Baby measured a week behind what we thought...

Had my first appointment today, thought I was 7 wks 5 days but baby measured at 6 wks 4 days. A little confused, but doc said the heartbeat looked strong!! I have to wait for another US after I switch from the fertility docs to my regular OB. I just want to hear the heartbeat and get a number. Husband and I are excited to at least see the flicker of the heart on the screen!!