No, I don't think standing for long periods of time will cause a miscarriage. Babies hang tough. What isn't so tough is our body. And we have added weight and extra hormones. Standing for 5 hours is going to make anyone tired and possibly sore, but is not dangerous. Now, as you get further along, you may want to address with your boss having some breaks thrown in. I doubt it would lead to Braxton hicks contractions (false labor) but for some, I imagine it could.