A blood test can only tell you if you are pregnant or not but the Dr. Can do a pelvic exam and tell you about how far along you are comparing to your dates of conception.
Can A Blood Test Tell You Approx How Far Along You Are Pregnant?
When you get your first sonogram they will measure the baby and look at its development. They will then tell you how far along you are.
Yes a blood test will most certainly give you a very good approximation then a later you/S will better tell you how old is the pregnancy.