Ultrasound Accuracy: Understanding Fetal Measurements & Dating

Question
Lmp 5/27/12 due date 3/3/13...Which ultrasound is more accurate? A 8 week ultrasound or a 13 week ultrasound? Also I had an ultrasound at 32 weeks the baby was measuring 34 weeks and about 5 lbs the doctor says im having a big baby. He will send me for another ultrasound at 38 weeks.. Will I deliver earlier since baby is big?

Answer
An 8-10 week transvaginal (not abdominal) ultrasound is the most accurate, to within +/- 1 day.  The size of a baby does not correlate to when it will be born.  It should be allowed to come when it's ready.  Ultrasound at 32 weeks can be off by 2 weeks in either direction and up to 2 pounds either direction in weight. So, these measurements cannot be used to confirm a large baby because they can be so very wrong.  The only way to actually determine whether a baby is large is by performing consistent measurements weekly over the course of the pregnancy to establish a true growth rate.