Most self-insurance companies have a maternity 'rider' that you can just add on to your existing policy - all you have to do is ask.
However, - until the healthcare reforms change stuff, be prepared to pay for the policy for anywhere from 6 mo - 1 year before you can become pregnant. it's their way of actually getting money out of you.
I looked into this through Bluecare - BCBS and it was like an additional $500/mo to add on maternity. so definitely check around.
You also might want to find out about state insurance - depending on your income level you might qualify. Illinois has insurance that they can provide for incomes of various levels - it amounts to an HMO, so you don't have a choice about your Dr and where you deliver - you have to pick from their list, but something is better than nothing, right?
Good Luck