Alright well, I can only speak from my experience but I never had a blood test or scan before 20wks so it was used as my dating scan. I was a bit lost on my dates as I was in Japan and then sort of put the pregnancy in the back of my mind. I was pretty sure she was conceived the day I returned or sometime during the week but had no clue when my last period was... anyway! My ~20wk scan gave me an estimated due date of 31st of March and my DD was born 2 weeks before that on the 17th, spontaneously at 6lb 9oz (2.9kg). I'd say it was pretty spot on, she acted like she was a little early but not too early... if that makes sense? If she was 40wks when born, that means she was conceived sometime before I went to Japan which is possible I suppose although (TMI) I did have light bleeding a couple of days of my trip but then could that have been implantation??... ah, who knows!!

Wow, this really wasn't helpful in the end! I am fairly sure that the dating scan was correct although I would love to think that DD has been to Japan haha. Who knows! If it was me, I would take your hospital bag just in case because you never know when babies are going to make their appearance no matter how accurate your dates are!