For parents-to-be, knowing the due date of a baby is paramount, but knowing the conception date is also important, especially if the pregnancy is unplanned. Finding a conception date based on the due date is not 100 percent accurate and relies on a couple of assumptions in the calculation. It will also not be able to tell exactly when conception occurred even if you recall having intercourse around that date, because conception can happen up to three days after intercourse.
Subtract 280 days from the due date. When calculating a due date, doctors generally assume a 28-day menstrual cycle and add 280 days onto the date of the first day of your last menstrual period.
Add 14 days if your average period is 28 days, which is the likely date of ovulation. If you know your ovulation is 16 days after the first day of your period or 11 days, add those numbers instead. Many women keep detailed records of their menstrual cycle, in which case knowing the date of ovulation is easier because you can look back at the records. If you are unsure of your ovulation time or even your average menstrual cycle then use the assumed 28-days. After delivery of the baby and resumption of menstrual periods, you can start to keep a record of period dates and learn about signs of ovulation; this not only helps you understand your body better but can also aid in future pregnancies.
Find the date after subtracting 280 days then adding the appropriate number to get your ovulation date. This is the closest conception date you will get based on the due date.