When did you see the first heartbeat?

So I thought I was 6 weeks and I went to the Dr last week and had a vaginal sonogram. You see the sac and you can see something very small inside but he couldn't find a heart beat. The doctor thought I was maybe 5 weeks instead of 6. Going back tomorrow just wondering when everyone seen their baby heart beat for the first time??? I'm kinda nervous even through my doctor told me not to worry he has no doubts just that I'm a week earlier than I thought. I'm having all the pregnancy symptoms also... :/