My husband was not insured until he physically started his job here. That said, I would imagine it depends upon the employers plan if they are covered before the job starts. I would not assume many places would do that but who knows? As for pre-existing conditions... again, I suspect it depends upon the employer/insurer. I don't think our insurance company would have considered it pre-exisiting, but DH works for the Smithsonian - which is part of the US govt.
The other thing, though, is that many states offer free health care for pregnant women, children, and mothers. I arrived in the US 4 months before my DH and I had insurance for myself and my 2 young boys through the state of Maryland. I was really very pleased with it, too. I don't know if all states provide this, though.