This is fine. It's certainly not in poor etiquette. People who assume it's because you're pregnant are the rude ones. If anyone is rude enough to ask, just smile and say, "Our financial situation changed and we feel it's a better idea to marry earlier than later." You really don't owe them an explanation. "Because we wanted to" is reason enough.
However, if you've already invited people to the later wedding, and then don't invite them to the smaller and earlier wedding ... then, yeah, that's pretty rude. Don't get me wrong, I totally understand your reasoning for needing to do so, but it's still not too polite. The exception would be if you are ONLY inviting your absolute nearest and dearest to the small wedding - like, your parents, siblings, grandparents and closest friends, maybe a total of 20 people or so. In this case, you can legitimately say that you didn't have enough money to invite everyone you originally wanted to, and people should understand.
But if you have already invited 200 people to your big wedding, and then invite only 120 to the smaller wedding - then that's rude. Those 40 people would probably be offended, and with good reason. In that case, you'd be more polite to postpone the wedding until you can save up enough money to invite the 200 people you'd originally promised an invite to (or cut it back to those super-close 20 people).
But if you haven't already invited anyone yet (verbally or with a paper invite or Save the Date card), then do what you wish.
As far as planning everything in time - take care of the basics first (photographer, church, reception hall, catering, license, music, chairs and tables, outfits), and leave the little stuff for later so that you can scrap it if time/money runs out (favors, flowers, limos, etc.). Figure out what you really want, and what you could stand to do without if push came to shove. You need to feed, seat and entertain your guests - they won't mind if you wind up getting cheap and fast invites, or if you use your own car instead of a limo, or if you don't offer favors. They WILL mind if you run out of food or don't rent enough chairs for everyone to sit down.