I don't think you should size up, unless you do it just by one size or so, assuming you could alter it back down (I ended up doing that one year simply because I lost a lot of weight, and the alteration worked fine.)
While pregnancy does make you giant, it kind of only does it at the tail end of the pregnancy. If the wedding is in June and you are still not KU now, you probably won't even be that large in June anyway. If the dress is flowy in the midsection then you're probably ok. But see if you can hold off ordering it for another month.
ETA: as the other posters said, I forgot about rib/boob expansion because it wasn't really a factor for me when I was pregnant given that I have a small chest to begin with. So it depends on your body type. If you have normal proportions or are a bit top-heavy then you should realize that as soon as you are KU your boobs will expand further. So one size up could be necessary even in the first month or two of pregnancy. When I was pregnant all of my dresses and tops suddenly fit a lot better (until the belly finally came along around 5-6 months in).