Blood test to determine how far along I am

Beth
Okay so I had a MC on September 29th. Bled from 9/29-10/9 then stopped. Started spotting on 10/19 until I started bleeding super heavy again on 10/25 and that lasted until 11/3. I know when approximately when I ovulated/conceived. My doctor's office is sending me for a blood hcg to see how far I am. Thing is I know when I ovulated and most likely when I conceived. And how can the hcg determine how far I am since there's such a broad range for levels for each week??? Just confused. I guess part of my thing is I'd like to be seen sooner since I just had a MC but I know my office won't do that unless there's something wrong. I guess I can just go with it and hope they schedule my first appointment for late December. At this point, I believe I'm due around August 3-5. Only time will tell