I am in the same situation now. I am the matron of honor at my best friends wedding in September. My DH & I have been TTC since last July. My BF became engaged in September of last year. She had known we were TTC since before she became engaged, but then after expressed her feelings that she would be "highly disappointed" if I became pregnant before the wedding.
Last week I text her and let her know we were still trying, hoping I would have her full support. Her response was that she would support me because I am her best friend, but then she went on to say "When you get pregnant, you have no idea what it is going to do to your body, you have no idea how you're going to feel. And waiting this close to September, you have no clue if it would come early." She then proceeded to tell me she thought it would be best if I waited a few more months, that way I would be "less pregnant". She also expressed her feelings of being worried about how I would look in my dress. And she worried that if I were put on bed rest, that I could miss the wedding. She then let me know that she also "always thought it was best to be married two years before you even think of having children" (I will be married two years in September, we have been together 11 years.) She then said she was stressed out even thinking about it. And then she said "the one selfish thing I will say about this whole thing is I want to be celebrated and have my wedding time just like you did."
We are not going to stop trying until March. I skipped last month, because I didn't want to be 9 months pregnant at the wedding. We ordered dresses Saturday, and I did go up two sizes. I asked her what she thought about that at the bridal store because I was unsure what size I should get, (I am typically an 6-8, I ordered a size 10) but she proceeded to give me a look like she was upset I even asked her and did not want to tell me what she thought I should do. Instead she just said that "you know they can't take it up more than two sizes." And that was that.
From the outside looking in, I will say do not wait on anyone! If they are a true friend they would understand. I had two pregnant bridesmaids in my wedding, and if something had happened close to my day, I would have been more concerned about them than "my day". Of course your wedding day is an important time, and you want the important people in your life there, but at the same time, health is more important and I wouldn't have been upset. BUT, since I am the one going through it now, I actually feel like I am the one being selfish for not waiting to try.