I was in a similar position and I did tell because I couldn't not tell because of my conscience. I figure the same as Gracie said, if you're right for it then it shouldn't matter, this doesn't mean that they will think that way though. There is IMO still discrimination with this. I looked for some temp work whilst pg but got knocked back and I know myself it was because I was pg. You do what you feel is right, for me it's a moral and ethical thing and I feel I'm lying if I don't.