How do you determine the due date of the baby? how accurate is the assumption and why does it always change everytime a scan is conducted

Estimate at best: Baby comes when it comes. The first date is estimated from the last full menses. Scans rely on using the statistical average size of the infant at any point to better predict a due date but 2 scans grant more useful data that increases the accuracy, Since babies also differ in size/growth rate all forms of predicting due date are educated guesses,80% deliver within a week +/- of this date.