how many weeks am I? getting conflicting dates...

Bethany

So, I have my first midwife appt on the 21st -- some apps calculate me at 6 weeks and others say I'm at 7.. I'm so confused because I know I conceived the night of march 11th/early the 12th.  I want to follow along with the stages of development but not sure which week I'm at exactly.. if it's plus 2 weeks from conception wouldn't I be at 7 weeks?