Just put it completely out of your mind! I had the same thing during my second pregnancy - and the placenta fixed itself with no problem. So RELAX! (I know. I know. Easier said than done, but try!) OBs have to do the "full disclosure" thing; otherwise they end up getting sued down the road. This means that they are telling new moms EVERYTHING that could possibly ever even remotely be an issue. (With my oldest son, every single time I had an ultrasound I was told that this, that, and the other thing "was a sign of Down Syndrom." I wasn't in the risk category for Down Syndrome, and genetic tests done at 11 weeks gestation had already verified he had no genetic problems. But at every appointment there was "something" that was a sign of Down Syndrome. If I hadn't had the test done, I would've been a nervous wreck!)
If your OB is nonchalant, then try to take your cue from her. You're fine. You're baby is fine. And you are going to have a happy, healthy, wonderful pregnancy!