From my experience I would wait til you know you are 7 weeks. My dr would have let me go in any time, but I waited til 7 weeks because early scans caused me a lot of anxiety last time. I had gone in at 5+4 last time, thinking I would see a heart beat, and the sac was completely empty. They weren't sure if my dates were off (I knew they were not because I chart) or if things just weren't developing. I had to wait a week and at 6+4 there was a heartbeat, but baby was a week behind. Finally at 7+4 all was caught up and on target! This time I waited til 7+1 and baby was a day behind, but that is within the margin of error and doesn't really mean anything. So just from my experience I would say 7 weeks!!