Not sure what to think...

Ok so since we were ttc and I know when I ovulated, I know when I actually conceived... I told my dr that the date I ovulated was 4/27 (based on opk's for a whole 12 days since apparently I ovulate late into my cycle) and he's still dating the pregnancy back to my LMP which was 4/8...which puts me about 2.5-3 weeks later... So what I'm wondering is: when I go get my first sono/heartbeat, are they even going to be able to see/hear anything significant since they THINK I'm 10 weeks by then but I'll really only be 8..?