If lmp and 12 week ultrasound correlate, would the u/s be considered more accurate since the lmp is known? or is there still 1 week margin of error?

+/-1 week.: First trimester sonography is more accurate than even certain menstrual dates due to inherent variation in ovulation. The degree of accuracy at 12 weeks is within a week of the true due date. Take Folic Acid 1-5 mg daily and good luck!

LMP for dating : If in the first trimester, we go with the LMP unless the sono is over a week off. I we give the ultrasound date, but say "consistent with dates" the and still use the LMP date.