First Ultrasound

Kelly • Finally prego with baby #1
So the first day of my last period was 8/17. Pretty sure I O'd 2 weeks after . So technically , my first US should measure more about 5-6 weeks than 7-8 weeks right? I know they count from last period , and I'd love to be 8 weeks- but I didn't ovulate that day ..soo?? 
But I was jw if others exp'd the same - measuring less than expected on 1st ultrasound . Tx!