Newborn Fever Checker

For a baby under 3 months old, treat it as one. The firm rule used by pediatricians is that a rectal temperature of 100.4°F (38°C) or higher in an infant under 3 months (12 weeks) needs medical care right away — call your baby's doctor immediately or go to the emergency room, day or night. The reason is not that every young baby with a fever is gravely ill, but that at this age a fever can be the only outward sign of a serious infection, and a newborn's immune system and reserves are limited, so problems can move fast. Because you can't safely tell a harmless fever from a dangerous one at home in a baby this young, the standard is to have the baby evaluated rather than watch and wait. Do not give fever medicine to bring the number down and hold off on calling — that can mask exactly the sign the doctor needs to act on.

For a baby under 3 months, a rectal temperature is the accurate, standard method — it's the reading the 100.4°F (38°C) rule is based on, and forehead or ear devices can miss or misjudge a fever at this age. To take one: use a digital thermometer labeled for rectal use, put a little petroleum jelly on the tip, lay your baby on their back or tummy, gently insert the tip about half an inch to an inch (until the metal tip is inside), hold it in place until it beeps, and never force it. Clean it afterward and keep a rectal thermometer separate from any oral one. An armpit (axillary) reading can be used as a quick screen, but if it suggests a fever in a young baby, confirm it rectally — or simply call, because a young baby who seems unwell warrants a call even with a borderline number. For older babies and toddlers, rectal, temporal-artery (forehead), or ear thermometers can all be used; whatever you use, tell your doctor which method and what the reading was.

Because young infants don't mount and show illness the way older children do. A newborn's immune system is still developing, so an infection that would produce obvious symptoms in an older child can, in a baby, show up as very little — a slightly raised temperature, feeding less, being unusually sleepy or fussy, or a change a parent can only describe as "not themselves." Some seriously ill newborns even run a low temperature rather than a high one. That's why the guidance for this age doesn't wait for a dramatic fever: any temperature of 100.4°F (38°C) or higher gets an immediate evaluation, and a baby under 3 months who is feeding poorly, hard to wake, breathing oddly, or simply not right should be seen even if the thermometer looks reassuring. Trust changes in how your baby is acting as much as the number.

In a baby older than about 3 months who is alert, drinking or feeding, and comforts when the fever eases, a mild fever is usually the body's normal response to a passing viral illness, and the priority is comfort rather than chasing the number down to normal. Keep them hydrated with regular feeds or fluids, dress them lightly, and let them rest. Fever-reducing medicine (acetaminophen, or ibuprofen for babies 6 months and older) can be used for discomfort, but the dose depends on your baby's weight and age, so confirm it with your pediatrician or pharmacist — and never give any fever medicine to a baby under 3 months without talking to the doctor first. Call the pediatrician if the fever climbs above 104°F (40°C), lasts more than about 3 days, keeps returning, or if your baby starts to seem unwell, stops drinking, is hard to wake, or shows any danger sign. When you're unsure, a call is always reasonable — this tool helps you gauge urgency, but your pediatrician makes the call.

Some signs mean get care now regardless of how old your child is or exactly how high the fever is: trouble breathing or fast, labored, or pausing breathing; being very hard to wake, limp, floppy, or not responding normally; a seizure; a stiff neck, a bulging soft spot, or a severe headache; a rash of tiny red or purple spots that does not fade when you press a glass or finger against it (this can signal a dangerous bloodstream infection); signs of dehydration such as no wet diaper for 6 or more hours, a dry mouth, a sunken soft spot, or fewer or no tears in a baby old enough to make them (many newborns don't produce tears until around 1 to 3 months, so its absence means less in the youngest babies); pale, blue, grey, or mottled skin or lips; a fever above 104°F (40°C); or a fever that lasts more than about 3 days. Any of these is a reason to call 911 or go to the emergency room, or to contact your pediatrician urgently. A fever is the body fighting infection — but these signs suggest the illness itself needs immediate attention, and infection in a child can occasionally progress to sepsis, which is a medical emergency.

Yes. This checker runs entirely in your browser: the age and temperature you enter are evaluated on your own device and are never sent to a server, saved, or shared — close the tab and they're gone. There's no account, nothing to delete, and no tracking of what you entered. What you get back is general educational information based on published AAP, MedlinePlus, and CDC guidance; it is not a diagnosis, not a temperature reading, and not a substitute for your baby's pediatrician. Its only job is to help you decide how quickly to seek care — and to put the emergencies first.