I had an anterior placenta at 12 weeks, felt movement about 18 weeks. When I went for 19 week scan my placenta has moved to posterior. I asked Ob and they said that it can move around in early pregnancy and they are not too bothered about positioning until later on, unless it is very low and likely to cause probs. I didn't think that it could move and that once it implanted it was stuck there but obviously not, unless bub has been swinging from it!