A pediatrician's guide to fever in babies under 3 months. Learn what counts as a fever, when to go to the ER, how to take an accurate rectal temperature, and what to expect from the sepsis workup.
Medically Reviewed by: Dr. Melissa Tribuzio, MD, Board-Certified Pediatrician
Last Updated: May 3, 2026
In a baby under 3 months, a rectal temperature of 100.4°F (38°C) or higher is a medical emergency. Call your pediatrician right away. They may direct you to the ER. If you cannot reach them within a few minutes, go straight to the ER. At this age, fever may be the only sign of a serious infection. A few hours can matter. Do not wait it out. Do not give Tylenol. Have your baby seen by a doctor.
A fever in a baby under 3 months is a rectal temperature of 100.4°F (38°C) or higher. There is no clinical "low-grade fever" category in this age group. The threshold is a single number, and the response is the same every time: urgent evaluation by a doctor.
Pediatricians use the word newborn for babies in the first 28 days of life. For the purposes of this article, we include all babies up to 3 months old, since the urgent approach to fever is the same across this entire window: 100.4°F (38°C) rectal in the first 3 months means call now — especially if your baby is less than 60 days old.
If your baby is under 3 months and has a rectal temperature of 100.4°F (38°C) or higher, call your pediatrician immediately. They may direct you to the ER. If you cannot reach them within a few minutes, go straight to the ER. Do not wait for a callback. Do not retake the temperature an hour later to see if it goes down. Do not give a fever-reducing medication first.
Go to the ER right away — even if the temperature is below 100.4°F — if your baby has any of these warning signs:
For babies in the first 3 months, do not wait for additional warning signs once a fever is present. The fever itself is the warning sign.
A young baby's immune system is still developing. They cannot localize an infection the way an older child can. That means they may not have a cough, a runny nose, or any clear source for the fever. In babies under 3 months, fever can be the only sign of a serious bacterial infection. The most common is a urinary tract infection (UTI). The most serious is meningitis. About 1 in 10 babies with fevers in this age range turns out to have a UTI; bacteremia and meningitis are much rarer (meningitis is fewer than 1 in 2,000).
A young baby cannot tell you they feel sick. The most dangerous infections at this age can look unremarkable on the outside. So pediatricians treat the fever itself as the alarm. The AAP's 2021 clinical practice guideline — "Evaluation and Management of Well-Appearing Febrile Infants 8 to 60 Days Old" — is built around exactly this reality. Even babies who look fine still get evaluated, because the fever may be the only clue.
At Blueberry Pediatrics, we often hear from parents in the first hours after they take their baby's temperature for the first time. Our role on those calls is to confirm the temperature is a true rectal reading and help you decide what to do next. For a baby under 60 days old, that means going to the closest emergency room. For older babies in their first 3 months, we may instead route you to your pediatrician.
For babies in their first 3 months, a rectal temperature is the most accurate method. It is the one the AAP recommends as the basis for any fever decision. Forehead and armpit thermometers can miss a true fever in this age group. They should not be used to rule out an emergency.
Here is how to take a rectal temperature safely:
If your forehead or armpit thermometer reads anywhere near 100°F, retake the temperature rectally before deciding what to do. A reading from another method is a screen, not an answer. One of the most common questions we hear from parents of newborns is whether to trust the forehead thermometer the hospital sent them home with. The honest answer: in the first 3 months, it is not enough to make a fever decision on its own.
When you arrive at the ER, the team will move quickly. They will check your baby's vital signs, maybe place an IV, and begin tests. What happens next depends on your baby's exact age and lab results:
The lumbar puncture is the test parents most often worry about. It is done with a thin needle to draw a small amount of fluid from the lower back. ERs that care for babies do this test regularly and use numbing medicine. It is the most reliable way to rule out meningitis quickly. If the team recommends it, it is because the alternative — missing meningitis at this age — is far worse than the test itself.
Some babies under 3 months with a fever are admitted to the hospital for at least 24 to 48 hours while bacterial cultures finish growing. That waiting period feels long, but it is how the team confirms it is safe to send your baby home.
What temperature is considered a fever in a newborn?
A rectal temperature of 100.4°F (38°C) or higher. There is no "borderline" or "low-grade" fever in this age group. If the rectal reading is 100.4°F or higher, call your pediatrician immediately.
What should I do if my newborn has a fever?
Call your pediatrician right away. They may direct you to the ER. If you cannot reach them within a few minutes, go straight to the ER. Do not give Tylenol or ibuprofen first. Do not wait to see if the temperature drops.
Should I take my newborn to the ER for a fever?
Yes. For any baby under 3 months with a rectal temperature of 100.4°F (38°C) or higher, the answer is the ER. Every time — even if your baby looks fine. Your pediatrician will help you get there quickly and may call ahead.
Can teething cause a fever in a newborn?
No. Teething does not cause a true fever (100.4°F or higher), and most babies do not start teething until around 6 months of age. If your newborn has a temperature of 100.4°F or higher, do not assume teething is the reason. Have your baby evaluated.
How do you take a newborn's temperature?
Use a digital rectal thermometer with a small amount of lubricant. Lay your baby belly-down or with legs tucked up, gently insert the tip about half an inch, and hold it until it beeps. Rectal is the most accurate method for babies in their first 3 months.
Is 99.5°F a fever in a newborn?
A rectal reading of 99.5°F is not a fever. The fever threshold is 100.4°F (38°C) rectal. If the reading came from a forehead or armpit thermometer, retake it rectally — those methods can read low. If your baby seems unwell at any temperature, call your pediatrician.
Why is fever dangerous in newborns?
A young baby's immune system is still developing, and fever may be the only sign of a serious infection like a UTI, bacteremia, or meningitis. About 1 in 10 babies with fever between 8 and 60 days old turns out to have a UTI; bacteremia and meningitis are much rarer (meningitis is fewer than 1 in 2,000). That is why pediatricians treat any fever in this age group as urgent.
Can you give Tylenol to a newborn for a fever?
Not without a doctor's evaluation first. In babies under 3 months, fever is the signal that brings them to medical care. Giving Tylenol can lower the temperature briefly and hide the sign the ER team needs to see. Ibuprofen is not given to babies under 6 months under any circumstances without a doctor's approval. See our baby fever guide for guidance on older infants, and the Tylenol dosing guide for ages 3 months and up.
For a confirmed fever in a baby under 3 months, telehealth is not the right tool. The right tool is the ER. We will say so plainly when you call.
What our pediatricians can do, quickly and at any hour, is help you confirm whether the temperature is a true rectal reading. We can talk you through the next steps. We can stay on the line while you reach your local ER. After the ER visit, we can also help with follow-up calls, with questions about discharge instructions, and with the coming weeks of the newborn stage.
If you are unsure whether your baby has a fever, or you are not sure how to take a rectal temperature, talk to a Blueberry pediatrician. We would rather help you measure once correctly than have you guess.
Children's Hospital of Philadelphia. Febrile Infant — Emergent Evaluation Clinical Pathway. https://www.chop.edu/clinical-pathway/febrile-infant-emergent-evaluation-clinical-pathway
This article is for general parent education and does not replace medical advice from your child's clinician. If your baby is under 3 months and has a rectal temperature of 100.4°F (38°C) or higher, call your pediatrician or 911 immediately.
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