I had this happen to me..I had my first ultrasound to find out how far I was..I was only 5+6 and they couldnt see but a little tiny speck..I went back 9 days later which I was 7+1 and saw the little one and the heart beat :) the tech told me the first time its usually about 6+3befbefore actually seeing more than a speck and a sack
I think around six weeks, when heartbeat starts. I waited till 8 weeks to gav mine & that was dr choice just to date pregnancy.
It depends on what you are looking to see...the sac shows up first at just a week or two..then a fetal pole I've seen that at 5 weeks but can be later...then a kidney bean looking sprout at around 8 weeks or so. I have seen these numbers vary by a few days or even more & be perfectly healthy. :) good luck