Just a sac :/

So yesterday I went for a sono and the ob has estimated that I am about 7 weeks while I go with 6 weeks bc I found out the diameter of my sac and I heard that if u add 28 to the millimeters (12) it shows how far along you are. He expected a heartbeat while I didn't but the wierd part is that we didn't see anything but the sac! I thought maybe because he did the sono instead of the tech maybe he could've missed something (he did it quicker than usual) but at this point I thought it would be hard to miss. Any suggestions ?