What it is
100.4°F rectal, under three months old: go in now. That is the whole rule. Call 911 if the baby is also limp, blue, or hard to wake. Otherwise go straight to an emergency room, not urgent care, and call the pediatrician on the way.
Why so strict? A newborn's immune system cannot wall off an infection. A bladder, blood, or brain infection can look like a bit warm and sleepy for hours. About 1 in 10 babies under two months with a fever has a serious bacterial infection. Doctors cannot tell which baby by looking, so they test. The younger the baby, the more tests: urine, blood, and under 28 days usually a spinal tap and a hospital stay.
The number comes from a rectal thermometer. Under three months, that is the only reading that counts. Forehead and underarm readings miss fevers and should not reassure you.
What's typical
A baby brought in fast usually does well. Most spend one to two days in the hospital on antibiotics while cultures grow, then go home. Many turn out to have a virus and nothing more. That is not a wasted trip; it is the system working.
A newborn's normal rectal range is about 97.7 to 100.3°F. A temperature under 97.7°F (36.5°C) that stays low after a warm blanket and skin-to-skin is also a reason to go in. Low temperature can be a sign of infection too.
After two-month vaccines, a low fever in the next 24 to 48 hours is common. Still call the pediatrician. They will tell you whether this one can be watched at home.
What's known
The AAP's 2021 guideline on febrile infants 8 to 60 days old sets out the tests and the thresholds. Emergency teams ask that you not give fever medicine before you come; it hides the number they need. Blankets do not cause a true rectal fever.
What helps
- Buy a rectal thermometer before the birth and learn to use it: a dab of petroleum jelly, half an inch in, baby on her back with legs up.
- Note the number and the time.
- Keep a card in the diaper bag with the baby's birth details. ER staff will ask about the birth, GBS, and feeding.
- Ask a partner or friend to drive. You should not be driving while scared.
When to call someone
- Nowrectal temperature of 100.4°F (38°C) or higher under 3 months. Go to the ER. Call 911 if the baby is limp, blue, having trouble breathing, or will not wake.
- Nowrectal temperature under 97.7°F (36.5°C) that stays low after warming, in a baby who is sleepy or feeding poorly.
- Todaya baby who feels warm but reads under 100.4°F and is feeding, peeing, and acting normal. Call the pediatrician and recheck in an hour.
- At your next visitask what thermometer they want you to use and what to do after shots.
Who helps
Tools: Postpartum Prep Checklist · Birthworker Directory
Related
- When to call the pediatrician (and when to go in)Baby Basics
- Newborn breathing, congestion, and the noseBaby Basics
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Sources
- American Academy of Pediatrics — Clinical practice guideline: evaluation and management of well-appearing febrile infants 8 to 60 days old (2021)
- American Academy of Pediatrics, HealthyChildren.org — Fever and your baby
- NHS — Fever in babies and children
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