During the first eight to ten weeks or so of fetal development, there are a lot of visible changes occurring as the egg develops into a growing fetus and the various organs and structures develop, so the earlier the scan, the better the chance of seeing something that gives a good idea of gestation. After this, there are less visible changes, just the size of the baby increases, and the sixe of the baby is a notoriously innacurate way of determining due dates.

However, the due "date" is really a misnomer - an astoundingly small percentage of babies are actually born on their due date. There is a four week "window" where the baby is ready to be born, and the due "date" is simply the day that falls into the middle of that period.

So, I would take your earliest ultrasound EDD, and calculate the period two weeks either side of that - that is when the baby is "due".

Some hospitals will tell you that ten days past the due date is "too overdue" and that the baby should be induced. In actual fact, research shows that there is no increase in problems up to 14 days after the due date, so don't feel pressuredinto having an induction that you probably don't need. In actual fact, you can push past the fourteen day mark if you monitor the baby and the placental function closely - even early ultrasounds are not 100% accurate, and babies who stubbornly do not come on the day or week we have chosen may well not be due as a result of an inaccurate dating scan.