Ultrasound

So I went and got my first ultrasound last friday and she said I was measuring at 4-5 weeks. By my LMP, I would have been 8, but I have PCOS and didn't get a september period. I had a prenatal appointment booked for the 30th of this month but now they want me to go back for an ultrasound on the 29th? Isnt it still too early to see anything? I believe I'm 4 weeks and 6 days, and at 4 weeks 3 days all they seen was the yolksak? What difference will it be at 5+6