How Many Weeks??

I took math in school and can count. I know how to differentiate between days, weeks, months on a calendar. But yet, I can't figure out how many weeks I'm pregnant. And that's how you tell time in pregnancy, by weeks not months.

Every time I visit the Doctor, which is monthly. According to his calculations he always adds a
week; making me more pregnant than I think I am.

On December 10th, he told me I was 13 weeks pregnant.

On January 20th (6 weeks later) he told me I was 20 weeks pregnant....uuuuum, where did the extra week come from?

On March 2nd (again 6 weeks later) he told me I was 27 weeks pregnant....another week added!

Heeeeello, Doc! What kind of a calendar are you using?