Having Nacho Cheese during pregnancy is generally safe as long as the Nacho Cheese has been heated up really good prior to putting the Nacho Cheese on the Nachos.
The cooking or heating of the Nacho Cheese will usually kill off the Listeria bacteria which is what is a problem during pregnancy.
So make sure you get the Nacho Cheese pretty hot before eating it to kill off the listeria so that it doesn't harm your baby or pregnancy.
When pregnant avoid any cheese that is unpasteurized or made with unpasteurized milk to avoid any harm to the unborn baby or risk having other problems with your pregnancy.