Doctors blood tests

Bee
Hey guys
If your doctor takes a blood test to confirm how far you are, are they measuring how far you are roughly from date of conception? 
For example blood test came back as 4-5 weeks. I'm wondering if that means I conceived 2 weeks ago (4 weeks was from last period). Or if I conceived 4 weeks ago in the previous cycle so I would really be 7 weeks by last period?

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