Chance of seeing HB at 6 week ultrasound?

I have an ultrasound scheduled for Friday. I will be 6 weeks +3 days. I was originally scheduled to have it at eight weeks, but moved it up because I was concerned that my hCG levels did not double in 48 hours. I am worried now that it will be too soon to see the heartbeat and I will actually just be more worried after the ultrasound. Has anyone had an ultrasound that early and seen the heartbeat? What do you think are the chances, or should I just try to push it back a little? Trying not to stress out, but after two losses in the last six months it's pretty hard.