Gestational sac further along than baby?

Amie

Hi just wondering if anyone else has experienced this? I had my first scan last week and the baby was measuring at 7 weeks 4 days with a heart beat of 159. This seems to match up perfectly with my dates yet the sac was measuring at 9 weeks? The doctor did not concerned with this at all but I was just wondering if anyone else has had this and how it developed.