Hi Jan,
I have felt like I was going to pass out numerous times during this pregnancy. I have never got to the point of actually fainting, but i have come very close. I asked my midwife what I should do, and I was told to have something to raise my blood sugar on hand (museli bar, fruit etc) and to always have a bottle of water with me. And, not to stand for long periods, because the blood pools in my feet.
I find shopping centres the worst for me, because of the lack of fresh air.
Unfortunately nothing has really helped me... except for getting some fresh air and sitting down.