The "breeder" I got my pewter from said she gave calcium to gravid females, but she used a syringe and put it in the dead rat's mouth. I thought it was odd, and had never heard it before. Since then I have found out that she isn't exactly reputable and has sold sick animals (I got lucky I guess). I know at least in humans that calcium inhibits the absorption of iron, so I wouldn't mess with the natural balance that is a whole prey item.