irregular periods or no periods help

if you have irregular periods and was supposed to measure 11 weeks but only measured 8 weeks 2 days at dating scan. does that mean i ovulated later ? also does it mean it’s accurate and that i’m actually 8 weeks because i read that it’s really 6+2 gestation because the 2 weeks for lmp but that doesn’t occur for me since i am 3 weeks off lmp… and doctors don’t do dating scan from conception so can someone explain is my dating scan accurate and can i settle on knowing it’s correct ?