Newborn Fever: When to Call the Doctor, When to Go to the ER, and How to Take a Baby's Temperature

There is a specific number every new parent should know before they leave the hospital: 100.4°F (38°C), taken rectally, in a baby under 3 months old. That is not a fever to "watch and see" — it is a medical emergency, and the guidance is the same across every major pediatric source: call your baby's doctor right away, and if you cannot reach them, go to the emergency room.

The reason is not that the number is dramatic. It is that a newborn's immune system is still catching up, and a small fever can be the only outward sign of an infection that needs urgent evaluation. Older kids get to "ride out" a mild fever at home. Newborns do not. Here is exactly what to do, how to check the temperature correctly, and what warning signs to watch for — with every claim tied to a source you can open yourself.

01What counts as a fever in a newborn

For young babies, the fever line is drawn low and drawn hard. Stanford Medicine Children's Health puts it plainly: "Call your baby's healthcare provider right away if your baby is younger than 3 months old and has a rectal temperature of 100.4°F (38°C) or higher. This is an emergency."

Seattle Children's uses the same threshold — "a fever is a body temperature of over 100.4 degrees F (or 38.0 degrees C)" — and says for babies in this age range to "call right away" for any fever. Nemours KidsHealth is just as blunt: if your baby "is younger than 3 months old with a temperature of 100.4°F (38°C) or higher ... If you can't reach your doctor, go to the emergency room (ER)."

The reason that specific threshold matters is that a low-grade fever in a young infant is treated as a possible early signal of a serious bacterial infection. The American Academy of Pediatrics' 2021 Clinical Practice Guideline on Infant Fever — the guideline your pediatrician is working from — sets fever at "at or above 100.4 degrees Fahrenheit (38 degrees Celsius)," and notes that more than 10% of febrile infants in the 8-to-60-day age range turn out to have a urinary tract infection, with smaller numbers having pneumonia, bloodstream infection, or (very rarely) meningitis. That is why the evaluation is thorough and the phone call is not optional.

02How to take a newborn's temperature

For a baby, the number you get depends on how you get it — and the method that decides an ER trip needs to be the accurate one. University of Rochester Medical Center, citing the AAP, says: "For best results in babies and toddlers up to 3 years of age, the American Academy of Pediatrics advises taking the temperature in the rectum." An armpit (axillary) reading, in the same guidance, "may be used as a first way of checking to see if a child may have a fever. If this shows a fever, the temperature should then be checked by the rectum or forehead."

In other words: an armpit or forehead reading can raise the flag, but for a baby under 3 months, a rectal reading is what your pediatrician will want, and it is the one you should trust before deciding to go in.

How to take a rectal temperature, step by step

03When to call versus when to go straight to the ER

For a baby under 3 months, "call your pediatrician" and "go to the ER" are not two different responses — they are the same response, in order. Call first; if you cannot reach a real person quickly, go. Nemours KidsHealth is explicit on that fallback: "If you can't reach your doctor, go to the emergency room (ER)."

But some warning signs skip the phone call entirely. Nemours KidsHealth says to "go to the ER if your child has a fever and: is crying nonstop or is extremely cranky or fussy; is sluggish and hard to wake up; has purple spots that look like bruises; has blue lips, tongue, or nails; has a stiff neck, severe headache, or bad belly pain; is limp or refuses to move; has trouble breathing; or has a seizure."

Seattle Children's adds to that emergency list: severe breathing difficulties or rapid breathing, seizures, a rash (other than chickenpox), frequent vomiting for more than 24 hours, no urination for 8 or more hours, or a fever above 104°F. In a baby under 3 months, though, you do not need to see any of those to act — the fever itself is enough.

• Call now — or go to the ER if you cannot reach anyone — for any rectal temperature of 100.4°F (38°C) or higher in a baby under 3 months old.
• Go straight to the ER for a baby of any age who has a fever plus trouble breathing, blue lips or nails, a stiff neck, purple spots, a seizure, or is limp / hard to wake.
• Call the same day (per Seattle Children's) for a baby 3 months to 2 years old with a fever that lasts more than 24 hours without other symptoms — or sooner if they seem unwell.

04What NOT to do

The internet is full of well-meaning fever "tricks" that make things worse. The consensus from every source below is short and consistent:

• Do not give a newborn (under about 3 months) any fever medicine without your pediatrician telling you to. In this age range, the pediatrician needs to see the fever — bringing it down at home first can hide the signal they need to make decisions.
• Never give aspirin to a child. Nemours KidsHealth: "never give aspirin to a child" because of the link to Reye syndrome, a rare but serious illness.
• Skip cold baths, ice packs, and alcohol rubs. Nemours: avoid "cold baths or ice packs because they can make kids shiver," and avoid "rubbing alcohol because it can cause poisoning when absorbed through the skin."
• Do not bundle a hot baby. Seattle Children's advises against "excessive clothing or bundling" — heavy layers trap heat in exactly when the body is trying to release it.
• Do not alternate acetaminophen and ibuprofen on your own. Seattle Children's: "Don't alternate between acetaminophen and ibuprofen; choose one and maintain consistent dosing." (For a newborn, ibuprofen is off the table entirely without a doctor — it is not recommended under 6 months.)

05Common reasons a newborn's temperature climbs

It is worth knowing that not every high reading is an infection — though in a baby under 3 months, you still call. Stanford Medicine Children's Health names three common contributors to a raised temperature in a newborn: infection (which "may or may not" produce a fever in a newborn), overheating from too much clothing or too warm a room, and dehydration from not getting enough breastmilk or formula.

That is why one of the first things a pediatrician will sometimes ask is how many layers your baby has on, and what the room temperature is. Stanford recommends keeping the room around 72-75°F and dressing your baby in light layers. But — and this is the point — you do not diagnose "just overheated" from the couch. In a baby this young, the doctor makes that call.

06Signs your baby is dehydrated

Fevers speed up fluid loss, and a dehydrated newborn is its own reason to call. Nemours KidsHealth says to seek attention if your child "seems dehydrated, like urinating (peeing) less, not having tears when crying, or being less alert and less active."

For a newborn specifically, the easiest thing to track is wet diapers. If your baby has had noticeably fewer wet diapers than usual (in the first week or two, most babies have at least six wet diapers a day once feeding is established), or if their mouth looks dry, or the soft spot on top of their head (the fontanel) looks sunken — that is worth a phone call, fever or not.

07What to expect if you go in

It helps to know why the workup for a young baby with a fever looks more intense than it does for an older kid. The AAP's Clinical Practice Guideline — the roadmap for well-appearing infants 8 to 60 days old — organizes evaluation into three age brackets (8-21 days, 22-28 days, and 29-60 days). Depending on the bracket, the workup can include a urine test, a blood test with cultures, inflammatory markers, and — for the youngest babies — a spinal fluid sample. The guideline explicitly aims to balance a thorough evaluation with "avoiding unnecessary tests or hospitalizations" for babies who look well.

Translation: your team is not being alarmist. They are following a national playbook that treats a small fever in a young baby as an important signal — because for the small number of babies with a serious infection, catching it early changes everything.

08Older babies and toddlers, briefly

For an older baby with a fever, the calculus loosens. Seattle Children's says for babies 3 months to 2 years, "contact your doctor if fever persists beyond 24 hours without other symptoms." Beyond that, the guidance shifts toward comfort: offer fluids, dress in light layers, let them rest, and use fever medicine (acetaminophen for babies 3+ months, ibuprofen for 6+ months) only if your child is uncomfortable — not to chase a normal number on the thermometer.

The core rule stays the same at every age: watch how your baby is acting, not just what the thermometer says. A baby who is alert, drinking, and interacting is a very different situation from one who is limp, refusing to feed, or hard to wake — even at the same temperature.

09One more thing before the first cold

The single most useful thing you can do before your baby's first fever ever happens is to have a real digital rectal thermometer in the drawer, know how to use it, and have your pediatrician's after-hours number saved in your phone. That is the whole prep list. If you want the rest of the newborn learning curve — feeding, sleep, diapering, what is normal and what isn't — our newborn basics guide covers it, sourced the same way this page is. And if you are still pregnant, our free week-by-week planner puts the small pieces of prep like this one in the weeks before your baby comes home, so you are not learning them at 2 a.m.

Questions parents actually ask

Newborn Weeks is planning help, not medical advice — always follow your own doctor, midwife, or pediatrician.