I wouldn't say they shouldn't run evidence has shown they can run. I believe they have a physicians consent, and if they are in good shape. Then, I am fearful of them falling and having injury that can cause harm to them and or the baby. It's a risk either way if you're an expectant mother or not there's always chances of getting injured. I'm not sure, but I wouldn't recommend it if you haven't been running all along. WOW! that incredible!!!