First of all, the heartbeat does not matter at all to gender. My girl's heartbeat was actually the lowest of my 5 children and I was actually concerned at times with how slow it was compared to the others. Her heartbeat was in the 120-130 and I'd be like OMG is she dying LOL?? And they'd be, "no that's normal." A couple of my sons occasionally had HB as fast at 180 beats!!!
Secondly, you cannot tell on the basis of where they pick up the heartbeat, where the baby's placenta is. The baby moves around a lot and there's no way to know where the baby implanted.