OH and I started trying TTC in Feb, thinking "meh...it'll take a good few months" fell pg the next month. Our wedding was booked for end of May nd i'd had the dress a year. Shop bought, so no chance of alterations. Was 11 weeks on the wedding day and was bursting out of it! No-one knew except me, him, best man and maid of honour, not even parents etc. When we told people, they then could SEE that I was pregnant but not before I said, IYSWIM.
I don't want to hijak (sp?) the thread with the above, but literally just went through this a couple of months ago 
I felt great on the day, the adrenaline kept me going, was a late ceremony, so didn' have to rush around in the morning, and was the last one off the dancefloor. It was a real pleasure to reveal to everyone during my speech that I was pg....everyone's jaw dropped, and it just magnified the day!
If I were you, i'd be more "worried" about if you get pg in September, then you'd have a 5 month bump! There are, however, some stunning maternity wedding dresses!!
Oh, and to answer the origional qu, I 'popped' out at 14 weeks.