how far am i?

if my last period was november 20th, how far would i be? my period should have been december 21st, i am pregnant, but doctor said that i was about 4 weeks and 5 days on wednesday the 21st of december. but i’m not sure i’m that far ? HCG level on first draw was 51 (12/21) next draw HCG level was 117 on wednesday (12/23)