Confused about pregnancy dating


Hopefully someone can clear this up for me. Based on LMP (which was actually a chemical pregnancy not a true period), I should be 7 weeks today. Based on my first dating scan last week, the gestational sac measured 5 weeks 3 days, so only 4 days behind where I thought I was. No problem.

Today we saw and heard the heartbeat (114 bpm), but the baby is measuring 6 weeks exactly crown to rump.

The doctor didn’t seem concerned at all, but I’m confused as to how far along I really am. Am I only as far as the baby measures or do I continue to go based off my LMP and the baby is just behind?