I flew at 9wks pregnant, and started bleeding the day after we got home from a three-day trip. Everything ended up being fine (went through the rest of my pregnancy with no complications, had a very easy homebirth), but they weren't able to find out exactly what caused the bleeding. The doctors' (I went to the ER when I started bleeding, as per instructions) and midwives' best guess was that the change in air pressure (and possibly the altitude sickness I had...we lived in FL at the time and went camping high in the mountains in CO) could have caused it. I was told not to fly anymore at all during my pregnancy. I'll probably always be paranoid about flying while pregnant (probably wouldn't even fly at all while pregnant again), just because it was extremely mentally stressful for me to go through everything I did at the ER (to the point where I was physically ill because of it).