Empty sac

Hi!

Has anyone actually saw an embryo on later stage after seeing an empty sac at first (at 5-6 weeks)?

I am pretty sure about the gestational weeks, because my menstrual cycle is very regular. So I am 6w +\- 2 days as per the LMP. I went for my first ultrasound today and all we saw was empty sac. The Doc said it is okay it can still has an embryo some days later, but with my first there was a little bean and a heartbeat around this week so it sounds impossible to me :(