The expected date of confinement (eds) is usually estimated by a formula called:

It is called the Naegele's run the show. It predicts an expected due date in light of the beginning of the lady's last menstrual period. You just extra nine months and seven days to land at the due date.

It is a standard method for ascertaining the due date for a pregnancy while expecting a gestational age of 280 days at labor.