question...

This is my second baby. the doctors office is putting me at 7 weeks 4 days today going by my last period but my cycle days were around 35 days and I didnt get my positive until 6 days after AF was due. all of this makes me think I might be 6w4d instead. I have an ultrasound coming up Thursday to figure out how far I am and check things out so I'll have a better idea then but I'm curious if any of you had this experience and if you ended up being not quite as far along. my first baby I had 28 day cycles and I was pretty certain how far along I was before my ultrasound so this is new for me! what do you ladies think?