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. If you need to sit for a few minutes --- I think you'd be entitled to a break after 4 hours for sure. Personally, I say make as much money as you can before the baby arrives. It's get harder once baby is here. You can keep your eye open for a less strenuous job too if need to. Hang in there.