Hi and welcome!!
When it comes to assessing dates, the earlier the ultrasound the more reliable and accurate it is esp. at 6 weeks. and they do normally take it as it.
in saying that if you say you deff. know when your last known menstral period is then they take your word for it...but remember, the calculation they do based on your LKMP is based on a 28 day cycle, and like DaintreeDream said, not eveeryone has this cycle.
the way the cycle works...is EVERYONE ovulates 2 weeks before their menstral cycle...if you have a longer cycle, its the TWW ('2 week wait') period that is longer, or if you have a shorter cycle this time is shorter.
just means you add extra days or subtract extra days on the little formula. I have the formula and alterations somewhere in my pile of uni mess next to me, if you wanted me to get it out =)
All the best and good luck!!
wishing you a happy and healthy preg!!!!!