everybody has different experiences, my OB told me to not go anywhere more than 50miles from home during my pregnancy, but only because he doesn't want me to get in any trouble too far away from home. However, with my first pregnancy I went on an 11 hour flight to Europe. I was about 15-16 weeks pregnant at the time. I'd say, wait until the 2nd trimester, but than you should be fine. Get up, walk a lot, because of swelling of your legs and the chance of blood clots, and drink A LOT, to keep yourself hydrated.
I can't speak to the pp regarding her m/s in the first trimester, but can only share my experience.
As for the electric blanket, not sure it's about the magnetic field, but it's about increasing your body tempreture. That's why you shouldn't go into hot tubs or take too long hot baths. you don't want to increase your body tempreture.
hope it helped a bit.