I think JenJay has a good point. If Anna the bride is really serious about this no-pregnancy thing--and I think it's worth one more conversation just to be sure--then to me the OP's wishes and Anna's are in conflict. At the very least I think the OP should tell Anna, "Look, I have no intention of trying hold off a pregnancy until after your wedding. If that means you want me to step down now, so be it. If you'd rather wait and see, and possibly ask me to step down later, so be it." So at least Anna knows what's going on and can make an informed decision.
And I definitely think Anna's terms are ridiculous. But, to me it seems dodgy for a person to agree to them, while secretly having no intention of fulfilling them. I can definitely see how this varies with the situation, so in some cases it might be appropriate. Maybe the OP has reason to suspect that, in a few months, Anna will calm down and be embarrassed about her no-pregnancy edict, and grateful that it didn't lose her any friends. I think that's a judgment the OP really has to make herself, while knowing that she's taking the risk that Anna is totally serious and consistent, and things may come to a head later--at a time when the OP should be basking in the glow of her own happy news.
There's also the principle of the thing, which I know is a dangerous place to go. Everyone has different things that rub them the wrong way. For me, if I really wanted to get pregnant and was trying to do so, it would be quite hurtful to hear my supposed friend call pregnant women "fat," say that my potential pregnancy would present an aesthetic problem for her wedding, and try to dictate what I did with my family planning. Stressed or not, those are just plain mean things to say. And it would be hard for me, personally, to "unhear" them, or to shrug them off even if they turned out to irrelevant (that is, if I didn't become pregnant in time to affect the wedding). I might want to distance myself from someone who would say those things, and then see how she acted once her stress had decreased--an apology would definitely be in order. But, this is definitely a case where people's mileage may vary, and someone else might truly shrug it off.