Confused on how many wks I am?

Nicki • #1🍼 due 12-5-18

Why does the doctor tell me I am 9 weeks....I should be 7 weeks bc I didn’t have sex those 2 weeks prior it was only around I know they count from my last period but when I go for my first ultrasound on May 11th will the ultrasound show that I am 7 weeks or 9? My sis was trying to explain but I just don’t get it