The EDD can be calculated as The date of conception + 266 days The last menstrual period (LMP) + 280 days (40 weeks) for women with regular, 28-day menstrual cycles The LMP + 280 days + (cycle length - 28 days) for women with regular menstrual cycles other than 28 days duration When periods are regular and recorded contemporaneously, the menstrual history is relatively reliable.