The date on which a baby’s birth is expected, traditionally calculated by doctors as 280 days (40 weeks) from the beginning of the woman’s last menstruation, even though fertilization probably did not take place until two weeks later, at the time of ovulation; also called expected date of confinement (EDC) by doctors. To calculate a due date, start with the day on which the last period began (say July 15), add nine months (making it April 15), and add seven days (making the due date April 22). By contrast the average period of gestation is 266 days (38 weeks), figured from the date of fertilization. If the fertilization date is known, subtracting two weeks from the due date gives a more accurate expected delivery date. Because the 40-week due date calculation is imprecise, obstetricians may give a more accurate date toward the end of the pregnancy, guided by a variety of tests and examinations, including when the first heartbeat is heard.