Depending what kind of coverage you want and how much you're willing to pay for it there are a number of options.
I decided to go with William Russel's platinum. I worked the cost versus benefit out by looking at what the worst case scenario was for cost of maternity care and delivery as well as other things like physio, then looking at how much 2 years of insurance cover would cost. It worked out well enough, and I've been able to claim a lot throughout my pregnancy and haven't had to compromise on my choice of hospital, rooms or doctors. I can even have physio sessions nearly every week of my pregnancy and still be under the limit.
It also gave me the option of being able to move countries and not lose my insurance cover (which is what happened when I left the UK), especially important to me at this time. Sure, I'd love to be able to put a claim in after every visit to the doctor rather than waiting until after delivery to claim, but we're fortunate enough to be in the position that waiting for a few months isn't a problem.
There are insurance brokers around town like E-sure and Kwiksure who can help you look at the cover benefits versus the premiums as well.
Good luck.