Heartbeat

Today was my first in-person appt. Based off their computation they thought I was at 12w before going in. Today it was discovered that I am only 6w 4d! We were not able to hear the heartbeat yet. Is this common?? Can the baby still be okay and healthy, regardless of not hearing a heartbeat at this stage? I’m kind of going crazy with all these thoughts. Maybe hearing from other pregnancy experiences, I can relax a little.