2nd Ultrasound Question...

Liz
Hiya,
​Just a quick question! 
​I went for my 1st u/s at 7wks 4days according to LMP. However the u/s said I was 6wks 2days. Frustrating to go backwards, but ok!
​So my Dr told me to book back in for my 2nd u/s in 5wks time, or at the 11-12wk mark. 
​I'm going to be 9wks on Fri according to the 1st u/s (10wks 2days by LMP) but now I'm confused...when should I book my next u/s for?! 
​Sorry, that was longer than I thought!! Advice appreciated! Thank you ladies! :) x