Dating behind but heartbeat strong

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Had my first scan today but instead of 8+2 she said everything is measuring more like 6+5 which makes no sense but had a strong heartbeat of 157. I have to schedule a transvaginal us in 1 to 3 weeks to see if it was just the quality and fact that it was an abdominal us. So instead of beginning of October I'm due more towards the middle of October based on this info. I guess I'll find out more once I have the follow up scan in the next few weeks. Anyone had this happen where you measure almost 2 weeks behind where you thought based on dates?