Just wanted to say thank you for your informative and thoughtful reply. It is much appreciated.
Dear Annabelle72-
There is an order to when things appear on ultrasound. The first feature imaged is a gestational sac. A after the gestational sac first appears a yolk sac can be seen. Then a week later a heart beat. I agree with your RE that it might just be too soon to see a heartbeat - it is not until 6 weeks 4 days of 6 weeks 5 days that a heartbeat can be seen 100% of the time in normal pregnancies. In other words, even at 6 weeks 2 days not all normal healthy pregnancies will image a fetal heart, and if you are only 5 weeks 6 days, or 6 weeks, because of a slight delay in implantation, the time to seeing a fetal heart will be delayed yet another couple of days.
If you are looking for published percentages, there is an old paper from 17 years ago that looked at this - Rempen A. Diagnosis of viability in early pregnancy with vaginal sonography. J Ultrasound Med 1990;9:711-6. These statistics may be off a couple of days as IVF pregnancies were not considered.
Also, Dr. Celso Silva and I wrote a chapter on early pregnancy loss in ACOG PRECIS GYNECOLOGY III that addresses this issue.
I know this is going to make for a long week for you - but a repeat ultrasound in a week will tell the story. I think your chances of a normal pregnancy are a little better, maybe 2/3 - 1/3, but really it is 0% or 100& for you, and only time will tell.