child's physician for medical concerns. PediaPulse is a product of ProParenting Pulse LLC.

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References

  1. Pantell RH, Roberts KB, Adams WG, et al. Evaluation and management of well-appearing febrile infants 8 to 60 days old. Pediatrics. 2021;148(2):e2021052228.
  2. Cioffredi LA, Jhaveri R. Evaluation and management of febrile children: a review. JAMA Pediatrics. 2016;170(8):794-800.
Ebenezer Adebiyi, MD, MPH, FAAP

Ebenezer Adebiyi, MD, MPH, FAAP

Board-Certified Pediatrician · FAAP · Founder of PediaPulse

Dr. Adebiyi built PediaPulse to help parents understand what they are observing so they can have better, more informed conversations with their own doctors.