Congratulations on the wedding and pregnancy!!
I'm size 10 and 20 weeks pregnant. In the past couple of weeks, my bump has grown much bigger, and I definitely have a noticeable bump. So, by 25 weeks, I'll be bigger again. I tend to find that I'm also extra big (bloated!) on some days too, so clothes that might be comfortable on one day may not be so great the next.
Not only that, my boobs are now MUCH bigger.
If I were you, I'd definitely speak to the wedding dress shop and see what they say. And, the further in advance, the better too. As others have said, it's not just a case of going up a size or two, as you won't be the same size all over.
There are some designers that specialise in maternity. Some styles may be more forgiving too. My SIL was pg (although only 12 weeks) when she got married, and had a lace up style corset at the back rather than buttons/zip. She could also loosen it slightly later on in the day after she'd eaten, etc. Don't underestimate how much you might want comfort as well as style!