There are so many things which affect EDD's! With both of my last 2 pg's I got a couple of EDD's even though I knew when my LMP was for sure. With DD's pg I was given about 3 EDD's spanning about 1 1/2 weeks apart. With this pg, I know I got my LMP between 10th and 19th April though I am not sure the exact date of it starting. I had an u/s at what I estimated to be about 7 weeks. This scan said the pg was 7w4d. OK, that would be about right maybe a bit more than I though I could be, but hey what is a few days? My nuchal trasluscency scan (supposed to be done between 11-14weeks apparently) I had when I was just 11w2days according to dating scan. This u/s said baby measured 12w1d. A whole week out, quite a fair bit out from what I was guessing when my LMP was due. So either baby has had a huge growth spurt in the past few weeks or I was waaayyy out with my guestimations. I am assuming that my morphology scan will be telling me that baby is measuring even bigger than before. I tend to have fairly big babes. And long ones too. I'm saying my EDDis about the 17th of Jan. My first scan puts my EDD at around the 21st Jan. A week either way I'm not too worried. I will just be aware of baby in the weeks leading up to my EDD.
They say the earlier you can have a scan the better, I think about 6-7weeks is the best time.