In general, a fever
in a newborn is a rectal temperature of 100.
4f or higher (38.0 celsius). If you are checking the temperature by other means (such as under the arm), it may register lower by up to a degree than the baby's real temperature. Fever is most urgent in the first couple of months, when the baby's immune system is at it's lowest. As your baby gets older, threshold for fever gets higher.