How accurate are doctors at measuring how far along you are?

Al

Let’s say you could be anywhere from 10-13 weeks and doctors said you measured 13 weeks not 10. Would that be pretty accurate?

Our birth mother we are adopting from did not keep track of her periods so now I’m anxious about the due date(I have no idea why). Doctors said September 3rd (my birthday).

This was 3 weeks ago at her first appointment so if doctors measurements are correct she would be 16 now.