Dr. Allen Worrall of Alaska, USA, has this to say about the role of the early first trimesterultrasound scan. The First ultrasound always sets the gestational age. Subsequent ultrasounds determine the growth of the baby. You never change a due date based on a second or third ultrasound (assuming in all cases of course that the first ultrasound was done correctly, by someone who knows what they are doing, and using a machine that accurately makes the measurements).
Why is the first ultrasound always the most accurate in determining gestational age? Because we use the size of the baby to determine gestational age, and we do not all weigh exactly the same at birth. The closer we get to term, the more our "genetic heritage" is manifesting itself to make us small, average, or large. Or to put it another way: at around 8-10 weeks gestation we all are about the same size for a given gestational age. At 34-36 weeks gestation some of us are going to be small for our gestational age, some of us are going to be just about the size expected for our gestational age, and some of us are going to be large for our gestational age.
Mean Sac Diameter (MSD) is not as accurate as CRL. MSD is best used at very early gestational age when all you can see is the yolk sac, perhaps a heart beat, but you really cannot see the embryo yet. We are talking about 5w and a few days, by high resolution transvaginal scan.
From me: that was beautiful; you could not have put it better.