Early Pregnancy Ultrasound: Fetal Pole & Heartbeat at 6 Weeks - What to Expect

Question
I've read that most people who have early u/s can see a fetal
heartbeat and fetal pole by 6 weeks.  I'm 6w3d (certain of
conception date due to monitoring and IUI, that date is based on
ovluation/iui date) and we did not see a fetal pole or heartbeat.  
They asked me to come back in 3 days but it's so hard waiting.  I
did see the sac and yolk but that's it.  Should I be worried or is
6w3d still a bit early and is there still hope?

Answer
Julie,

There is still hope.  The rule of thumb is if by seven weeks there is no decetable heartbeat, then the pregnancy is most likely not viable, unless there is questionable dating.  So, as hard as it is, try and think positive for the next three days.  It is a great sign that you were able to visualize the yoke sac.  Good luck!