I know of ONE stand alone maternity plan, here in Texas. It costs $350 a month, you have to wait 3 months before conceiving for the pregnancy to be covered, and it has a $5,000 deductible (you pay the first $5,000 in medical bills).
Effectively, it's a prepayment plan. You don't need insurance for that, you can bank the $350 x 12 monthly payments, and the $5,000 deductible, and pay your own birthing costs. If something really bad happens, your regular health insurance will still cover an emergency c section, and you can still add baby to your regular health insurance, the day he is born, to cover HIS hospital costs.