Travel it's two months between 12 week and 20 week scans (give or take) and what gets me is that once you get to the 20 weeks, you're still only half way!! I have a gender scan booked for 16 weeks, precisely so that the long wait is broken up a bit.
Have you considered getting a doppler? I got one last week and have managed to pick up the heartbeat (I have to be very strict with myself though, and not let myself panic if I can't find it as it is still very early) I still think I'm getting some little tiny movements, though I could also be imagining it I suppose!
I think 16 weeks is when the MW will generally start checking for a heartbeat, so can't be too difficult to find. Mine at 12 weeks with DS tried and couldn't find it, but only because she knew I had my scan that afternoon so wouldn't have to worry for too long!