are early ultrasounds more accurate than lmp
Generally yes, but it needs to be quite early (before 8 weeks). There is approximately a 10% error and it only goes up. So, counting from conception if you have an u/s at 5 weeks (commonly considered 7 weeks) you could be off by 3.5 days. At 20 weeks, you could be close to 2 weeks off based on the u/s. This is, of course, approximate.
With DD we had an early u/s (7-8 weeks) and her due date was 12/31 (based on LMP it would have been 12/20 but we knew it was later than that based on when I finally tested pg). My water broke 12/31 and she was induced and born 1/2. I don't know how long she would have gone if left alone.
With DS we had an early u/s (7-8 weeks) and his due date was 8/22. Because of vacationing at the time of conception we KNEW the earliest he could have been due was 8/26 and probably a few days later. He was born 8/31, but we did have my membranes swept and used cohosh.
With DS1 we didn't have an early u/s. I had been tracking my cycles, but not charting and while the 12 week ultrasound, 20 week u/s and my LMP all agreed with a 7/28 due date we thought that a later date was more appropriate given my menstrual cycle history and when I tested positive with home pregnancy tests. According to the u/s he was born at 43 weeks, but he had no signs of post-dates and the midwife feels our guess was much closer: that he was born at 41w6d.