When does the heart beat show on a ultrasound?

With my first child I was 7w3d when I had my first u/s. My second child I was 9 weeks. This coming Monday I will have an u/s @ 6w6d and hope to see a heartbeat. I heard from my Ob/Gyn that at 6w1d there should be a heartbeat noticeable on trans. vag. u/s.