This time round i had a scan at 6w, the measurements were behind and we could only get a flicker of a heartbeat with an internal ultrasound, from the measurements she put my dates back to 5w2d, my Ob was not impressed and checked again the following week and moved my dates forward 4 days again. She said its so subjective when you are measuring .somethings of a mm. it can all differ in the angle its it.
hope the next test put your mind at rest xx