I really wouldn't be concerned (I know its easy to say and difficult to do especially with your history). If the doc had been any way concerned they would have done something about it. We had same issue when we went to see our OB at 16 weeks and he couldn't find heartbeat .... after a few goes he switched to small little US machine he had in office and there was the little tyke kicking and head banging away! Turns out bub was lot higher than he had thought which is why doppler didn't pick it up. Before he used the US machine and saw where bub was placed he said that placenta could be to front which can also mask the sound. While it did give me a fright first of all when we couldn't hear anything at least it meant I got to get another look at our growing baby