Hoping I'm off on my dates....

So we went for a dating ultrasound because I wasn't 100% sure I was correct on the EDD. Doctors guessed I would be about 7w+3d at this point, but at the ultrasound the tech said I looked more like 6weeks. She said it's a viable pregnancy as of now, we a gestational sac plus a yolk sac, but no heartbeat could be seen. Praying when I go back in a week that the baby can be seen and there's a heart beat. Has this ever happened to anyone else that their dates were wrong??