Flashback to 9 weeks!:) which reminds me of a question it was this day that my OB said i was 9 weeks…

Flashback to 9 weeks!:) which reminds me of a question it was this day that my OB said i was 9 weeks and 6 days which was on a Wednesday so I assumed I would be changing weeks every thursday then she tells me I will change weeks every Monday so the following monday is when i will turn 10 weeks even though on the wednesday before i was 9 and 6. Does she not know how to do math or have I been 4 days off since I was 9 weeks pregnant?