The NHS dating scan can move your due date

Your GP or midwife will usually give you an estimated due date from your last period, but the 12-week dating scan measures the baby directly and often becomes the official due date on your records if it differs from the period-based estimate by more than a few days, since a direct measurement is generally more accurate than counting from a possibly irregular cycle.

Your due date sets your maternity leave and pay dates

In the UK, statutory maternity leave and pay rules are calculated relative to your due date, not the birth itself: you must tell your employer you are pregnant by the 15th week before your due date (the "qualifying week") to claim Statutory Maternity Pay, and leave can start any time from 11 weeks before the due date. Keep your due date confirmation letter, since employers and the DWP use it as the reference point.