A temperature of 100.4°F (38°C) or higher, taken rectally, is considered a fever in a 5-month-old baby. This is the same threshold used for all infants and children, regardless of age. At 5 months old, a fever is worth paying attention to but is generally less urgent than it would be in a newborn under 3 months, where any fever typically requires immediate evaluation.
How to Take Your Baby’s Temperature
For babies from birth to age 2, a rectal thermometer is the most accurate way to measure core body temperature. It’s considered the gold standard in pediatrics. You can use an armpit (axillary) reading as a quick screening tool, but it’s the least accurate method for estimating core temperature in children. If an armpit reading looks elevated, follow up with a rectal reading to confirm.
The numbers that count as a fever depend on where you measure:
- Rectal, ear, 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
Normal rectal temperature ranges from about 97.9°F to 100.4°F, so anything at or above that upper limit qualifies as a fever. A reading just below the threshold, like 100.2°F rectally, isn’t technically a fever but is worth monitoring closely, especially if your baby seems off.
Why Age Matters With Infant Fevers
Pediatricians treat fevers very differently depending on how old a baby is. For infants under about 8 weeks, any fever of 100.4°F or higher is treated as potentially serious. These very young babies may need blood work, urine tests, and sometimes a spinal fluid sample to rule out bacterial infections, because their immune systems are still immature and infections can progress quickly.
At 5 months old, your baby is past that highest-risk window. A fever at this age is most often caused by common viral infections. Your pediatrician will typically assess how your baby looks and behaves overall rather than relying on the temperature number alone. A baby who is feeding well, making eye contact, and staying alert with a fever of 101°F is generally less concerning than a baby who is limp and unresponsive with the same temperature.
Signs That Need Prompt Medical Attention
The fever number itself matters less than how your baby is acting. Call your pediatrician or seek care if you notice any of these alongside a fever:
- Unusual sleepiness: sleeping far more than normal, or being hard to wake up
- Floppiness or limpness: less muscle tone than usual, or seeming “ragdoll-like”
- Dehydration signs: fewer wet diapers, crying without tears, dry mouth, or a sunken soft spot on the head
- Breathing trouble: fast or labored breathing, or a persistent cough
- Refusing to eat: missing two or more feedings in a row or eating very poorly
- Forceful vomiting: not just spit-up, but vomit that shoots out, especially if your baby can’t keep liquids down for 8 hours
- Inconsolable crying: crying that’s more intense than usual and nothing calms them down
A fever above 104°F (40°C) at any age warrants a call to your doctor regardless of how your baby is acting.
Keeping Your Baby Comfortable
Fever is your baby’s immune system doing its job, so the goal isn’t necessarily to eliminate it. Focus on keeping your baby comfortable and well-hydrated.
Offer breast milk or formula frequently. At 5 months, your baby isn’t eating solid foods or drinking water yet, so milk feedings are the main source of hydration. If your baby is vomiting, a pediatric electrolyte solution like Pedialyte can help. Skip fruit juice entirely.
Dress your baby in one layer of lightweight clothing. It’s tempting to pile on blankets when they have the chills, but bundling can actually trap heat and push the fever higher. One lightweight blanket for sleep is enough. Keep the room at a comfortable temperature, and use a fan if it feels stuffy.
A lukewarm sponge bath can help bring the temperature down, but it works best when paired with fever-reducing medicine. On its own, the cooling effect tends to be temporary and the temperature bounces back. Never use cold water, ice, or rubbing alcohol, as these can cause shivering, which actually raises core body temperature.
Fever-Reducing Medicine at 5 Months
Acetaminophen (Tylenol) is the standard fever reducer for babies this age. Dosing is based on your baby’s weight, not their age, so check with your pediatrician or pharmacist for the right amount if you’re unsure.
Ibuprofen (Motrin, Advil) is not recommended for babies under 6 months old. It hasn’t been established as safe for this age group, and the FDA has not approved its use in infants younger than 6 months. Your baby will be eligible for ibuprofen in about a month, but for now, stick with acetaminophen if medicine is needed.
You don’t need to give medicine for every fever. If your baby is feeding normally, sleeping reasonably well, and seems comfortable, it’s fine to monitor without medication. The purpose of fever reducers is comfort, not hitting a specific number on the thermometer.