How to calculate the date of conception backwards?

Use an app: There are many apps available where you can enter your due date and get the date of conception (look for "pregnancy wheel". Alternatively, if you know what week you are today, convert the weeks into days (for instance, 18 weeks and 3 days equals 129 days), then subtract 14 days (129-14=115), then count back that number of days. This will give you your conception date.

Backwards?: It doesn't work like that. You can't go backwards because you need the age of the fetus to do the counting. You can only figure that out approximately. In other words, if your baby seems to be full term, you can count backwards and subtract about 9 months. Counting forwards, though, your date of conception would be about 2 weeks from the first day of your last menstrual period.