with dc1 and dc2 they started it from the first day of your last menstrual period. i was kind of banking this time around on being more pregnant than the nhs due date calculator thinks i am (because then i could feel good about not feeling so sick, whereas if i am only 4 weeks as said calculator thinks then of course i wouldn't be sick yet). this is the only explanation i can think of, especially as i know this many days past my last due date the calculators before definitely had me as more pregnant iyswim.
Until you have your dating scan at 12wks it's done on LMP. However, i'd been monitoring my periods so I could tell my OB that my cycle was averaging 35 days not 28 so we both knew I wasn't as far along as the LMP implied. I also told the lady doing the scan, and my dates matched up better with the size of the baby. Do you know roughly how long your cycle is? I have my fingers crossed for you that you don't get sickness!
Has anyone else had their midwife tell them something different to the NHS calculator? The NHS calculator says 27th August, but midwife says 25th August (this is just going off dates, not had scan yet).