Worried about first ultrasound!

I went to a pregnancy center a week ago and they said I was 8 weeks pregnant but I counted from the first day of my last period and I'm 7...test positive but they suggest that I wait to get an ultrasound at week 10 (but really ill be week 9). I'm so worried that there will either be no baby, the baby will have no heart beat, or the baby isn't 9-10 like its suppose to... :(