If you've never had a miscarriage before, it should be safe to fly. The other theory you've heard has to do with the fact that approximately 20% of all pregnancies end in miscarriage. Once you've had two or three miscarriages in a row, this risk increases to more than 30%. If you are going to have a miscarriage, it's much more likely to happen in the first trimester than later in pregnancy. By flying, you aren't increasing your risk of miscarriage, but it can be logistically and emotionally difficult if you experience a miscarriage away from home.