Unlike older infants and children, newborns don't often develop a fever. Fever in a newborn always warrants investigation, although some causes are more serious than others. It can indicate an illness in an infant. Call your pediatrician whenever the baby runs a fever in the first three months of life.
You should worry any time a baby younger than 1 month of age has a core temperature of 100.4 or greater. If your baby is older than 1 month, 100.8 F constitutes a fever, according to the University of British Columbia's Learn Pediatrics website. A rectal thermometer is the most accurate way to determine your baby's core temperature. A newborn can't hold a thermometer in his mouth and less invasive methods, such as tympanic thermometers, which fit inside the ear canal, aren't always accurate in tiny babies.
Fighting off Infections
Any newborn who has a fever should be evaluated by your physician for a possible infection, even if she doesn't appear sick. Infection can be very dangerous for a newborn because her immune system isn't developed enough to fight it off. The degree of fever isn't an accurate indication of how sick a newborn is. Call your doctor for any elevation over 100.4 F.
An overdressed infant could have an inaccurately high temperature. A high temperature caused by overdressing should drop to less than 100.4 F if you remove a layer or two of clothing and then retake his temperature after a few minutes. Overdressing or overheating a room generally won't raise a baby's rectal temperature greater than 101 F according to a Wolters Kluwer Health article. If your baby's temperature is over that point, assume he has a fever and call the doctor.
Dehydration and weight loss can also cause the fever. A breastfed newborn may not be getting nursing effectively or often enough to get the fluid she needs. If the baby has a fever and shows signs of dehydration such as listlessness, dry mucus membranes, and a sunken fontanel, she needs immediate medical evaluation.