I would recommend the kind of plan where you pay a co-pay rather than a % of the bill with a yearly cap on out of pocket expenses. I paid $50 per day, maximum $250 for my hospital stay and maternity care was included basically free. My daughter's tab came to $200,000 for 3 weeks of care. I haven't yet seen a bill for my portion of that but I know it will be much, much smaller.
You might also look into a medical savings account. I put money into that out of every pay check tax free, so they take $100 out of my paycheck, but it only feels like $70. I then use that to reimburse myself for all my medical expenses- so I dont have to use after tax dollars. It works out nicely. And if there's any money left over you can use it for other stuff- like over the counter medications, first aid supplies, etc.
As for the preexisting condition thing, I don't think they would see it that way if you had it previously, just if you are pregnant currently and have it. For me, my high blood pressure has continued beyond the pregnancy so they would probably see that as a preexisting condition, but not the preeclampsia.