If it were me I would definitely try for the promotion. Like you say, what have you got to lose? If you are planning on going back to work anyway, why not go back in a position that you are more deserving of and have worked hard for?
I'm in a tricky work situation at the moment which I will post about after I reply to this. So I fully understand the complexities of the fact there isn't supposed to be any discrimination yet there clearly, clearly is in every job. But if you apply for the position you are showing them YOU don't believe in the need for discrimination. That you might be pregnant, but you are capable of doing the job and can be counted on when you return to work.