Honestly, depending on how accurate you want it to be, that could be very hard to determine, because not every child is born exactly the same number of days past conception (even ones that are "full term"). Which is why a due date is an approximate - full term can be anywhere from 37 to 42 weeks, or about 280 days, but that's give or take about 14 days or so on either side of that....
So if you want to know for the sake of sentimental purposes, I would go with 280 days back from the day of birth. If you want to know for paternity reasons, you would be much better off with a dna test, because other than saying "I only had sex on __ day, so that must be when I got pg" there's really no way to tell the exact day of conception.
at 3:03 AM on Apr. 9, 2011