During the interview process if a candidate discloses that she is pregnant and this affects the employers decision on whether she should be hired, can the employer be sued?

As long as the job doesn't involve something that would be dangerous for the woman to do like heavy lifting or be around chemicals or stand all day or something, yes, it shouldn't affect the hiring or not-hiring... how would you know if it was the fact the person was pregnant though that she didn't get hired?

Yeah, you can't prove it unless you have proof. They have truth on their side (or at least what the perceived truth is.)

I would think it would be their word against yours, unless you have written proof or something else then you don't have a case.