Use a digital rectal thermometer; 100.4°F (38°C) or higher means fever in a newborn—call your pediatrician now.
What Counts As A Fever In A Newborn
For babies under 3 months, a rectal temperature of 100.4°F (38°C) or higher equals fever. Ear and forehead tools aren’t reliable in babies this young. If your baby is younger than 3 months and you see 100.4°F or more, contact a doctor now. See the AAP’s guide to taking a child’s temperature for methods that match each age.
Temperature Methods And What They Mean
| Method | Use Under 3 Months? | Fever Level |
|---|---|---|
| Rectal (digital) | Yes, preferred | 100.4°F (38°C) or higher |
| Underarm (axillary) | Only if rectal isn’t possible | 99.0°F (37.2°C) or higher |
| Forehead/Ear | Not reliable in this age | Confirm with a rectal reading |
How To Tell If A Newborn Has A Fever (Step-By-Step)
1) Watch Behavior First
Is your baby feeding less, sleeping more, or extra fussy? Those clues support a reading, but don’t prove a fever.
2) Get The Right Tool
Choose a digital thermometer set aside for rectal use only. Keep a tiny tub of petroleum jelly or water-based lube, tissues, and a pen.
3) Set Up
Wash your hands. Clean the tip with alcohol or soap and water; rinse and dry. Place your baby on the back with hips supported, or tummy down across your lap.
4) Take A Rectal Temperature Safely
Apply a pea-size dab of lube to the tip. Gently insert the tip 1/2 inch to 1 inch (1–2.5 cm), no farther. Keep one hand steady on the thighs or ankles. Hold the thermometer in place until it beeps.
5) Read And Record
Write the exact number, the method (rectal), and the time. If it’s 100.4°F (38°C) or more in a baby under 3 months, call now. If it’s close and your baby seems unwell, recheck in 10–15 minutes after removing extra layers.
Rectal Temperature: Safe How-To
Use a dedicated rectal thermometer so you don’t mix it with oral use. Insert gently; don’t force. If you feel resistance, stop. After the reading, clean the tip with alcohol or hot soapy water, rinse, dry, and cap it. Label the device “RECTAL” with tape or a marker. The NHS page on fevers in children also explains home checks and care.
Common Signs That Support The Reading
A thermometer gives the answer. Still, these signs can point you toward checking:
- Warm chest or back to the touch
- Flushed cheeks or sweating
- Irritable or hard to settle
- Feeding poorly or fewer wet diapers
- Sleepier than usual
None of these prove a fever on their own. Always take a temperature to be sure.
When To Seek Care Or Go Now
Call your baby’s doctor right away for any rectal temp of 100.4°F (38°C) or higher in a baby under 3 months. Seek urgent care now if any of these show up with or without a fever:
- Fast or difficult breathing, grunting, or a pause in breathing
- Blue lips or a pale or mottled look
- Stiff neck, weak cry, or not waking as usual
- Bulging soft spot, a seizure, or repeated vomiting
- A widespread rash, or a rash that doesn’t fade when you press a glass on it
- Signs of dehydration: few wet diapers, dry mouth, no tears
What You Can Do At Home While You Call
Keep layers light. Open a window or use a fan across the room for air flow. Offer breast milk or formula more often; small, steady sips keep babies hydrated. Skip cold baths or rubbing alcohol. Those chill the skin but don’t help the body. Never give aspirin. Don’t give acetaminophen in babies under 3 months unless your clinician tells you to. Don’t give ibuprofen under 6 months.
Feeding And Hydration Tips
Newborns take tiny volumes. Offer frequent breastfeeds or small bottles rather than large spaced feeds. Watch diapers: six or more wets in 24 hours after the first week is a good sign. If latching is tough while sick, hand-express a few drops to start the flow, then try again. A slow-flow nipple can help bottle-fed babies coordinate suck and swallow when they feel off their game.
Dressing And Room Setup
Choose a single layer of cotton with a light sleep sack. Skip hats indoors unless your clinician advised one. Set the room around 20–22°C (68–72°F). Use a fan or open window for air movement, not a draft. Hold your baby skin-to-skin if they’re unsettled; your chest helps steady temperature and breathing.
If The Number Is High But Baby Seems Okay
Call anyway if under 3 months. While you wait, keep feeding, keep layers light, and watch behavior. If the number climbs or new signs arrive, head in.
If The Number Is Borderline Or Low
Recheck after 10–15 minutes with fewer clothes. If hands and feet feel cool, warm your baby on your chest for a few minutes and retest. Sudden temperature drops can be a sign of illness too; call if your newborn seems unwell even without a fever.
Urgent Signs And Next Steps
Use this quick table to match a red flag with the safest action.
Red Flags And Actions
| Sign | What You’ll See | What To Do |
|---|---|---|
| Breathing trouble | Fast breaths, ribs pulling in, grunting | Call emergency services |
| Poor feeding/dehydration | Few wet diapers, no tears, dry mouth | Call now; go in for evaluation |
| Seizure or hard to wake | Shaking or limp, not responding | Call emergency services |
What Causes Fever In Newborns
Most fevers in this age come from infections. Viruses lead the list. Urinary tract infections can show up with fever as the only sign. Less often, bacteria in the blood, lungs, or brain can be the cause, which is why newborn fevers get prompt checks. Vaccines given at 2 months can bring a mild rise in temperature later that day. Because this age is fragile, report 100.4°F (38°C) or higher even after shots unless your clinician gave different instructions.
Touch Isn’t Enough: Why Thermometers Matter
Hands can’t feel a one-degree rise. Skin gets warm from crying or cuddling and cool from a breeze or a wet swab. A digital thermometer is quick and gives a number you and your doctor can use.
Picking A Thermometer You’ll Use
Choose a digital model with a flexible tip. Skip glass mercury models. If you keep a forehead scanner at home for older kids, that’s fine for them, but confirm newborn readings rectally. Store the baby’s rectal thermometer in a small zip bag with alcohol wipes and a tiny lube packet so it’s ready.
How To Calm A Wiggly Baby During A Check
Pick a quiet spot. Swaddle the upper body with the arms free or let your baby suck on a finger or pacifier. Talk in a soft voice. A second pair of hands can steady the hips and legs. The reading takes seconds.
Quick Reference: Age And Action
- 0–28 days: Rectal temp 100.4°F (38°C) or higher? Contact a doctor now or go to the nearest emergency department.
- 29–90 days: Same cutoff; call your baby’s clinician now for advice and next steps.
- Over 3 months: Follow your clinician’s guidance; focus on comfort, hydration, and behavior.
Newborn Fever Myths To Skip
- “Teething causes high fevers.” Not in this age. Teeth don’t bring big spikes.
- “A cold bath brings the number down fast.” It causes shivering and stress. Go with light layers and room-temp air.
- “More blankets will sweat it out.” Overheating raises skin heat and makes babies miserable.
- “Touch can tell.” Only a thermometer tells you when to call.
Hygiene And Storage Tips
Clean the tip before and after each use. Alcohol wipes work well. Label one digital thermometer for rectal use only so it never ends up in a mouth by mistake. Keep spare batteries on hand. Store the kit where you change diapers so checks fit into the routine.
What To Expect At The Clinic Or ER
Staff will repeat the temperature and check breathing, heart rate, and hydration. They may run tests on urine, blood, or spinal fluid. Many babies this age go home the same day if the exam and tests look reassuring. Some need antibiotics in the hospital. The team will guide you through steps and answer questions.
After The Visit: Home Monitoring
Keep a log at home. Write temperatures, times, the method you used, feeds, and diaper counts. Give medicine only if your clinician told you to and follow the dose on the label. Stick with safe sleep on the back, in a bare crib. Set a follow-up plan before you leave the clinic so you know whom to call, where to go, and what to watch over the next day or two.
Caring For Yourself While You Care For Baby
Fever in a tiny baby brings stress. Call a trusted friend for a ride or a hand with a diaper bag. Bring a charger, diapers, and a blanket to keep your baby warm during waits.
Plain Facts To Act On
- Use a digital rectal thermometer for newborns.
- 100.4°F (38°C) or higher equals fever in this age.
- Call your baby’s clinician right away for any newborn fever.
- Light layers, feeds, and calm cuddles help while you get care.