Accuracy of 9 week & 12 week ultrasounds?

j&mmadison

Having a discussion with a friend about her due date.  Her cycles are irregular, so she can't base her due date on LMP. She had u/s at 9 weeks and at 12 weeks. They were off by only 4 days, so her due date has stayed the same. Her 20 week was maybe a week ahead...but growth can be different rates by that time. It's my understanding that the 9 and 12 week u/s would be quite accurate and only off by a few days at the most. Is this correct?