Slow heartbeat?

Hey ladies I had my first visit yesterday with our first baby! I was what I thought 7-2 but he measured me at 6-5. I tracked my ovulation and pretty well pin-pointed how far along I SHOULD be. I also thought the flickering heart beat looked slow but the doc said we are probably only seeing half of the actual beats. He said it was about 150, but it sure didn't look like it. He said he would see me again in 4 weeks. So that is good. Anyone had this happen at all? I am just afraid that the heartbeat isn't strong enough and that's why the measurements are off. Ps. Here is a picture of our little tink tink.