A fever in a baby is a rectal temperature of 100.4°F (38°C) or higher. This single number is the standard threshold used across pediatric medicine, regardless of whether your baby is two weeks old or ten months old. What changes with age is how urgently that fever needs medical attention.
Fever Thresholds by Measurement Method
The 100.4°F threshold applies specifically to rectal and forehead (temporal artery) readings. Other measurement methods have slightly different cutoffs because they read a bit lower or higher depending on the body site:
- Rectal or forehead: 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
- Ear: 100.4°F (38°C) or higher
Armpit readings are the least reliable of these options. If an armpit reading suggests a fever, it’s worth confirming with a different method before deciding what to do next.
Which Thermometer to Use
For babies younger than 3 months, a rectal thermometer is the most accurate way to measure temperature. This matters because at that age, even a low-grade fever can signal a serious infection, so you need a number you can trust. Forehead thermometers are considered comparably accurate and involve less discomfort, making them a reasonable alternative.
Ear thermometers aren’t recommended for newborns or young infants. They become more reliable after about 6 months of age, once the ear canal is large enough for a consistent reading. For a quick screening of an older baby who seems warm, an armpit reading works as a first pass, but treat it as a rough estimate rather than a definitive answer.
Why Age Changes Everything
A fever of 100.4°F in a one-month-old is treated very differently than the same temperature in a nine-month-old. The younger the baby, the more limited their immune system’s ability to localize and fight infections, which means fever in the early weeks of life can indicate something that needs immediate evaluation.
Here’s how age brackets generally break down:
Under 8 weeks old: Any rectal temperature of 100.4°F or higher needs prompt medical evaluation. Babies this young should not be given fever-reducing medication at home. The fever itself is the signal that something needs professional assessment, and masking it with medication can delay diagnosis. The American Academy of Pediatrics has specific clinical guidelines addressing well-appearing infants between 8 and 60 days old with fever at or above 100.4°F, reflecting how seriously this age group is treated.
2 to 3 months old: A fever still warrants a call to your pediatrician, even if your baby seems comfortable and is feeding normally. At this age, doctors will want to know about the fever promptly and may ask you to bring your baby in for an exam.
3 to 6 months old: A low fever with an otherwise happy, feeding baby is less alarming, but temperatures reaching 102°F or higher, or a fever that lasts more than a day, should prompt a call to your doctor.
Over 6 months old: Fever is more commonly a sign of routine viral illness. How your baby acts matters more than the exact number on the thermometer. A baby with a 101°F fever who is playful, drinking fluids, and making wet diapers is typically less concerning than a baby with a 100.5°F fever who is limp and refusing to eat.
Fever-Reducing Medication
Acetaminophen (Tylenol) can be given to babies 8 weeks and older. Ibuprofen (Motrin, Advil) is not safe until 6 months of age unless specifically directed by your baby’s doctor. Both are dosed by weight, not age, so check the packaging carefully or ask your pediatrician for the correct amount.
The goal of medication isn’t to eliminate the fever entirely. Fever is part of how the body fights infection. The point is to bring your baby’s temperature down enough that they feel comfortable, can sleep, and will continue drinking fluids. If your baby is resting comfortably despite a mild fever, medication isn’t always necessary.
Keeping Your Baby Comfortable at Home
Hydration is the most important thing you can do during a fever. Babies should continue breastfeeding or taking formula as usual. If your baby is vomiting, an oral electrolyte solution like Pedialyte can help replace lost fluids. Fruit juice is not recommended for babies.
Dress your baby in one layer of lightweight clothing, even if they have the chills. Bundling them in extra blankets or heavy clothes can trap heat and push the fever higher. For sleep, a single lightweight blanket is enough. Keep the room at a comfortable temperature. If it feels warm or stuffy, a fan can help circulate air without blowing directly on your baby.
Signs of Dehydration to Watch For
Fever increases fluid loss, and babies can become dehydrated faster than older children. Watch for these warning signs: fewer wet diapers than usual, few or no tears when crying, sunken eyes, and unusual drowsiness or irritability. In young babies, the soft spot on top of the head (the fontanelle) may appear sunken if dehydration is setting in. Any of these signs alongside a fever warrants prompt medical attention.
Red Flags Beyond Temperature
The number on the thermometer is only one piece of the picture. Pay closer attention to how your baby looks and behaves. A fever paired with any of the following needs urgent evaluation: difficulty breathing, a rash that doesn’t fade when you press on it, persistent vomiting, extreme sleepiness or difficulty waking, a weak or high-pitched cry that sounds different from normal, or a stiff neck. Babies who appear “floppy” or unusually limp should be seen right away.
Fever following immunizations is common and typically appears within 48 hours of the shot. It’s usually mild and resolves on its own, but if your baby is under 2 months old, let your pediatrician know rather than assuming the vaccine is the cause.