Missing Yolk Sac?

So all the waiting has me so nervous. I see my doctor on Thursday but am wondering if anyone can comment on this. Is it abnormal that you can't see the yolk sac in this picture? Or is it completely normal that you don't always see it? Maybe it's the angle? This was taken transvaginally at 7w3d. Heartbeat was at 154. My understanding is that without a yolk sac there would be no heartbeat. Anyone? Thank you!