Generally with a Christmas ham it is covered with a layer of skin & fat. If you peel that off & eat a few slices straight from the top, straight after it is cut then you will be fine. The skin & fat have protected the meat from any listeria germs, but ones it has been exposed to the air then you should steer clear. Of course prior to eating it, make sure it has been kept very cold & hasn't sat out for very long before it is carved. Also make sure you eat it right after carving so it's not sitting out in the warm air.

This is how I did it for the three Christmas' that I was pregnant, under doctor's instruction from the first pregnancy