work announcement

So I am in a little different boat with my line of work. I know most people wait for many reasons to tell work they are pregnant however there are certain aspects of my job I shouldn't do if I become pregnant. I work with lead dust. It is not a critical part of my job but I'm required twice a week unless I have some medical reason as to why I can't. So with that said how do I inform them of that possibility without having to tell them every time it's a possibility? Or just when it's a positive? I mean the risk is low but for a growing baby pretty sure it's a little more sensitive to handle.