Aw congratulations J2011! I'm in a similar position, having had a 10 week MC and a MMC diagnosed at 8 weeks, both last year. Now almost 38 weeks with a perfectly healthy little girl, and very much looking forward to meeting her!
Being something of an over-thinker, I have racked my brains for what could be different this time for months... but I've come to the conclusion it must've been bad luck. My GP had very kindly put me forward for the first round of tests after my second MC, because there's a history of early menopause in my family (I'm 34), and they showed nothing - in fact, they showed I was actually pretty healthy/normal.
Since my losses I've discovered at least four people in my circle of friends who had two or more losses before having their first baby, so sadly I don't think it's all that uncommon. It's just one of those things people don't really talk about, so you never get any idea of how common it can be.
Best of luck with third tri, may it all go smoothly for you! XXX