Actually it is just basic Biology. The vast majority of human females miscarry their first pregnancy, although often it happens early enough in the process that they never realize they were pregnant. Add to that the emotional strain Mia would have been under after moving away suddenly and a miscarriage is more than plausible. I think it adds more depth and complexity to both her hesitance to interact with the MC when he arrives (miscarriages typically are accompanied with feelings of self-blame, and since her relationship was a secret she wouldn't have been able to talk to anyone about it and would have wrestled with those feelings for a very long time), and to her need to be in a committed monogamous relationship and not in a harem. For me, yet another example of realism in the Mia games that sets them apart from the average game story, but I get that some people are not into that in their sex games. To each their own, I guess.