I had a scan right at 5 weeks exactly, we saw the sac and the yolk sac. I needed the scan to make sure that it was not an ectopic. I beleive what you see depends more on your HCG levels. Normally, from what I have heard, your levels need to be 1000-2000 before anything can be seen.I may have been earlier than 5 weeks. I was so early so that the machine could not even measure the size of the sac so she said that I had to be 4-5 weeks along. We are having another one done tomorrow, 6 weeks 6 days to look for proper growth and and a heartbeat. Here is a picture of my ultrasound. the small amound of whiteness on the right hand side of the sac is the yolk sac. (I hope that its okay that I post the pic here)
https://i142.photobucket.com/albums/r84/jen51903/babyshouse.jpg