1 Last Menstrual Period: Using the first day of your last menstrual cycle, gestational age can be determined (plus or minus 2 weeks.) The doctor adds 280 days to the first day of your last menstrual cycle to determine a due date.2  Based on that number and today’s date, gestational age is estimated. Last Menstrual Period: Using the first day of your last menstrual cycle, gestational age can be determined (plus or minus 2 weeks.) The doctor adds 280 days to the first day of your last menstrual cycle to determine a due date.