I just asked my OB about this, because two separate u/s have told me different things. He said the first one (I thought I was 6w3d but according to the u/s I was 5w4d) is the most accurate. He told me that the early ones are more accurate because at that stage all babies grow at about the same rate. By 12 weeks (my 2nd u/s) babies have started to grow at different rates depending on how much the mother eats/how much exercise she does etc. SO I guess I am a week further back than I thought