6w4d no embryo

I just got back from my first ultrasound and im trying to be hopefuly but its very hard. I was told there is a gestational sac and a yolk sack measuring at early 6 weeks but no embryo was seen. I was fully expecting to hear the heartbeat and to not even see an embryo was devastating. They told me to come back in a week for another ultrasound. Has anyone had any experience with this? I am certain of how far along I am because I tracked ovulation and my cycles are regular.