For a 6-month-old baby, a fever is a rectal temperature of 100.4°F (38°C) or higher. This is the standard threshold used by pediatricians, and rectal readings remain the most accurate way to measure temperature in young children. If your baby hits or exceeds that number and seems unwell, it’s worth a call to your pediatrician.
How to Get an Accurate Reading
A rectal thermometer gives the most reliable temperature for any child under 3 years old. At 6 months, your baby also has a few other options. Ear (tympanic) thermometers become accurate starting at 6 months of age, so this is the earliest you can use one and trust the result. Forehead (temporal artery) thermometers work for babies 3 months and older. Armpit readings are the least reliable method and are best used as a quick screening. If an armpit reading looks concerning, follow up with a rectal temperature to confirm.
A digital thermometer is all you need. When taking a rectal temperature, lubricate the tip with petroleum jelly, insert it about half an inch, and hold it in place until it beeps. The whole process takes under a minute.
What Usually Causes a Fever at This Age
Almost all fevers in babies are caused by infections, and viruses are responsible about 10 times more often than bacteria. Common culprits include colds, flu, and other respiratory viruses. Roseola is a classic example in this age group: a baby may run a fever for 2 to 3 days with no other symptoms, then develop a rash once the fever breaks.
On the bacterial side, bladder infections are the most common cause of fever without obvious symptoms in girls. Meningitis, a bacterial infection of the membranes covering the brain and spinal cord, is rare but serious and requires immediate medical attention.
One thing that probably isn’t causing your baby’s fever: teething. Despite the persistent belief, research consistently shows that teething does not cause true fevers. A teething baby may be fussy or drool more, but if the thermometer reads 100.4°F or above, an infection is far more likely.
When a Fever Needs Medical Attention
The urgency depends partly on your baby’s exact age within the 6-month window. If your baby is closer to 3 to 6 months old, call your pediatrician for any temperature of 100.4°F or higher, or for a lower temperature if your baby seems visibly unwell (unusually sleepy, refusing to eat, difficult to console). Once your baby is past 6 months, the guideline shifts slightly: call if the fever is above 100.4°F and lasts longer than one day. For any infant, a fever lasting more than three days warrants a call regardless.
Temperature alone doesn’t tell the whole story. How your baby is acting matters just as much. A baby with a mild fever who is still making eye contact, feeding reasonably well, and having wet diapers is in a very different situation than a baby with the same temperature who is limp, inconsolable, or refusing all fluids.
Keeping Your Baby Comfortable
Dress your baby in one layer of lightweight clothing. It’s tempting to pile on blankets when your baby has the chills, but bundling can trap heat and push the fever higher. For sleep, one light blanket is enough. Keep the room at a comfortable temperature, and if it feels stuffy, a fan can help circulate air.
Hydration is critical. Babies lose fluids faster when they have a fever, and the higher the temperature, the greater the risk of dehydration. If you’re breastfeeding or formula-feeding, offer smaller, more frequent feeds. Watch for warning signs of dehydration: fewer than the usual number of wet diapers (or none for three hours), a dry mouth, no tears when crying, sunken eyes, or skin that stays pinched up when you gently squeeze it instead of flattening back immediately. If you notice these signs, contact your pediatrician right away.
For medication, acetaminophen (Tylenol) is safe for babies 6 months and older and can help bring down a fever and ease discomfort. Ibuprofen (Advil, Motrin) is also an option starting at 6 months, but should not be used in younger babies unless specifically directed by a doctor, as it hasn’t been established as safe below that age. Always dose by your baby’s weight, not age, and follow the label instructions.
Febrile Seizures
Febrile seizures are convulsions triggered by fever, and they’re most common between 6 months and 5 years of age, peaking between 12 and 18 months. So your 6-month-old is just entering the window of risk. A family history of febrile seizures increases the likelihood, and some childhood vaccines can slightly raise the risk as well.
During a febrile seizure, a baby typically shakes all over and loses consciousness. The most common type lasts a few seconds to 15 minutes, doesn’t repeat within 24 hours, and affects the whole body rather than one side. Complex febrile seizures last longer than 15 minutes, happen more than once in a day, or involve only one side of the body.
If your baby has a seizure, keep them safe by placing them on a flat surface away from hard objects, turning them gently on their side, and not putting anything in their mouth. Call your pediatrician as soon as the seizure ends, even if it lasted only a few seconds. Call 911 if the seizure lasts longer than five minutes or is accompanied by vomiting, a stiff neck, breathing problems, or extreme sleepiness afterward.
One important thing to know: giving fever-reducing medication at the start of a fever may make your baby more comfortable, but it will not prevent a febrile seizure. These seizures are triggered by the rapid rise in temperature, not the peak number itself.