After babies are born, the nursery staff takes axillary (armpit) temperatures to
make sure that infants are not becoming chilled due to the temperature in the
nursery. Since parents learn by watching the nurses, they may assume that they
should continue using axillary readings once their babies go home. Because
pediatricians watch carefully for fever in the first two months of life, it is
important that you take rectal temperatures if you think your baby has a fever.
The reason for this is because this method is more accurate and our approach to
fever is based on rectal temperatures. (A fever in a young infant is defined at
a rectal temperature of 100.4º F or higher. Also, because babies do not always
get a fever when they are sick, you should call your doctor right away if you
think your newborn is sick even if he does not have a fever.)
— Howard J.