July 19, 2013 (40 weeks)
This would lead to the possible conclusion that your ex is the dad, but it is not foolproof, since there are only three days between Guy A and the first possible conception date
. A conception calculator
is only as good as the method by which you got your due date
-- was it by ultrasound? How early was the ultrasound in the pregnancy? (The earlier the more accurate, and the later, well, if it was past the first trimester, it could easily be off by 3 days.