First day of your last period: Typically, pregnancies last for about 40 weeks. So a sure-shot way to estimate the due date would be counting 40 weeks (280 days) from the first day of your last menstrual period.
Conception date: The last menstrual period method may not work for you if your periods are irregular. In that case, you can calculate your estimated due date by counting 38 weeks (266 days) from the date of conception.
