Congratulations! I feel you on the scared part — we found out that I was pregnant three months after the wedding, which while more socially acceptable is still really tough on our plans and budget.
To answer some of your questions:
Your dress will still fit. Corset-back = ADJUSTABLE. Just eat healthily and until you’re not hungry anymore, and don’t fall victim to the "Well, I’m pregnant and deserve it" argument to eat that entire carton of ice cream in the freezer (not yet, anyway). 🙂 You’re not really supposed to gain much weight in this trimester anyway. Lots of women lose weight due to morning sickness, honestly.
Your officiant — I assume this person is a conservative Christian of some manner if you signed an agreement not to have premarital sex (which is not crazy, btw, but true to his beliefs). I’d say ask yourself two questions:
1. Am I going to want to continue to be in a good relationship with this person after the wedding? IE, is he your personal pastor/a friend, etc?
2. Has this person seemed to be overly judgmental/vindictive/disrespectful with his beliefs?
If he’s not a loony, I’d say tell him. Any Christian worth his salt will not refuse to marry an expectant mother and her baby’s father — this is what he will WANT to see happen now more than ever. If he is a loony, well… tell him after you’ve told family because no guarantees about what he will feel to be his "duty" to share with your mom with or without your consent.
And your mom — I have seen a lot of really conservative mothers, even ones who really disapproved of their future son-in-laws, come around and be happy for their pregnant-shortly-before-marriage daughters. Does your mom like your FI? I’d say prepare for some rough times, but all is not lost. Blood is thicker than water after all, and unless your mom has some seriously messed-up priorities, I think that sooner or later she will just want to love on you and her new grandchild.
It seems you’re worried that people will think you have made a mistake — well, from their perspective, you did. Nothing that can be done about that now. What I will bet they want to see from you now, though, is a dedication to doing the right thing (get married!). God bless. 🙂