Babies are never due when they say they are due. It's always a few days (maybe even weeks) before or after the due date. So maybe you should just keep things the way they are. But if you feel too worried about it then maybe it's best to just change the wedding date. Good luck!
This is a tough situation to deal with. I whole heartedly believe that you have to decide what is most important to you and FH and go with it. This is a very special time for you and you should do what you want so you don't look back and have any regrets.
I know when I asked my SIL to be a bridesmaid in my wedding she said "Sure, as long as I am not pregnant." I asked her in December '07 and our wedding is July '09. Talk about stealing thunder!! Everyone was over for Christmas and let me tell you there was a huge baby buzz. I understand talking about babies is always exciting but in my situation I was newly engaged and I totally felt like she stole my thunder. So, while for the few months she was my bridesmaid she wanted my bridesmaid dresses to be a certain way....b/c she was planning on being pregnant. I almost agreed untill I realized I didn't want to change the bridesmaid dress that I had always wanted (strapless A-line) because of a possible prgnancy. She agreed and stepped down. Since then I am so happy that I picked what I wanted and didn't bend. I want to look back at my day and be happy with my choices knowing they were made for me and FH, not to make someone else happy.
I know when she was planning her wedding a year and a half ago she got everything she wanted. That's all we talked about. Now that it's my turn I feel like she kind of ignores the fact. She always has something better that she is doing to talk about. Plus, to top it off when I told her the date of my bachelorette party she said she would come as long as she didn't have another wedding to go to!! Oh and by the way, my SIL isn't pregnant and about a month ago she made the comment when asked that she & my brother weren't planning on having a baby for a while..Errr!!
So because of all this I definitley sympathize with you!! I know we have different situations but I know how you feel. Everyone is excited and wants attention when they are planning their wedding. Her pregnancy doesn't make your day any less special. It just gives you a few obstacles you may have to deal with. Just do what makes you happy.