What are the chances?

I was wondering what the percentage of miscarrying were around I am. This is our 3rd pregnancy. We lost the first to at 5 weeks on dec.2.2006 and at 7 weeks on Nov.10.2007. I am currently 15 weeks pregnant, but iI dont go to the doctor until March 6th (my 3rd appointment). We saw and heard the heartbeat (saw during my u/s for downs testing on feb 6th and heard by doppler by my OB/GYN on feb 7th). On the 6th the heartbeat was 160 bpm and on the 7th it was in the 150s. Lately I have just been terrified for some reason, more than usual. I am just curious. Thanks in advance.