I would personally not be comfortable with that, for myself or for someone in a facility I was in charge of. That said, while I believe there was a relatively recent study about a correlation between heavy exercise and miscarriage, it was in the first trimester and most of those studies are based on recall. Honestly it probably isn't enough in itself to be heavy physical exertion or trauma, but I would be concerned about the potential for a secondary fall or injury and I think at some point it could potentially increase your risk for preterm labor and for the cord wrapping. But I am not a doctor, that would just come to mind. I think there are a lot of factors that potential trauma affecting the fetus would be based on, but when there are controllable risks, you might as well play it safe. As well I think many people eventually experience looser joints in pregnancy (maybe not until 3rd trimester for most) which would increase the risk of injury to hips, knees, etc and also stressing the joints repeatedly prior to this could lead to more pain later on.