The standard method is to add 280 days to the first day of your last menstrual period. How this concept works: the 280-day cycle works on the typical 266-day gestation period plus fourteen days, which would be the time from your first day of your period, until the time when you likely ovulated. This is not an exact science because this method assumes you ovulated exactly fourteen days after you started your period, and that fertilization took place on that exact day. So even the day your doctor tells you is just an approximation.