Your Baby’s Temperature: Fever A true fever is especially worrisome in the first few months of life. That's because the infant's immune system is immature and not as effective in fighting off infections as it will be after 3 or 4 months of living outside of the womb. So it's very important to learn how to correctly take your baby's temperature and understand what signifies a true fever. What Is Considered a Fever? Doctors usually say a baby has a fever if her temperature rises about 1.5-2 degrees (Fahrenheit) above its normal value -- which is typically 98.6 but can vary by a degree or so in either direction. Learn what your baby's normal temperature is by taking it a few times when she is not sick. And as a rule of thumb, once the body temperature hits 100.4, that's a fever. What Causes a Fever? There can be many causes for an infant's fever. Dehydration is one. Or the infant might be overbundled with clothes in a relatively warm environment. (The rule of thumb is to dress your baby in only one more layer of clothing than you are comfortable in.) But, most commonly, fever is caused by an infection. The body's immune system senses the "foreign invader" -- such as bacteria or a virus -- and sends a chemical message to the temperature center instructing it to crank up the heat inside the body. This has many advantages: • Some bacteria and viruses don't like the higher temperature and are then more easily destroyed by the immune system. • The higher body temperature helps the body to ward off the infection. In this way, fever is actually our friend: It lets us know that an infection may be brewing, and it helps to fight off the infection. When to Worry About a Fever: Any fever in the first months could indicate a significant infection requiring immediate attention. But fever is only part of the story. An even more important question is if your newborn looks sick, fever or not. If he looks sick in any way -- irritability, listlessness, lethargy, poor feeding, respiratory distress, rash, vomiting, diarrhea, etc. -- contact your pediatric provider, even if the temperature is not high enough to be called a fever.