No heartbeat at 6 week ultrasound...

I'm a little worried even though my doctor told me all looks good for now. But talking to some girls on here who had ultrasounds around the same time I did and say they got to see the heartbeat. I have had 3 miscarriages and am so scared it will happen again. I had an ultrasound at 6 weeks and 2 days and all we saw was the yolk sac and the start of a baby but my doctor didn't push to see the heartbeat, just said it was still to early. I'm wondering if anyone else had had an ultrasound this early without seeing a heartbeat or is my doctor just trying to not worry me... Ugh... Dr waking out a little.