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A true fever is a cause for concern in the first few months of a baby’s life. That’s because the infant's immune system isn’t yet as good at fighting infections as it will be at 3 or 4 months of age.

It’s vital to learn how to take your baby’s temperature and to know how to spot a fever.

When Is It a Fever?

Most doctors say a baby has a fever if her temperature rises to 100.4 F or above when measured with a rectal thermometer. Take a few readings when she’s well to figure out what her normal temperature is.

What Causes Fever?

Many causes for a baby’s fever exist, but the most common is infection.

As in older children and adults, the body’s immune system senses a “foreign invader,” such as bacteria or a virus, and tells the brain to crank up body heat. Some germs don’t like the added warmth, and the immune system takes care of them easily. This kind of fever lets you know that an infection may be brewing, and helps protect against it.

Another possible cause is being too warm. Dress your baby in one more layer of clothing than you find comfortable.

When to Worry

An unusually high temperature in the first months could mean infection. All children under age 2 months with a true fever should see a doctor. Call your doctor about fevers for at least the first 4 to 6 months of your baby’s life.

At your baby’s next check-up, ask the doctor if he has a “fever policy.” That will give you a better idea about when you need to call. If you’re worried, take the better-safe-than-sorry approach.

Fever is only part of the story. A more important question is: Does your newborn show symptoms of illness? Call the doctor if your baby is:  

  • Irritable
  • Inactive
  • Sluggish
  • Not eating
  • Has trouble breathing
  • Has a rash
  • Is vomiting
  • Has diarrhea
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