I'm not female, but if I were, then my answer to your question would depend on whether you're bump is visible or not. If it is, then I would inform your date before you meet, paying a lot of attention to my words. If the bump is not yet visible, I would say nothing on the first date. If the relationship continues and progresses, then once the relationship reaches a certain point, I would bring it up carefully.

That's what I would do. Good luck!