My question is, is it so important that you can't wait until you get married to get pregnant. Regardless of having sex before marriage and how you feel about it, your marriage should come before your children, not the other way around. Your marriage will show your children what to expect from life, the world around them, truth and security from your marriage, good and bad alike.
At this point, since you do not have children yet (i'm assuming) you're marriage is the most important thing right now, and you can wait a few weeks to get pregnant (believe me, the four months before your wedding will go incredibly fast. Plus, you do not want the stress of "oh my gosh i'm pregnant" ontop of the multiple levels of (healthy and necessary) stress that accompagny marriage--questions, doubts, planning, figuring out this is what you really want to commit to and making yourself ready mentally, emotionally, spiritually to commit.
That is a lot of stress on your body. And I don't recommend it for you or for a potential baby in the womb.
Even if you don't care about all that, you will almost certainly be beginning to show by then and your dress won't fit, especially if it's a form-fitting dress.
If you want to stop taking birth control pills so you can start a family right away, but are concerned about getting pregnant before your wedding, check out the natural family planning thread in the birth control forum.