The u/s goes by size and apperance, tho they do enter in last menstral. According to menstral, id be 12 weeks, according to u/s I am 10 weeks, which goes with the day I kno I got pregnant. Go by u/s if they are early, but in 2nd trimester and on they get les accurate bc every baby grows according to genetics, food, medical, ect
The u/s measures by the date of your last period. Are you counting 6 weeks from when you believe you conceived?
if the ultrasound says ur 8 weeks and 4 days then u r most likely 8 weeks and 4 days... how did u figure that ur 6 weeks?