Could mercury cause a miscarriage?

Maybe. Mercury (and the biologic form methylmercury) can be very toxic to a pregnancy and cause abnormalities and miscarriage. On the other hand, it is relatively difficult to come in contact with it these days unless you eat a bunch of fish (or certain types of fish: shark, king mackerel, tilefish, swordfish). If you broke a thermometer and touched mercury, that wouldn't be a problem.