Obstetricians work on the assumption that ovulation has occurred 2 weeks after the LMP. As the baby gets bigger, it starts to express its individual growth potential. This is presuming that the pregnancy is actually at this stage of development. Even average babies can differ by up to 2 to 3 "weeks of growth". Dating scans also reveal other important information such as: How accurate is the ultrasound examination? Scan Conception dating an EDC implies it is considered ovulation was not 2 weeks after LMP women in Moskva Naughty say 3 weeks or whatever is established to be the case by scan Conception dating scan. Scan Conception dating scan or my dates - which should I believe? Most dating scans are done with a trans-abdominal transducer and a fullish bladder. These scans are only used to estimate the due date of your baby if this is all you have to go on.