I think in my case I knew the placenta had moved, because when I had my midwife check-ups and they use the doppler to check the heartbeat, they had trouble finding the heartbeat directly i.e. not through the placenta. To get the direct heartbeat they had to put the probe right up under my ribs...so I figured that my placenta couldn't be all the way in the front as well as down below covering my cervix too. So I wasn't worried.

I was told at my morph scan to get rescanned at 32w along, but then the doc didn't order the scan until I was 34w. I think they don't check before 28w because it's not until about then that the uterus starts its major growth.

Let's hope you get the all-clear.