The same happened to me I thought I was 7 weeks, but I was only 5 and the doc saw the sac, but no baby. Two weeks later when I was really 7 weeks they saw the baby's heartbeat. I think this is quite common. Hopefully in a couple of weeks when the baby is bigger they can detect it on the ultrasound. Right now I am 29 weeks, so it is possible to have hope. I know it is a confusing and painful time and I will pray for angels to comfort you.