This happened to me when I had a molar pregnancy. They are more common the older you get. I had the sac and nothing inside. My hcg's went extremely high and ended up around 500,000 before they did a d&c. When I went in at 7 weeks they just saw the empty
sac. Then at 10 weeks I had another sono and the sac was filled with growths and so they had to do an immediate d&c. It is something you might want to read up on just in case. My doctor hadn't seen one before so wasn't quick to catch it.