Hi, I had a previa with my first pregnancy. Unfortunately my placenta didn't move so I automatically had to have a c-section. I know sometimes it does move. My doctor explained it like placing a dot on a balloon...the bigger you get, the more it can move, but if you have a complete previa I think the chances are slimmer. I was checked for movement at 20 weeks, 25 weeks and 30 weeks. Hope this helps :)
And when can I repeat the ultrasound ?