Use a digital rectal thermometer for the most accurate newborn reading; call your pediatrician for 100.4°F (38°C) or higher.
Newborns can run warm during feeds or cuddles, and that’s normal. What you need is a reliable way to tell when a number signals trouble. This guide keeps things simple: which thermometer to use, how to get a safe reading, and what to do with the result.
What Counts As A Fever In Newborns
For babies under three months, a rectal reading of 100.4°F (38°C) or more is a fever that warrants a call to your pediatrician. Axillary and forehead readings run lower than rectal, so treat higher numbers on those methods as a prompt to confirm rectally. Daytime swings of half a degree are common, so look at the method and the whole picture, not a single blip.
Thermometer Methods For 0–3 Months
| Method | Use For | Notes |
|---|---|---|
| Rectal (digital) | Best accuracy; go-to for newborns | Use water-based lube; insert 1/2–1 inch; keep hand on baby. |
| Axillary / Armpit | Quick screen | If ≥99°F (37.2°C), confirm with a rectal reading. |
| Forehead / Temporal | Helpful screen | Quality devices help; confirm rectally for ≥100.4°F. |
| Ear / Tympanic | Not ideal under 6 months | Can read off in tiny ear canals. |
| Oral | Not for newborns | Use later when a child can hold the probe under the tongue. |
| Glass mercury | Never | Avoid for safety and accuracy. |
Checking A Newborn’s Temperature At Home: Safe Method
Rectal thermometers read closest to core temperature in this age group. Set up before you start so the whole process takes under a minute and feels calm for you and your baby.
Prep In 30 Seconds
- Pick a clean digital thermometer labeled for rectal use only.
- Wash your hands and the probe tip; dry the tip.
- Apply a pea-size dab of petroleum jelly or water-based lube.
- Lay baby face up on a firm surface, hold the ankles with one hand, and steady the hips with the other.
Step-By-Step: Rectal Reading
- Turn on the thermometer.
- Place the tip just inside the anus, about 1/2 to 1 inch.
- Hold the thermometer between two fingers while your palm rests on baby’s bottom. Keep baby still.
- Wait for the beep, then remove and read.
- Clean the tip with soap and water or alcohol, rinse, and dry.
If You Need A Quick Screen First
An armpit or forehead scan can help you decide whether to proceed to a rectal check. With an armpit reading, dry the skin, place the tip deep in the pit, lower the arm snugly, and wait for the signal. If the armpit number is 99°F (37.2°C) or higher, do a rectal confirmation. Forehead readers work best on a calm baby in a room without drafts; wipe sweat if present.
When you want the official thresholds and red flags, see the guidance from AAP HealthyChildren and the NHS page on how to take your baby’s temperature.
When The Number Means Action
Call your pediatrician for any rectal reading at or above 100.4°F (38°C) in a baby under three months, even if the baby looks fine. Call sooner if the baby looks unwell, feeds poorly, is hard to wake, breathes fast, has a new rash, or you notice fewer wet diapers. If the number keeps climbing or you feel alarmed, seek urgent care.
While You Wait For Advice
- Keep baby lightly dressed. Skip heavy blankets.
- Offer breast milk or formula more often to prevent dryness.
- Use a fan for gentle air movement, not direct cooling.
- Avoid cold baths and ice packs.
- Never give aspirin to a child.
Safe Gear And Clean Technique
Pick a simple digital model with a flexible tip and a backlight for night checks. Label it “rectal” with tape so it never goes in the mouth. Store it in a clean case. Wash the tip with soap and warm water after each use; allow it to dry before putting the cap back on. Replace batteries when startup icons look faint.
Positioning Tips That Help
Many parents prefer the diaper-change setup: knees gently toward the chest, hips steady, your forearm across the thighs for control. For a baby who startles, try a side-lying position with knees tucked and a hand at the hip. Talk or hum during the check; your voice settles the moment.
Common Mistakes To Avoid
- Using a forehead or ear device as the only method for a young infant.
- Taking a reading right after a warm bath or long cuddle; wait 15 minutes.
- Switching methods across readings when you’re watching a trend.
- Inserting the probe too shallow or too deep; aim for 1/2–1 inch.
- Reading while the baby wails or kicks; movement can skew numbers.
- Keeping a glass mercury thermometer at home.
Smart Tracking And Comfort Care
Write down the time, method, and number each time you check. Note feeding, wet diapers, and any medicines given. Share that log during calls or visits. Most fevers come from viral colds; your job is fluids and comfort while you watch your baby’s energy and breathing. Room temp water on a washcloth for a gentle wipe-down feels good; skip alcohol rubs.
Quick Reference: Readings And What To Do
| Reading | What It Suggests | What Parents Can Do |
|---|---|---|
| Rectal 97.7–99.5°F (36.5–37.5°C) | Usual range for many babies | No action; check again only if baby seems off. |
| Rectal 99.6–100.3°F (37.6–37.9°C) | Borderline | Recheck in 15–30 min; keep baby lightly dressed. |
| Rectal ≥100.4°F (38°C) | Fever | Call your pediatrician now. |
| Armpit ≥99°F (37.2°C) | Screen suggests fever | Confirm with a rectal reading. |
| Forehead ≥100.4°F (38°C) | Possible fever | Confirm with a rectal reading; act on the rectal number. |
Medication Notes
For babies under three months, do not give fever reducers unless your clinician tells you to do so. If medicine is advised later, use the exact dose from your clinician based on weight, not age, and use the syringe that came with the bottle. Call if a dose is spat out or vomited.
How Often To Recheck
If your baby felt warm and the first reading was normal, wait 3–4 hours unless new symptoms show up. If you got a borderline number, repeat the same method in 15–30 minutes while the baby is resting. After a dose of fever reducer given with medical advice, wait at least an hour before rechecking. Do not wake a baby who is sleeping comfortably just to take a number; wake for poor feeding, fast breathing, or if your clinician asked you to monitor overnight.
Picking A Thermometer That Works
A basic digital model with a short, flexible tip and a clear beep is enough. A backlight helps during night feeds. A waterproof body makes cleaning easier. Some models store the last number; that snapshot helps during phone calls. A quiet mode is handy for light sleepers. If your device uses coin batteries, keep a spare out of reach of children.
Label One Thermometer For Babies
Set aside a dedicated unit just for rectal use and mark it with tape. Keep it with your diaper supplies so you are not searching when you need it. If a caregiver visits, show them where it lives and how you use it.
Normal Swings Through The Day
Most babies sit somewhere between 97.7 and 99.5°F during the day when checked rectally. Numbers drift up in the late afternoon and evening. Feeding, crying, and swaddling can push the number up a bit for a short stretch. Teething can cause fussiness and drool, yet it should not push a rectal reading to 100.4°F or higher. When you see a higher number after a bath or contact nap, let the baby cool off for 15 minutes and check again using the same method.
Feeding And Hydration Clues
Diapers and behavior often tell the story. Past the first week, aim for about six or more wet diapers in 24 hours. Watch for a dry mouth, fewer tears when crying, sunken soft spot, or sleepiness that does not lift after a feed. If those show up with a fever, you need a same-day call.
Myths And Handy Facts
- Teething does not cause high fever. Look for another reason if the rectal number hits 100.4°F.
- Bundling raises skin warmth but not a true fever; a rectal check sorts it out.
- Ear devices are better later; tiny canals make readings less reliable in young infants.
- Old forehead strips are not accurate; use digital readers only.
- Vaccines can bring a mild fever for a day or two; call if the number hits 100.4°F or your baby seems unwell.
When Not To Take A Reading
Numbers right after a warm bath, car seat nap in a hot car, or time by a heater can be misleading. Wait 15–30 minutes and try again. If your baby just finished a long cry, soothe first and recheck later. Switching between armpit, forehead, and rectal during the same hour makes trends hard to read; pick one method for that stretch.
Newborn Temperature Checklist
Before You Start
- Calm space, good light.
- Digital thermometer, labeled for rectal use.
- Small amount of lube and wipes.
During The Check
- Steady hips; insert 1/2–1 inch.
- Hold steady until the beep.
- Read, clean, and log.
After The Check
- Rectal ≥100.4°F (38°C)? Call now.
- Baby looks unwell at any number? Seek help.
- Unsure about the reading? Repeat rectally in 10–15 minutes.
You know your baby best; if something feels wrong, seek care. Trust calm instincts too tonight.