If you’re a parent, you know the look of a feverish child. The flushed skin and drowsy expression are classic fever flags, not to mention the warm head. You take his temperature and your suspicion is confirmed. But should you call the doctor?

It depends how high the thermometer reading is, plus a few other variables.

Kids' temperatures vary based on age and other factors. They can even go up or down based on activity level, according to the Cleveland Clinic, which defines a fever as 100.4 F or higher.

Related: Checking Kids for Fevers: Which Kind of Thermometer Is Best?

Remember a fever itself is not an illness — it’s a symptom of another problem. KidsHealth says a few potential causes are:

  • Infection — Most fevers are caused by infections, and a fever is the body’s defense.
  • Overdressing — The child might be wearing too many layers.
  • Immunizations — Kids sometimes get low-grade fevers after vaccinations.

Still, it’s scary to watch the temperature reading climb, and you want to make sure you’re doing the right thing for your toasty tot. The Cleveland Clinic, KidsHealth and the AAP offers this advice on when to worry and when you shouldn’t.

When to relax

Don't worry if:

  • The fever is under 102.5 F (unless your baby is under three months).
  • Your child is playing, eating and drinking normally.
  • The fever lasts less than three days.
  • The fever is low and happened after your child was immunized.
  • Your child’s skin color looks normal.

Kids with fevers under 102.5 F often don’t need medicine unless they’re uncomfortable, KidsHealth says. But call a doctor before giving fever reducers like acetaminophen or ibuprofen to children under 2. And don't give aspirin to kids or teens, especially during an illness, according to KidsHealth. This can increase the risk of a rare but serious illness called Reye Syndrome.

You can make feverish kids feel better by dressing them in light clothing and covering them with a light blanket or sheet. Make sure they drink plenty of fluids — KidsHealth says if they won’t drink water or eat soup, try ice pops. Also let them get plenty of rest.

Related: Are You Giving Your Kids Their Medicine Correctly?

When to call the doctor

Call the doctor if:

  • Your child is younger than three months old and has a fever of 100.4 or higher. Even a slight fever can be a sign of a serious problem in young infants according to the American Academy of Pediatrics (AAP).
  • The fever lasts more than three days.
  • The fever is more than 104 F.
  • It does not come down after your child takes fever-reducing medication.
  • Your child isn’t acting like himself, seems drowsy or isn’t drinking fluids.
  • Your child recently had a vaccination and has a fever higher than 102 F for longer than two days.
  • Your child has other symptoms, like a sore throat, headache, vomiting, diarrhea, a rash or a stiff neck.
  • Your child has a seizure.

One last tip: If your child has a fever, he shouldn’t go to school or child care, KidsHealth says. Experts say it’s safe to return once the fever has been gone for 24 hours.

Related: Stomach Flu Symptoms: Is It a Virus or Something Else?

Angela is a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide. Prior to joining SafeBee, she was the features editor for Boston.com at The Boston Globe, overseeing health, travel, entertainment, business and lifestyle coverage. Before moving to features, she was the news and homepage editor, covering stories such as the Boston Marathon bombing, Red Sox World Series victories, presidential elections, a papal inauguration, and more. Her favorite safety tip: Clean your phone! The average cell phone has 18 times more germs than the toilet handle in a men’s restroom.