A fever in a newborn is a rectal temperature of 100.4°F (38°C) or higher. Unlike fevers in older children and adults, which are usually harmless and can be managed at home, a fever in a baby under 3 months old is treated as a serious medical concern that requires prompt evaluation. Newborns have immature immune systems, which means infections can spread quickly and produce fewer obvious warning signs.
The 100.4°F Threshold
The number that matters is 100.4°F (38°C), measured rectally. This is the cutoff used by pediatricians and emergency departments. Below this number, your baby’s temperature is considered normal, even if they feel slightly warm. At or above it, your baby needs medical evaluation, regardless of how well they appear to be acting.
This threshold applies to all babies under 3 months old, but urgency increases the younger the baby is. A fever in a baby under 28 days old is treated with the highest level of concern because very young newborns are more vulnerable to serious bacterial infections, including bloodstream infections and meningitis. Their immune systems haven’t yet developed the ability to localize infections the way older children’s can, so bacteria can spread through the body with few outward signs.
Why Rectal Temperature Is the Standard
For newborns, a rectal thermometer gives the most accurate core body temperature reading. Forehead, underarm, and ear thermometers can vary significantly from the true core temperature, and those differences matter when a single degree determines whether your baby needs emergency care. Studies comparing axillary (underarm) and rectal readings have found statistically significant differences between the two, meaning they can’t be used interchangeably.
To take a rectal temperature, apply a small amount of petroleum jelly to the tip of a digital thermometer and insert it about half an inch into the rectum. Hold it in place until it beeps. If you get a reading of 100.4°F or higher, don’t recheck or wait to see if it comes down. That single reading is enough to act on.
True Fever vs. Overheating
Not every warm baby has a fever. Newborns are small and lose the ability to regulate temperature easily, so overbundling, a warm room, or being held skin-to-skin for a long time can temporarily raise their temperature. This is called hyperthermia, and it’s different from a true fever. A fever happens when the body’s internal thermostat resets itself higher, usually in response to an infection. Overheating happens when the baby simply absorbs more warmth than they can release.
If your baby feels warm and you suspect overheating, remove a layer of clothing and wait 15 to 20 minutes before rechecking. If the temperature drops below 100.4°F, overheating was likely the cause. If it stays at or above that threshold, treat it as a fever.
What Causes Fever in Newborns
Most fevers in newborns are caused by viral infections. Common culprits include respiratory viruses like RSV (respiratory syncytial virus), influenza, and enteroviruses. Rhinovirus is the most frequently detected virus in febrile infants, though its presence doesn’t rule out a bacterial infection happening at the same time.
Bacterial infections are less common but more dangerous. Urinary tract infections are the most frequent bacterial cause of fever in young infants. More serious possibilities include bacteremia (bacteria in the bloodstream) and bacterial meningitis (infection of the membranes surrounding the brain and spinal cord). These are rare, but they’re the reason every newborn fever gets a thorough workup.
Herpes simplex virus (HSV) is another concern in the newborn period. About 1,500 to 2,000 newborns in the U.S. are affected each year. Risk factors include invasive monitoring during delivery, the presence of skin lesions, and seizures. Doctors maintain a high index of suspicion for HSV in febrile newborns because early treatment makes a significant difference in outcomes.
What Happens at the Hospital
If you bring a febrile newborn to the emergency department, expect a thorough evaluation. The goal is to identify or rule out serious bacterial infections, because newborns can look relatively well even when a dangerous infection is developing. The specific tests depend on your baby’s age.
For babies under 28 days old, the workup is extensive. It typically includes blood tests, a urine sample collected by catheter, and a lumbar puncture (spinal tap) to check the fluid around the brain and spinal cord for signs of meningitis. A chest X-ray may be taken if there are any respiratory symptoms. This can feel overwhelming as a parent, but each test serves a specific purpose: blood cultures catch bloodstream infections, urine cultures catch urinary tract infections, and spinal fluid analysis catches meningitis, all of which can be present without obvious symptoms in a newborn.
For babies between 29 days and 3 months, the evaluation may be slightly less extensive. Blood and urine tests are still standard. A lumbar puncture may be skipped if the baby looks well, has normal blood counts, and has a clean urinalysis, but this is a clinical judgment call made by the treating physician. If any test results come back concerning, the baby will be admitted to the hospital for IV antibiotics while cultures are monitored.
Fever After Vaccinations
Newborns receive their first hepatitis B vaccine shortly after birth, and the next round of vaccines typically comes at the 2-month visit. Low-grade fevers after vaccination are common and expected. For example, the pneumococcal vaccine causes fever in roughly 1 in 4 infant recipients within the first week. Rotavirus vaccine causes fever in about 1 in 5 babies. Hepatitis B vaccine causes fever in a smaller percentage, roughly 2 to 10 out of 100 recipients.
Most post-vaccination fevers appear within the first day or two and resolve on their own. However, for babies under 3 months old, the standard advice still applies: a temperature of 100.4°F or higher warrants a call to your pediatrician, even if the timing lines up with a recent vaccine. The doctor can help determine whether the fever is a normal immune response or needs further investigation.
Fever Reducers and Newborns
You should not give fever-reducing medication to a newborn without specific instructions from a doctor. Ibuprofen is not approved for babies under 6 months old. Acetaminophen (Tylenol) can be used in younger infants, but only at doses prescribed by a pediatrician, and never in the first few weeks of life without medical guidance. More importantly, giving a fever reducer before an evaluation can mask the fever and delay diagnosis of a potentially serious infection.
Signs That Need Immediate Attention
A fever alone is enough to seek care for any baby under 3 months. But certain accompanying signs raise the urgency further. These include poor feeding or refusal to eat, unusual sleepiness or difficulty waking, a weak or high-pitched cry, rapid or labored breathing, a rash with small purple or red dots that don’t fade when you press on them (petechial rash), skin that looks pale or mottled, and slow capillary refill, where pressing on the skin leaves a white spot that takes more than two seconds to return to its normal color.
It’s also worth knowing that sick newborns don’t always run fevers. Some newborns with serious infections have temperatures that are lower than normal rather than elevated. If your baby under 3 months seems unusually lethargic, isn’t feeding, or just doesn’t seem right, trust your instinct and seek care even without a documented fever.