A 5-month-old has a fever when their temperature reaches 100.4°F (38°C) or higher. If your baby hits that number, the first step is to stay calm and assess how they’re acting overall. A fever itself is a sign the immune system is working, and most fevers in babies this age are caused by common viral infections. What matters most is how your baby looks and behaves, not just the number on the thermometer.
How to Take an Accurate Temperature
For a baby this young, a rectal thermometer gives the most reliable reading. Temporal artery (forehead) thermometers are convenient but can be less precise, and ear thermometers aren’t ideal for small ear canals. To take a rectal temperature, apply a small amount of petroleum jelly to the tip, gently insert it about half an inch, and hold it in place until it beeps. If you’re unsure about the reading, wait a few minutes and try again.
When to Call Your Pediatrician
At 5 months old, your baby falls into a specific age window that pediatricians pay close attention to. Call your doctor if your baby has any temperature above 100.4°F, especially if they seem unwell. Even a temperature at or just below that threshold warrants a call if your baby is acting sick, unusually fussy, or more sleepy than normal.
If the fever lasts more than 24 hours, contact your pediatrician even if your baby seems otherwise fine. For children under 2, a fever persisting beyond a day is a standard reason to check in.
Signs That Need Immediate Medical Care
Most fevers don’t require an emergency room visit, but certain symptoms alongside a fever mean you should get care right away:
- Breathing trouble: rapid breathing, flaring nostrils, or skin pulling in between the ribs with each breath
- Color changes: skin or lips that look blue, purple, or gray
- Unusual behavior: your baby is limp, unresponsive, hard to wake, or acting strangely withdrawn
- Constant crying: pain or fussiness that keeps getting worse and nothing soothes it
- Trouble swallowing: excessive drooling or spitting that seems different from normal
- Repeated vomiting: not just spit-up, but vomiting several times
Trust your instincts. If something feels off about the way your baby looks or acts, that’s reason enough to seek help.
Keeping Your Baby Comfortable at Home
While you monitor the fever, there are several things you can do to help your baby feel better. Dress them in one layer of lightweight clothing. Even if they have the chills, resist the urge to pile on blankets or extra layers. Bundling can actually trap heat and push the fever higher. For sleep, one lightweight blanket is enough.
Keep the room at a comfortable temperature. If it feels hot or stuffy, a fan can help circulate air. A lukewarm sponge bath can also bring some relief, but skip cold water, ice, or alcohol rubs. These can cause shivering, which actually raises the body’s core temperature. Lukewarm baths work best when paired with fever-reducing medicine; on their own, the temperature tends to bounce right back up.
The most important comfort measure is keeping your baby hydrated. Continue breastfeeding or formula feeding as normally as possible, offering smaller, more frequent feeds if your baby isn’t interested in full ones.
Watch for Dehydration
Fever increases fluid loss, so dehydration is a real concern in a baby this small. Watch for these warning signs:
- Fewer wet diapers than usual
- Few or no tears when crying
- A sunken soft spot on the top of the head
- Sunken eyes
- Unusual drowsiness or irritability
If you notice any of these, contact your pediatrician promptly. The soft spot on your baby’s head is a particularly useful indicator at this age. When a baby is well hydrated, it should look flat or slightly curved inward. A noticeably sunken soft spot signals they need fluids.
Fever-Reducing Medicine for a 5-Month-Old
Acetaminophen (Tylenol) is the go-to fever reducer for babies, but for children under 2, it should only be given with guidance from your doctor. Your pediatrician will recommend a dose based on your baby’s weight, not their age. Infant liquid acetaminophen typically comes in a concentration of 160 mg per 5 mL, and your doctor or pharmacist can help you measure the correct amount.
Ibuprofen (Advil, Motrin) is generally safe for babies 3 months and older who weigh at least 11 pounds, but check with your pediatrician before giving it for the first time. Never give aspirin to any baby or child.
One important note: the goal of medicine isn’t to eliminate the fever entirely. You’re aiming to bring the temperature down enough that your baby can rest, eat, and stay hydrated comfortably. Don’t wake a sleeping baby just to give fever medicine unless your doctor tells you to.
What to Keep Track Of
While managing the fever at home, it helps to jot down a few things so you have clear information if you need to call your doctor. Note the time and reading of each temperature check, when you gave any medicine and how much, how many wet diapers you’re seeing, and how much your baby is eating. Also pay attention to your baby’s behavior between temperature spikes. A baby who perks up and plays when the fever drops is generally in a very different situation than one who stays listless regardless.
Your pediatrician will ask about all of these details, and having them written down saves you from trying to remember in a stressful moment.