Fevers can be a very scary thing for parents, especially for first-time moms and dads. (How high is too high? Should I call the doctor immediately?)

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But every child will eventually experience a fever, no matter how careful you are, says pediatrician Tracy Lim, MD. And it’s important for parents to know what to do when this happens. That starts with taking your child’s temperature using any variety of thermometers available, from standard oral thermometers to the newer temporal artery scanners.

“You can use any of these devices, but a digital thermometer is generally all you need,” she says.

It’s most accurate to use a rectal thermometer for infants and young children. “If you feel uneasy doing this, use whichever device makes you most comfortable,” Dr. Lim says. “In older children, an oral temperature is most accurate, if the child is able to tolerate it.”

When to keep your cool

So what is a fever? Fever is defined as a temperature over 100.4 F (38.0 C). Normal body temperature is 98.6 F (37 C). Everyone’s body temperature varies throughout the day and can differ by age, activity level and other factors.

Don’t be alarmed if your child’s temperature varies. The magic number for fever is 100.4 F.

When should you not worry about your child’s fever? Dr. Lim says doctors tend not to worry about:

Fevers of less than five days if your child’s behavior is relatively normal. You don’t need to be concerned if your child continues to be playful and is eating and drinking normally. (He or she may seem more tired than usual).

if your child’s behavior is relatively normal. You don’t need to be concerned if your child continues to be playful and is eating and drinking normally. (He or she may seem more tired than usual). Temperatures of up to 102.5 F if your child is 3 months to 3 years of age, or up to 103 F if your child is older. These temperatures can be common, but not necessarily worrisome.

if your child is 3 months to 3 years of age, or up to 103 F if your child is older. These temperatures can be common, but not necessarily worrisome. Low-grade fevers if your infant or child was recently immunized. These can be normal if they last less than 48 hours.

When to call your doctor

Now for the important question: When should you be worried about a fever? Call a doctor when:

An infant younger than 3 months of age develops a fever. Fevers may be your infant’s only response to a serious illness. In newborns, a low temperature can also be a sign of serious illness. Call your pediatrician if your baby temperature drops below 97.7 rectally.

of age develops a fever. Fevers may be your infant’s only response to a serious illness. In newborns, a low temperature can also be a sign of serious illness. Call your pediatrician if your baby temperature drops below 97.7 rectally. Your child’s fever lasts more than five days. Your pediatrician may need to investigate further for underlying causes.

Your pediatrician may need to investigate further for underlying causes. Your child’s fever is higher than 104 F (> 40 C) .

. Your child’s fever does not come down with fever reducers.

Your child is not acting himself or herself , is difficult to arouse, or is not taking in enough liquids. Babies who aren’t wetting at least four diapers per day and older children who are not urinating every eight to 12 hours may become dangerously dehydrated, she says.

, is difficult to arouse, or is not taking in enough liquids. Babies who aren’t wetting at least four diapers per day and older children who are not urinating every eight to 12 hours may become dangerously dehydrated, she says. Your child was recently immunized and has a temperature above 102 F or a fever for more than 48 hours.

and has a temperature above 102 F or a fever for more than 48 hours. You are concerned. If you are uncomfortable with your child’s temperature or illness, call your doctor or nurse practitioner to discuss it.

What to do if a seizure occurs

Seizures are a very scary side effect of fevers in some children, Dr. Lim notes. “Febrile seizures” occur in 2 to 4% of all children under age 5. Not all seizures cause jerking movements in the body. Some seizures look like “passing out.” If your child develops a seizure:

Put your child on his or her side. Do NOT put anything in your child’s mouth. Call 911 if the seizure lasts more than five minutes.

If the seizure lasts less than five minutes, call your physician or seek immediate medical attention.

What to do about multiple fevers

If your child has persistent or multiple episodes of fever and a pediatrician cannot figure out what is causing them, they may refer your child to a specialist, Dr. Lim says. A pediatric infectious disease expert or pediatric rheumatologist may be able to get to the bottom of the issue.