If your baby is under 3 months old and has a rectal temperature of 100.4°F (38°C) or higher, call your pediatrician or go to the emergency room right away - even if your baby otherwise seems fine, feeding well, and acting normal. This is standard guidance from the American Academy of Pediatrics, not a judgment call to make on your own at home.
In babies this young, fever is treated as an emergency because it's one of the only early signs a serious infection can give - and a young baby's immune system can't always fight it off the way an older child's can.
Why Fever in Young Babies Is Treated Differently
A newborn's immune system is still immature. Infections that an older baby or toddler would fight off without much trouble can become serious quickly in the first few months of life - and a fever is sometimes the only obvious warning sign. That's why doctors don't wait and see with a young baby's fever the way they might with an older child's.
How to Take Your Baby's Temperature Accurately
For babies under 3 months, a rectal temperature is the most accurate way to check, and it's what pediatricians rely on to make decisions. Forehead strips, ear thermometers, and pacifier thermometers are convenient but less reliable in very young babies, and can miss a real fever.
What Counts as a Fever, by Age
| Age | What to do |
|---|---|
| 0–3 months | Any temperature of 100.4°F (38°C) or higher: call your doctor or go to the ER immediately |
| 3–6 months | 102°F (38.9°C) or higher, or a baby who seems unusually unwell: call your doctor the same day |
| 6+ months | Generally more leeway, but still call if your baby seems very unwell, has a high fever, or a fever lasting more than a day or two |
When in doubt at any age, call. It's always a reasonable reason to check in.
Other Signs to Watch For Alongside Fever
- Feeding poorly or refusing to eat
- Unusually sleepy, limp, or hard to wake
- Fewer wet diapers than usual
- A rash, especially one that doesn't fade when pressed
- Fast or labored breathing
Any of these alongside a fever is a reason to seek care promptly, not wait until morning.
Frequently Asked Questions
What temperature is considered a fever in a newborn?
A rectal temperature of 100.4°F (38°C) or higher is considered a fever at any age.
Is a fever in a newborn always an emergency?
For babies under 3 months, yes - any fever at that threshold warrants an immediate call to your pediatrician or a trip to the ER, even if the baby seems otherwise fine.
What thermometer is most accurate for a newborn?
A rectal thermometer is considered the most accurate for babies under 3 months; forehead and ear thermometers are less reliable at this age.
Sources
This guide was fact-checked against the following guidance on July 14, 2026:
Related Reading
Keep a record for the doctor
Milli lets you log temperatures and other health events, and generate an on-device PDF summary covering feeding, sleep, diapers, growth, health events and vaccines - useful to have on hand whether you're calling the pediatrician or heading to urgent care.