My babys rectal temperature is 99 f do i need to minus 1 degree from it for actual reading?

Not if...: If a thermometer is calibrated for rectal temperatures, then the 99 degrees is accurate and no adjustment is needed. If the thermometer is calibrated for oral or axillary temperatures, it might automatically add 1-2 degrees to what it senses, and thus display an incorrectly high reading (because it thinks it is in the mouth or armpit, when in fact you've put it in the rectum).