Congratulations! Seems scary now, but if you can just get through this (and it goes a lot faster than it seems) you'll be rewarded with...well, there are no words. Having a kid really is...darn. There really are no words. You won't regret it. ;)
As far as anyone else making such a huge mistake...well, I'd say everyone!
I'm not a fan of the popular modern notion that having babies is a matter of rational decision-making and is done sensibly. If that were the prime kick-in-the-butt, the human race wouldn't have gotten very far before dwindling out like a 4th of July sparkler. Kids are extraordinarily inconvenient in the short term. If we used our brain to make this decision, we'd never have 'em. Honestly, who do you know who's perfectly healthy...who can afford kids...who's in the perfect relationship...who'd make a perfect parent...who has the time to devote to properly raising them...who's at the right stage in their life...?
No, it's a darn good thing that having kids is the most natural thing in the world and an act that doesn't answer to logic, because there are ALWAYS rational reasons not to have babies...which I guess makes all of us mistakes of a sort!
P.S. I hope I haven't offended anyone with this opinion. I'm not saying having kids is the only reason to live, or making any statements about God or nature, or saying that everyone should be a parent, or that we should or shouldn't make hard decisions when it comes to whether or not to parent. I just believe that it's generally a good thing, and that when young people have kids these days many tend to frown on it. I believe society has a distorted view of childmaking - it's not, and never has been, really, a RATIONAL thing, though we like to believe all our decisions are based on reason. I also think having kids is generally unpopular these days because the economy sucks and new kids coming into the world are kinda seen as competition for the existing residents of it, But here's the thing - unless we find the key to immortality, kids are literally the wave of the future. We need 'em. Someone's gotta make 'em. And we're nowhere close to knowing enough about biology to "evolve" ourselves deliberately. So we do it like we always have - we have 'em when we have 'em.
Yikes, I didn't mean to get on my high horse...anyway, I'm so happy for you, though yes, it is terribly frightening. I'm thinking good thoughts for you. If you can, find yourself an obstetrician who's experienced with high-risk pregnancies. I had one and am very glad. I also found it odd that when I got pregnant, I seemed to enter this weird category of "the privileged sick" with the medical establishment - they really wanted to help, for once!