Anterior placenta anyone?

Last appointment I had at 11w3d, we couldn't hear the heartbeat using a Doppler. We ended up doing an ultrasound just to make sure everything was okay, and come to find I have an anterior placenta. Well, my next appointment is next week and I'll be 15w2d. I was wondering how often an anterior placenta shifts. Anybody else had this? It's killing me that I'm so far along and still haven't heard a heartbeat. Only seen it. Just am a little worried!