the due date is the date a pregnant woman expects to give birth. the expected date of delivery is not an exact date of delivery. scientists have calculated that only about 53% of women give birth on the expected date of delivery. since it is difficult for every pregnant woman to accurately judge the time of conception, medical regulations stipulate that the expected date of delivery is calculated from the first day of the last menstrual period. the entire pregnancy period is 280 days and 10 gestation months (each gestation month is 28 sky).