No heartbeat? Still a chance?

Just after some advise. I had a vaginal ultra sound today and they were unable to find a heartbeat and baby was measuring smaller then what was expected according to my period dates. I am booked in for a d & c on Monday and I want to be sure all hope is gone before I through with it so my question is... how far along could it still be considered too early to see a heartbeat through a vaginal ultrasound but still have a viable pregnancy? Thanks in advance.