Newborn temperature: how to check, what is normal, when to call

Newborn fevers are different from older-baby fevers. Until 3 months, a fever can be the only outward sign of a serious bacterial infection — meningitis, urinary tract infection, sepsis. The immune system is too immature to mount the usual signs (cough, runny nose, redness at an infection site). A baby under 3 months with a fever gets a full workup at the ER, not a wait-and-see at home.

This is the one rule every parent of a newborn should memorize cold. Everything else in this guide is about getting the number right.

The 100.4°F rule (non-negotiable)

The AAP and every pediatric emergency-medicine guideline in the US agree: any rectal temperature of 100.4°F (38.0°C) or higher in a baby under 3 months of age requires immediate medical evaluation. Not "wait until morning." Not "give Tylenol and see." Immediate.

The reason: roughly 7 to 12 percent of febrile babies under 3 months have a serious bacterial infection. The exam alone cannot rule them out. The workup typically includes a urine culture, blood cultures, and sometimes a lumbar puncture. Most of the time the baby is fine and goes home. The point of the workup is to find the small minority who are not fine, fast, before the infection progresses.

From 3 to 6 months, the threshold is still 100.4°F, but the workup is more individualized — your pediatrician will look at the baby and decide. After 6 months, fevers are managed more like an older child's.

Rectal is the only reliable method under 3 months

Every other method has known accuracy problems in newborns:

• Forehead (temporal artery) thermometers — convenient, but consistently underestimate fevers in babies under 3 months by 0.5 to 1.0°F. Useful for tracking trends in an older baby with a confirmed illness; not reliable enough to rule out a fever in a newborn.
• Ear (tympanic) thermometers — not recommended under 6 months. The newborn ear canal is too small and curved for accurate readings.
• Underarm (axillary) thermometers — generally underestimate by 0.5 to 1.0°F and are sensitive to clothing and ambient temperature. Acceptable as a screen — if axillary reads 99°F or higher, confirm with rectal — but not as a primary measurement.
• Pacifier thermometers — convenient but require 3+ minutes of consistent suck, which most fussy babies will not do.

A digital rectal thermometer is the standard. A reading takes 30 to 60 seconds and is uncomfortable but not painful for the baby.

How to take a rectal temperature

1. Wash the thermometer tip with soap and water; dry it.
2. Coat the tip with a small amount of petroleum jelly.
3. Lay the baby across your lap or on a changing pad, face-down or on the side.
4. Spread the buttocks gently with one hand. With the other, insert the thermometer tip 0.5 to 1 inch into the rectum. Stop the moment you feel resistance — do not push past it.
5. Hold it in place. Most digital thermometers beep in 30 to 60 seconds. Hold the baby's bottom closed around the thermometer to keep it from sliding out.
6. Remove, read, and clean. Note the time of day and the number.

If you are uncomfortable with rectal — most parents are the first time — practice once before you have any reason to need it. The technique is the kind of thing that goes much faster after the first time.

Normal range (and the surprising lows)

Normal newborn rectal temperature: 97.5 to 99.5°F (36.4 to 37.5°C). Babies run slightly warmer than adults — what would be a low-grade fever in an adult (99°F) is solidly in the normal range for a newborn.

Low temperatures matter too. Hypothermia is a sign of serious infection or cold stress in newborns. Anything under 97.0°F (36.1°C) rectally is worth a call, especially if the baby seems unwell or has been hard to warm up.

Things that can falsely raise (or lower) a reading

Before you panic about a borderline number, check:

• Overdressing. A newborn in a fleece sleep sack in an 80°F room can run 0.5 to 1.0°F warmer than usual. Strip down to a diaper, wait 20 minutes, retake.
• Recent feed. Active feeding can elevate temperature briefly. Retake 30 minutes after a feed for the most accurate read.
• Crying for an extended period. Vigorous crying for 20+ minutes can transiently raise temperature. Wait until the baby is calm, then retake.
• Room temperature. A baby just brought in from cold weather may read low; one just out of a warm car seat may read high. Wait 20 to 30 minutes in normal room temperature before measuring.

None of these adjustments matters if the rectal reading is 100.4°F or higher. That number always gets the same response: call. The adjustments matter for the 99°F-to-100.3°F gray zone.

What the ER will do

If you do end up at the ER with a febrile newborn, this is roughly what happens:

1. Triage and exam. Vitals, full physical, history of feeding, sleeping, peeing, pooping. Decisions are made quickly because febrile infants are triaged high.
2. Urine culture. Often by catheter — the only reliable way to get a clean sample at this age. UTIs are one of the most common causes of fever in newborns and the most easily missed without a culture.
3. Blood cultures and CBC. To screen for bloodstream infection and look at the white blood cell count.
4. Sometimes a lumbar puncture. Done if the workup suggests possible meningitis or if the baby is under 4 to 6 weeks. The procedure is safe and the spinal fluid analysis is the only way to rule out meningitis.
5. Sometimes a chest X-ray. If respiratory symptoms are present.
6. Admission for IV antibiotics while cultures cook (24 to 48 hours), in younger babies or babies with concerning findings.

Most babies go home within 48 hours with negative cultures and a viral diagnosis. The ER visit feels like a lot for a baby who turns out to be fine — and that is the point. The workup exists to catch the small minority of cases that would deteriorate without intervention.

Do not give fever reducers without instruction

Acetaminophen (Tylenol) is generally not given to babies under 3 months without a pediatrician's direction. Two reasons: it can mask a worsening fever and delay diagnosis; and dosing in newborns is weight-based and easy to get wrong. Ibuprofen (Advil, Motrin) is contraindicated under 6 months.

If your pediatrician or ER doctor instructs you to give acetaminophen, they will give a specific weight-based dose. Do not guess.

When the number is not the issue

A baby with a normal temperature can still need urgent attention if they are:

• Lethargic, limp, hard to wake
• Refusing feeds for 4+ hours
• Breathing faster than 60 breaths per minute at rest, with retractions or grunting
• Bulging fontanelle
• High-pitched, inconsolable crying that is unusual for them
• Color change to blue around the lips or extreme pallor

The full red-flags list lives in the newborn pillar. Memorize the short version. If anything on that list is happening, you do not wait for the thermometer reading — you call.

Based on the AAP 2021 febrile infant clinical practice guideline. The 100.4°F threshold is non-negotiable in babies under 3 months. When in doubt, call.