At your first prenatal visit, your doctor will evaluate where your baby is at in the development process and determine what your due date is based on this. Doctors are usually very accurate at this -- many times nailing the due date within a week, if not a few days. If you are not sure when you got pregnant, you can use the due date a few different ways to estimate your conception date, and this might help you identify the exact date of conception.
Count back 40 weeks from your anticipated due date. This is the most simple method of getting within a few days of your actual conception date, although the date you land on might not be the specific date.
Use a calendar or any past information available to you, including receipts and bank statements, to determine what you did on the nights close to your estimated conception date. If you were frequently sexually active during this period of time, it may be impossible to pin down the exact date. However, if you can recall a specific instance where you were sexually active, then there is the strong likelihood that was your conception date. If it is the only date you were sexually active within 2 days of your preliminary conception date, it's almost certainly the date of conception.
Count back 266 days from your anticipated due date. This number of days is the typical length of gestation for a baby.
Use Naegle's Rule to find the due date. This is often the most relied-upon result when different calculations differ. Use Naegle's Rule by counting back 3 months from the first day of your last menstrual cycle. Add 7 days to this number. This is your conception date.