I just has 2 scans to rule out ectopic pregnancy...first one at 5 and 2 - nothing was visible at all. They said come back in a week. So I went back at 6 and 2 and the gestational sac was visible and obvious.
So don't worry if you can't see it yet
Edit: I forgot to mention that at 6 and 2 it was measured and calculated that I was only 5 and 2...so I guess this means when I got my first scan I was only 4 and 2...too small to see - this could very well be where you're at