Newborn Temperature - Taking, Reading & When to Worry

Published - March 1 Last Updated: March 1, 2026

 At my daughter's 4-week checkup, I mentioned she "felt warm" that morning. The nurse immediately took her temperature: 100.6°F rectal. Within 2 hours, we were in the ER for a full sepsis workup. 

Turned out to be a virus, but any fever in a baby under 3 months requires immediate evaluation. Here's everything you need to know about taking and interpreting your newborn temperature.

Monitoring temperature is one aspect of newborn care—explore our complete newborn health guide for all health and safety topics.

Normal Newborn Temperature Range

What's Normal (97.5-100.3°F)

The normal newborn temperature range is 97.5-100.3°F (36.4-37.9°C) when measured rectally.

This is the baseline. Anything at or above 100.4°F is considered a fever.

Anything below 97°F is too cold and needs medical attention.

Slight Variations Are Normal

My daughter's temperature ranged from 97.8°F to 99.9°F on different days.

Small fluctuations throughout the day are completely normal. Bodies aren't thermostats set to exactly 98.6°F.

Don't panic over 0.2-0.3-degree changes.

Time of Day Affects Temperature

Body temperature tends to be lowest in the early morning and peaks in the late afternoon or evening.

A reading of 99.8°F at 5 p.m. might be 98.6°F at 6 a.m. the next morning.

This is normal variation.

Activity Level Effects

Crying, eating, being held skin-to-skin, or being bundled can all temporarily raise the temperature slightly.

If the baby feels warm, wait 10-15 minutes in a cool environment, then check the temperature.

Don't take the temperature immediately after a bath or while the baby is crying hard.

Why Temperature Matters in Newborns

Immature Temperature Regulation

Newborn babies haven’t yet developed the ability to control their body temperature effectively. They can't shiver when cold or sweat much when it’s hot. They depend on us to keep them at the right temperature. This is why monitoring newborn temperature matters.

Fever in Newborns Is Always Serious

In babies under 3 months, fever indicates infection until proven otherwise.

Their immune systems are immature. Bacterial infections spread fast. Fever is often the only sign of serious illness.

This is why any fever means an immediate ER visit. No exceptions.

Hypothermia Risk

Being too cold is also dangerous. A temperature below 97°F means the baby isn't warm enough.

Hypothermia affects heart rate, breathing, and blood sugar. It's a medical emergency.

Indicator of Illness

Combine temperature monitoring with recognizing other fever and illness signs.

Temperature is one piece of the puzzle. A sick baby with a normal temperature still needs evaluation.

But a fever in a newborn always requires immediate attention.

How to Take a Baby's Temperature

Rectal (Most Accurate for Newborns)

Rectal temperature is the gold standard for babies under 3 months.

It's the most accurate method. It measures core body temperature.

Yes, it feels awkward at first. You get used to it quickly.

Axillary/Armpit (Less Accurate)

Armpit temperature is less accurate—can be off by a full degree.

It's fine for a quick check, but if you get a high reading, confirm with rectal.

Don't add a degree to armpit readings. Just use rectal for accuracy.

Temporal Artery

Forehead/temporal artery thermometers are convenient but less accurate than rectal. They're fine for older babies, but not reliable enough for newborns when fever means ER. If you get a high reading, confirm rectally.

Ear (Not for Newborns)

Don't use ear thermometers on newborns. Their ear canals are too small for accurate readings. Save ear thermometers for when the baby is 6+ months old.

Which Method When

Under 3 months: Rectal only for accurate fever determination.

3-6 months: Rectal preferred, temporal artery acceptable with confirmation.

6+ months: Rectal, temporal, or ear all acceptable.

For newborns, stick with rectal. It matters.

Taking Rectal Temperature (Step-by-Step)

What You Need

• Digital rectal thermometer (mark it "rectal only")
• Petroleum jelly (like Vaseline)
• Tissues or wipes
• Calm baby (relatively speaking)

Never use old mercury thermometers. If they break, mercury is toxic.

Proper Technique

Step 1 - Wash your hands and clean the thermometer.

Step 2 - Put a small amount of petroleum jelly on the thermometer tip.

Step 3 - Lay the baby on its back on the changing table or lap.

Step 4 - Hold the baby's legs up gently (like diaper change position).

Step 5 - Insert the thermometer gently 0.5-1 inch into the rectum (about half an inch to one inch).

Step 6 - Hold thermometer steady. Keep the baby's legs still.

Step 7 - Wait for the beep (usually 30-60 seconds).

Step 8 - Remove, read, record.

Safety Tips

Never force the thermometer. If you face pushback, pause and approach it from another angle."

Keep a firm but gentle grip on the baby's legs. They'll squirm.

Never leave a baby unattended with a thermometer inserted.

Never insert more than 1 inch.

Lubrication

Use petroleum jelly every time. Make insertion comfortable and safe.

A little goes a long way. Just a small dab on the tip.

How Deep to Insert

For newborns - 0.5 inches (about half an inch)

For older infants - Up to 1 inch

More isn't better. You don't need to insert far to get an accurate reading.

How Long to Wait

Wait for the thermometer to beep. Usually 30-60 seconds.

Don't remove it early. Let’s finish measuring.

Digital thermometers signal when done.

What Is a Fever?

100.4°F or Higher (Rectal)

Fever is defined as 100.4°F (38°C) or higher when taken rectally.

This is the medical definition. Not 100.0°F. Not 100.2°F. Exactly 100.4°F or higher.

Below that is not technically a fever (though still call if baby seems sick).

Age Matters (Under 3 Months = Emergency)

Under 3 months - ANY fever is an automatic ER visit. Period.

3-6 months - Fever needs a same-day doctor’s evaluation.

Over 6 months - Fever is less concerning if the baby is okay, but still call the doctor.

Age makes a huge difference in how seriously a fever is treated.

Fever Without Other Symptoms

Even if the baby seems fine otherwise, a fever in a newborn means ER.

Don't wait for other symptoms. Fever alone is enough.

My daughter was happy, eating well, and alert. I still had to go to the ER because of the fever.

Why Fever in Newborns Is Serious

Newborns can have serious bacterial infections (sepsis, meningitis, UTI) with fever as the only symptom.

These infections can become life-threatening quickly. Hours matter.

This is why the threshold is so low and the response so aggressive.

When to Call a Doctor or Go to the ER

Understand when to call about fever and other emergency symptoms.

Any Fever in a Baby Under 3 Months

If rectal temperature is 100.4°F or higher, go to the ER immediately.

Don't call your pediatrician first. Just go to the ER.

Don't give fever reducers. Don't wait to see if it goes down. Go.

Temperature Over 100.4°F

This is the magic number. At or above this temperature, it's ER time.

Even 100.5°F. Even if the baby seems fine. Go.

Temperature Under 97°F

Too cold is also dangerous. Those below 97°F need medical evaluation.

This can indicate hypothermia, infection, or other serious issues.

Baby Acting Sick with Lower Temp

If the temperature is 99.5°F but the baby is lethargic, won't eat, or seems very sick, call the doctor immediately.

Trust your instincts. A sick baby with "normal" temperature still needs evaluation.

What to Expect at ER (Sepsis Workup)

When you go to the ER with newborn fever, they will do:

• Blood tests
• Urine culture (via catheter)
• Sometimes a spinal tap (to rule out meningitis)
• Often admission for 48-72 hours while cultures grow

This seems extreme, but it's necessary. They're ruling out life-threatening infections.

Keeping Baby at the Right Temperature

Maintain proper room temperature for sleep to prevent overheating.

Room Temperature (68-72°F)

Keep the baby's room 68-72°F for sleeping.

This range prevents both overheating (SIDS risk) and getting too cold.

Use the room thermometer to verify. Don't guess.

Dressing Baby Appropriately

General rule - Dress the baby in one more layer than you're wearing.

If you're comfortable in a T-shirt, baby wears a pair of jeans plus a sleep sack.

If you're in a sweater, baby wears a onesie, a sleeper, and a sleep sack.

Signs of Overheating

Check if the baby is too hot -

• Sweaty, damp hair
• Hot, flushed face
• Red cheeks
• Fast breathing
• Restless

If overheated, remove a layer and let the baby cool down.

Signs of Being Too Cold

Check if the baby is too cold:

• Cool hands and feet (these are often cool normally)
• Cool chest or back of neck
• Skin looks pale or bluish
• Lethargic

Add a layer if the baby seems cold.

Touch Test (Back of Neck)

The best way to check if a baby is comfortable: touch the back of their neck.

It should feel warm but not sweaty or hot.

Hands and feet are often cooler—that's normal. Check neck or chest instead.

Temperature regulation safety is part of overall newborn safety.

Common Temperature Mistakes

Adding Degree to Armpit Reading

Don't "add a degree" to armpit temperature and call it rectal equivalent.

Armpit readings are inherently less accurate. Math doesn't make them accurate.

If you need an accurate temperature, take it rectally.

Taking Temperature Right After Bath

Bath warms the baby up temporarily. The temperature taken immediately after the bath will be elevated.

Wait 15-20 minutes after bath before checking temperature.

The same goes for taking the baby's temperature while the baby is crying hard or right after nursing.

Bundling Too Much

Overdressing can cause the temperature to rise slightly.

If the baby feels warm, unbundle and wait 10-15 minutes before taking the temperature.

True fever won't go away by removing clothes. Overheating will.

Using Old Mercury Thermometers

Never use old glass mercury thermometers. If they break, mercury is toxic.

Use digital thermometers only. They're accurate, safe, and inexpensive.

Ear Thermometers on Newborns

Ear thermometers aren't accurate for newborns. The ear canals are too small.

Don't rely on ear readings for babies under 6 months.

After Vaccinations

Learn what to expect with fever after vaccinations.

Low-Grade Fever Is Common

Slight temperature elevation (99-100.3°F) after vaccines is normal.

It's the immune system responding. Usually 12-24 hours after shots.

This is expected and not concerning.

When to Worry

Even after vaccines, a temperature of 100.4°F or higher in a baby under 3 months needs evaluation.

Call your pediatrician. They'll advise whether ER is needed or if they can see the baby in the office.

Don't assume high fever is "just from vaccines."

Fever Reducers (Only If Doctor Says)

Don't give acetaminophen (Tylenol) or ibuprofen without a doctor's approval.

For babies under 3 months, call before giving any medication.

The doctor may advise giving acetaminophen for comfort, but ask first.

Choosing a Thermometer

Digital Rectal for Newborns

Buy a basic digital thermometer labelled for rectal use.

Mark it clearly "RECTAL ONLY" so you don't use it orally later.

They're inexpensive ($10-15) and accurate.

Multi-Use Thermometers

Some thermometers have different tips for rectal, oral, and armpit use.

These work fine. Just make sure you're using the right setting/tip.

Features to Look For

• Fast reading (60 seconds or less)
• Flexible tip (safer, more comfortable)
• Fever alarm (beeps differently if fever is detected)
• Memory function (recalls the last reading)
• Easy-to-read display

When to Replace

Replace thermometer if -

• It gives inconsistent readings
• The battery won't hold a charge
• Displaying is hard to read
• It's been dropped or damaged

Keep a backup on hand.

Frequently Asked Questions

Q: How do I take a rectal temperature safely?

A: Use a digital rectal thermometer with petroleum jelly. Lay the baby on its back, lift its legs. Insert the thermometer 0.5-1 inch gently. Wait for the beep. Clean the thermometer after. Mark, it "rectal only."

Q: Can I use a forehead thermometer on my newborn?

A: Temporal artery (forehead) thermometers can be used but are less accurate than rectal. For fever determination in babies under 3 months, verify with rectal reading.

Q: What if my baby's temperature is 100.3°F—almost a fever?

A: Under 100.4°F isn't technically a fever, but if the baby seems sick or the temperature is rising, call the doctor. Trust your instincts. “Almost fever" with illness symptoms warrants a call.

Q: Should I take my baby's temperature daily?

A: No need unless the baby seems sick. Taking the baby's temperature when the baby feels warm or acts ill is sufficient. Obsessive checking can increase anxiety unnecessarily.

Q: Can teething cause a fever?

A: Teething may cause slight temperature elevation (under 100°F), but doesn't cause true fever. If baby has fever, there's another cause—don't blame teething and miss a real illness.

Unsure when the temperature requires a call? Read our guide on when to call your pediatrician.

Reference 

American Academy of Pediatrics –

https://www.healthychildren.org/English/health- issues/conditions/fever/Pages/default.aspx/

nhs.uk -

 https://www.nhs.uk/baby/health/how-to-take-your-babys-temperature/

National Institutes of Health (NIH) | (.gov) https://pmc.ncbi.nlm.nih.gov/articles/PMC2819919/